Phylum Chordata Class Myxini Class … · What Is a Fish? In common (and especially older) usage,...

What Is a Fish?

In common (and especially older) usage, the term fish hasoften been used to describe a mixed assortment of water-dwelling animals. We speak of jellyfish, cuttlefish, starfish,crayfish, and shellfish, knowing full well that when we usethe word “fish” in such combinations, we are not referringto a true fish. In earlier times, even biologists did not makesuch a distinction. Sixteenth century natural historians classi-fied seals, whales, amphibians, crocodiles, even hippopota-muses, as well as a host of aquatic invertebrates, as fish.Later biologists were more discriminating, eliminating firstthe invertebrates and then the amphibians, reptiles, andmammals from the narrowing concept of a fish. Today werecognize a fish as an aquatic vertebrate with gills, limbs, ifpresent, in the form of fins, and usually with a skin covered

in scales of dermal origin. Even this modern concept of theterm “fish” is used for convenience, not as a taxonomic unit,because fishes do not compose a monophyletic group. Thecommon ancestor of the fishes is also an ancestor to theland vertebrates, which we exclude from the term “fish,”unless we use the term in an exceedingly nontraditionalway. Because fishes live in a habitat that is basically alien tohumans, people have rarely appreciated the remarkablediversity of these vertebrates. Nevertheless, whether appre-ciated by humans or not, the world’s fishes have enjoyed aneffusive proliferation that has produced an estimated 24,600living species—more than all other species of vertebratescombined—with adaptations that have fitted them to almostevery conceivable aquatic environment. No other animalgroup threatens their domination of the seas. �

C H A P T E R

26Fishes

Phylum ChordataClass Myxini

Class CephalaspidomorphiClass ChondrichthyesClass ActinopterygiiClass Sarcopterygii

507

Hammerhead shark near the Galápagos Islands.

508 PART 3 The Diversity of Animal Life

The life of a fish is bound to its bodyform. Their mastery of stream, lake,and ocean is revealed in the manyways that fishes have harmonizedtheir life design to the physical prop-erties of their aquatic surroundings.Suspended in a medium that is 800times more dense than air, a trout orpike can remain motionless, varyingits neutral buoyancy by adding orremoving air from the swim bladder.Or it may dart forward or at angles,using its fins as brakes and tilting rud-ders. With excellent organs for saltand water exchange, f ishes cansteady and finely tune their bodyfluid composition in their chosenfreshwater or seawater environment.Their gills are the most effective res-piratory devices in the animal king-dom for extracting oxygen from amedium that contains less than 1/20as much oxygen as air. Fishes haveexcellent olfactory and visual sensesand a unique lateral line system,which with its exquisite sensitivity towater currents and vibrations pro-

vides a “distance touch” in water.Thus in mastering the physical prob-lems of their element, early fishesevolved a basic body plan and set ofphysiological strategies that bothshaped and constrained the evolutionof their descendants.

Ancestry andRelationships of Major Groups of FishesThe fishes are of ancient ancestry,having descended from an unknown free-swimming protochordate ances-tor (hypotheses of chordate and verte-brate origins are discussed in Chapter25). The earliest fishlike vertebrateswere a paraphyletic assemblage ofjawless agnathan fishes, the ostraco-derms (Figure 25-14, p. 502). Onegroup of the ostracoderms gave rise to the jawed gnathostomes (Fig-ure 26-1).

The use of fishes as the plural form of fishmay sound odd to most people accustomedto using fish in both the singular and theplural. Fish refers to one or more individu-als of the some species; fishes refers tomore than one species.

The jawless agnathans, the leastderived of the two groups, includealong with the extinct ostracodermsthe living hagfishes and lampreys,fishes adapted as scavengers or para-sites. Although hagfishes have no ver-tebrae and lampreys have only rudi-mentary vertebrae, they neverthelessare included with the subphylum Ver-tebrata because they have a craniumand many other vertebrate homolo-gies. The ancestry of hagfishes andlampreys is uncertain; they bear littleresemblance to the extinct ostraco-derms. Although hagfishes and themore derived lampreys superficiallylook much alike, they are in fact so dif-ferent from each other that they havebeen assigned to separate classes byichthyologists.

Position in the AnimalKingdomThe fishes are a vast array of distantlyrelated gill-breathing aquatic vertebrateswith fins. Fishes are the most ancientand the most diverse of the monophylet-ic subphylum Vertebrata within the phy-lum Chordata, constituting five of thenine living vertebrate classes and one-half of the approximately 48,000 recog-nized vertebrate species. Although theyare a heterogeneous assemblage, theyexhibit phylogenetic continuity withinthe group and with the tetrapod verte-brates. The jawless fishes, hagfishes andlampreys, are the living forms thatresemble most closely the armored ostra-coderms that appeared in the Cambrianperiod of the Paleozoic. The living jawedfishes, cartilaginous and bony fishes, arerelated phylogenetically to the acantho-dians, a group of jawed fishes that werecontemporary with the placoderms ofthe Silurian and Devonian periods of thePaleozoic. The tetrapod vertebrates, theamphibians, reptiles, birds, and mam-

mals, arose from one lineage of bonyfishes, the sarcopterygians (lobe-finnedfishes). The evolution of fishes paralledthe appearance of numerous advances invertebrate history.

Biological Contributions

1. The basic vertebrate body plan wasestablished in the common ancestorof all vertebrates. Foremost was theevolution of cellular bone and thefirst endoskeleton. The vertebralcolumn replaced the notochord asthe main stiffening axis in most adultvertebrates and provided attachmentfor the skull, many muscles, and theappendages.

2. With the brain and spinal cordenclosed and protected within thecranium and vertebral column, theearly fishes were the first animals tohouse the central nervous system sep-arate from the rest of the body. Spe-cialized sense organs for taste,smell, and hearing evolved with a tri-

partite brain. Other sensory innova-tions include an inner ear with semi-circular canals, an electrosensory sys-tem, intricate lateral line sensorysystems, and extrinsic eye muscle.

3. The development of jaws with teethpermitted predation of large andactive foods. This gave rise to apredator-prey arms race that becamea major shaping element in vertebrateevolution through the ages.

4. The evolution of paired pectoraland pelvic fins supported by shoul-der and hip girdles provided greatlyimproved maneuverability andbecame the precursors of arms andlegs of tetrapod vertebrates.

5. Fishes developed the appropriatephysiological adaptations that enabledthem to invade every conceivabletype of aquatic habitat. The origin oflungs and air gulping in early lobe-finned fishes permitted limited pene-tration of semiterrestrial habitats andprepared for the invasion of land withthe evolution of tetrapods.

CHAPTER 26 Fishes 509

Coelacanth

Lungfishes

Sturgeons

Gars

Modernbony fishes

Sharks, skates,rays

Chimaeras

Lampreys

Hagfishes

Modern amphibians

Amniotes

Earlyamphibians

Gnatho-stomata

Placoderms

Agnatha

PermianCarboniferousDevonian

PALEOZOIC MESOZOIC

65 0225

CENOZOICSilurianOrdovicianCambrian

Vertebrata (craniata)

Ostracoderms

Chondrichthyes Holocephalans

Elasmobranchs

Acanthodians

Neopterygians

Modern neopterygians (teleosts)

Early neopterygiansChond

roste

ans

Actinopterygians (ray-finned fishes)

Sarc

opte

rygia

ns (l

obe-

finne

d fis

hes)

Commonchordateancestor

570Geologic time (My ago)

Figure 26-1Graphic representation of the family tree of fishes, showing the evolution of major groups through geological time. Numerous lineages of extinct fishes arenot shown. Widened areas in the lines of descent indicate periods of adaptive radiation and the relative number of species in each group. The fleshy-finnedfishes (sarcopterygians), for example, flourished in the Devonian period, but declined and are today represented by only four surviving genera (lungfishesand coelacanth). Homologies shared by the sarcopterygians and tetrapods suggest that they are sister groups. The sharks and rays radiated during theCarboniferous period. They came dangerously close to extinction during the Permian period but staged a recovery in the Mesozoic era and are a securegroup today. Johnny-come-latelies in fish evolution are the spectacularly diverse modern fishes, or teleosts, which make up most living fishes.


All remaining fishes have pairedappendages and jaws and areincluded, along with the tetrapods(land vertebrates) in the monophyleticlineage of gnathostomes. They appearin the fossil record in the late Silurianperiod with fully formed jaws, and noforms intermediate between agnathansand gnathostomes are known. By theDevonian period, the Age of Fishes,several distinct groups of jawed fisheswere well represented. One of these,the placoderms (p. 505), became ex-

tinct in the following Carboniferousperiod, leaving no direct descendants.A second group, the cartilaginousfishes of the class Chondrichthyes(sharks, rays, and chimaeras), lost theheavy dermal armor of early jawedfishes and adopted cartilage ratherthan bone for the skeleton. Most areactive predators with sharklike or ray-like body forms that have undergoneonly minor changes over the ages. As agroup, sharks and their kin flourishedduring the Devonian and Carbonifer-

ous periods of the Paleozoic era butdeclined dangerously close to extinc-tion at the end of the Paleozoic. Theystaged a recovery in the early Mesozoicand radiated to form the modest butthoroughly successful assemblage ofmodern sharks and rays (Figure 26-1).

The other two groups of gnatho-stome fishes, the acanthodians(p. 505) and the bony fishes, werewell represented in the Devonianperiod. Acanthodians somewhat resem-bled bony fishes but were distinguished

OsteichthyesChondrichthyesAgnatha

Teleostomi

Gnathostomata

Craniata = Vertebrata

Sarcopterygii(lobe-finned

fishes)Myxini

(hagfishes)Petromyzontiforme

(lampreys)Holocephali(chimaeras) Acanthodii†Placoderms†

Elasmobranchii(sharks, skates,

rays) Tetrapods

Limbs used forterrestriallocomotion

Unique supportive ele-ments in skeleton ofgirdle and limbs

Lung or swim bladderderived from gut, presence oflepidotrichia (fin rays)

Gills not attached to interbranchialseptum (as they are in sharks), bonyopercular covers

Part of second visceral arch modified assupporting element for jaws, teeth with dentine

Jaws from mandibular arch, 3 pairs semicircular canals, pairedappendages with internal skeletal supporting muscles, vertebraewith septum

2 or more pairs semicircular canals

Distinct head, tripartite brain, specialized sense organs,1 or more pairs semicircular canals, cranium, well-developed visceral skeleton

Loss of scales,teeth modified asgrinding plates

Placoid scales,claspers,cartilaginousskeleton

Naked skin withslime glands,degenerate eyes,accessory hearts

No pairedappendages,no dermal bone, long larval stage

"Ostracoderms"†

(anaspids)

†Extinct groups

Actinopterygii(ray-finned

fishes)

Figure 26-2Cladogram of the fishes, showing the probable relationships of major monophyletic fish taxa. Several alternative relationships have been proposed. Extinctgroups are designated by a dagger (†). Some of the shared derived characters that mark the branchings are shown to the right of the branch points. Thegroups Agnatha and Osteichthyes, although paraphyletic structural grades considered undesirable in cladistic classification, are conveniently recognized insystematics because they share broad structural and functional patterns of organization.


by having heavy spines on all finsexcept the caudal fin. They becameextinct in the lower Permian period.Although the affinities of the acanthodi-ans are much debated, many authorsbelieve that they are the sister group ofthe bony fishes. The bony fishes(Osteichthyes, Figure 26-2) are thedomi-nant fishes today. We can recog-nize two distinct lineages of bonyfishes. Of these two, by far the mostdiverse are the ray-finned fishes (classActinopterygii), which radiated to formthe modern bony fishes. The other lin-eage, the lobe-finned fishes (class Sar-copterygii), although a relic grouptoday, carry the distinction of being thesister group of the tetrapods. The lobe-finned fishes are represented today bythe lungfishes and the coelacanth—meager remnants of important stocksthat flourished in the Devonian period(Figure 26-1). A classification of themajor fish taxa is on p. 534.

Superclass Agnatha:Jawless FishesLiving jawless fishes are representedby approximately 84 species dividedbetween two classes: Myxini (hag-fishes) with about 43 species andCephalaspidomorphi (lampreys) with41 species (Figures 26-3 and 26-4).Members of both groups lack jaws,internal ossification, scales, and pairedfins, and both groups share porelikegill openings and an eel-like bodyform. In other respects, however, thetwo groups are morphologically verydifferent. Hagfishes are certainly theleast derived of the two, while lam-preys bear many derived morphologi-cal characters that place them phyloge-netically much closer to gnathostomesthan to hagfishes. Because of these differences, hagfishes and lampreyshave been assigned to separate verte-brate classes, leaving the grouping

“agnatha” as a paraphyletic assemblageof jawless fishes.

Class Myxini: HagfishesHagfishes are an entirely marinegroup that feeds on annelids, mol-luscs, crustaceans, and dead or dyingfishes. Thus they are not parasitic likelampreys but are scavengers andpredators. There are 43 describedspecies of hagfishes, of which the bestknown in North America are theAtlantic hagfish Myxine glutinosa (Gr.myxa, slime) (Figure 26-3) and thePacific hagfish Eptatretus stouti (N. L.ept, Gr. hepta, seven � tretos, perfo-rated). Although almost completelyblind, the hagfish is quickly attractedto food, especially dead or dyingfishes, by its keenly developed sensesof smell and touch. The hagfish entersa dead or dying animal through anorifice or by digging inside. Using two

Pores ofslime sacs

External gillopening

Caudal fin

Mouth surroundedby barbels

Mouth

Teeth ontongue

Olfactory sac

Teeth ontongue

Tongue

Nostril NotochordPharynxSpinal chord

Brain

Barbels

Mouth

Internal openingsto gill sacs

A B

C D

Figure 26-3The Atlantic hagfish Myxine glutinosa (class Myxini). A, External anatomy; B, Ventral view of head, showing horny plates used to grasp food during feeding;C, Sagittal section of head region (note retracted position of rasping tongue and internal openings into a row of gill sacs); D, Hagfish knotting, showing howit obtains leverage to tear flesh from prey.


toothed, keratinized plates on thetongue that fold together in a pincer-like action, the hagfish rasps away bitsof flesh from its prey. For extra lever-age, the hagfish often ties a knot in itstail, then passes it forward along thebody until it is pressed securelyagainst the side of its prey.

While the unique anatomical and physiologi-cal features of the strange hagfishes are ofinterest to biologists,hagfishes have notendeared themselves to either sports or com-mercial fishermen. In earlier days of commer-cial fishing mainly by gill nets and set lines,hagfish often bit into the bodies of capturedfish and ate out the contents, leaving behinda useless sack of skin and bones.But as largeand efficient otter trawls came into use,hag-fishes ceased to be an important pest.

Hagfishes are renowned for theirability to generate enormous quantitiesof slime. If disturbed or roughly han-dled, the hagfish exudes a milky fluidfrom special glands positioned alongthe body. On contact with seawater, thefluid forms a slime so slippery that theanimal is almost impossible to grasp.

Unlike any other vertebrate, thebody fluids of hagfishes are in osmoticequilibrium with seawater, as in mostmarine invertebrates. Hagfishes haveseveral other anatomical and physio-logical peculiarities, including a low-pressure circulatory system served bythree accessory hearts in addition to themain heart positioned behind the gills.

The reproductive biology of hag-fishes remains largely a mystery,despite a still unclaimed prize offeredmore than 100 years ago by theCopenhagen Academy of Science forinformation on the animal’s breedinghabits. It is known that females, whichin some species outnumber males 100to one, produce small numbers of sur-prisingly large, yolky eggs 2 to 7 cm indiameter depending on the species.There is no larval stage.

Class Cephalaspidomorphi(Petromyzontes):LampreysAll the lampreys of the Northern Hemi-sphere belong to the family Petromy-zontidae (Gr. petros, stone, � myzon,sucking). The group name refers to thelamprey’s habit of grasping a stonewith its mouth to hold position in acurrent. The destructive marine lam-prey Petromyzon marinus is found onboth sides of the Atlantic Ocean (inAmerica and Europe) and may attain alength of 1 m (Figure 26-4). Lampetra(L. lambo, to lick or lap up) also has awide distribution in North America andEurasia and ranges from 15 to 60 cmlong. There are 22 species of lampreysin North America. About half of thesebelong to the nonparasitic brook type;the others are parasitic. The genusIchthyomyzon (Gr. ichthyos, fish, �myzon, sucking), which includes threeparasitic and three nonparasitic species,is restricted to eastern North America.On the west coast of North America the chief marine form is Lampetra tridentatus.

Figure 26-4Sea lamprey, Petromyzon marinus, feeding on the body fluids of a dying fish.

Characteristics of Class Myxini

1. Body slender, eel-like, rounded,with naked skin containingslime glands

2. No paired appendages, no dor-sal fin (the caudal fin extendsanteriorly along the dorsal surface)

3. Fibrous and cartilaginousskeleton; notochord persistent

4. Biting mouth with two rows ofeversible teeth

5. Heart with sinus venosus, atrium,and ventricle; accessory hearts,aortic arches in gill region

6. Five to 16 pairs of gills with avariable number of gill openings

7. Segmented mesonephric kid-ney; marine, body fluids isos-motic with seawater

8. Digestive system without stom-ach; no spiral valve or cilia inintestinal tract

9. Dorsal nerve cord with differenti-ated brain; no cerebellum; 10pairs of cranial nerves; dorsal andventral nerve roots united

10. Sense organs of taste, smell, andhearing; eyes degenerate; onepair semicircular canals

11. Sexes separate (ovaries and testesin same individual but only one isfunctional); external fertilization;large yolky eggs, no larval stage


All lampreys ascend freshwaterstreams to breed. The marine forms areanadromous (Gr. anadromos, runningupward); that is, they leave the seawhere they spend their adult lives toswim up streams to spawn. In NorthAmerica all lampreys spawn in winter orspring. Males begin nest building andare joined later by females. Using theiroral discs to lift stones and pebbles andvigorous body vibrations to sweep awaylight debris, they form an oval depres-sion (Figure 26-5). At spawning, withthe female attached to a rock to main-tain her position over the nest, the maleattaches to the dorsal side of her head.As eggs are shed into the nest, they arefertilized by the male. The sticky eggsadhere to pebbles in the nest andquickly become covered with sand. Theadults die soon after spawning.

The eggs hatch in about 2 weeks,releasing small larvae (ammocoetes),which are so unlike their parents thatearly biologists thought they were a separate species. The larva bears aremarkable resemblance to amphi-oxus and possesses the basic chordatecharacteristics in such simplified andeasily visualized form that it has beenconsidered a chordate archetype (p. 501). After absorbing the remain-der of its yolk supply, the youngammocoete, now about 7 mm long,leaves the nest gravel and driftsdownstream to burrow in some suit-able sandy, low-current area. Thelarva takes up a suspension-feedingexistence while growing slowly for 3to 7 or more years, then rapidly meta-morphoses into an adult. This changeinvolves the eruption of eyes, replace-ment of the hood by the oral disc withkeratinized teeth, enlargement of fins,maturation of gonads, and modifica-tion of the gill openings.

Parasitic lampreys either migrate tothe sea, if marine, or remain in fresh

water, where they attach themselvesby their suckerlike mouth to a fish and, with their sharp keratinized teeth,rasp away the flesh and suck out bodyfluids (Figure 26-6). To promote the flow of blood, the lamprey injectsan anticoagulant into the wound.When gorged, the lamprey releases itshold but leaves the fish with a large,gaping wound that is sometimes fatal.The parasitic freshwater adults live 1 to 2 years before spawning and thendie; the anadromous forms live 2 to 3 years.

Nonparasitic lampreys do not feedafter emerging as adults and their ali-mentary canal degenerates to a non-functional strand of tissue. Within afew months they also spawn and die.

The invasion of the Great Lakes bythe landlocked sea lamprey Petromy-zon marinus in this century has had adevastating effect on the fisheries. Nolampreys were present in the GreatLakes west of Niagara Falls until theWelland Ship Canal was built in 1829.Even then nearly 100 years elapsed

MetamorphosisAmmocoete

larvae

Parasitic stage in lakes

Reproductionin streams

Migrationtowardslakes

Characteristics of ClassCephalaspidomorphi

1. Body slender, eel-like, roundedwith naked skin

2. One or two median fins, nopaired appendages

3. Fibrous and cartilaginousskeleton; notochord persistent

4. Suckerlike oral disc and tonguewith well-developed keratinizedteeth

5. Heart with sinus venosus, atrium,and ventricle; aortic arches in gillregion

6. Seven pairs of gills each withexternal gill opening

7. Opisthonephric kidney;anadromous and fresh water;body fluids osmotically andionically regulated

8. Dorsal nerve cord with differenti-ated brain, small cerebellumpresent; 10 pairs cranial nerves;dorsal and ventral nerve rootsseparated

9. Digestive system without stom-ach; intestine with spiral fold

10. Sense organs of taste, smell, hear-ing; eyes well developed in adult;two pairs semicircular canals

11. Sexes separate; single gonadwithout duct; external fertilization;long larval stage (ammocoete)

Figure 26-5Life cycle of the “landlocked” form of the sea lamprey Petromyzon marinus.


before sea lampreys were first seen inLake Erie. After that the sea lampreyspread rapidly and was causing extra-ordinary damage in all the Great Lakesby the middle 1940s. No fish specieswas immune from attack, but the lam-preys preferred lake trout, and thismultimillion dollar fishing industry wasbrought to total collapse in the late1950s. Lampreys then turned to rain-bow trout, whitefish, burbot, yellowperch, and lake herring, all importantcommercial species. These stocks weredecimated in turn. The lampreys thenbegan attacking chubs and suckers.Coincident with decline in attackedspecies, sea lampreys themselves

began to decline after reaching a peakabundance in 1951 in Lakes Huron andMichigan and in 1961 in Lake Superior.The fall has been attributed both todepletion of food and to the effective-ness of control measures (mainlychemical larvicides in selected spawn-ing streams). Lake trout, aided by arestocking program, are now recover-ing. Wounding rates are low in LakeMichigan but still high in some lakes.Fishery organizations are now experi-menting with the release of sterilizedmale lampreys into spawning streams;when fertile females mate with steril-ized males the female’s eggs fail todevelop.

ClassChondrichthyes:Cartilaginous FishesThere are nearly 850 living species inthe class Chondrichthyes, an ancient,compact, and highly developed group.Although a much smaller and lessdiverse assemblage than the bonyfishes, their impressive combination ofwell-developed sense organs, power-ful jaws and swimming musculature,and predaceous habits ensures them asecure and lasting place in the aquaticcommunity. One of their distinctivefeatures is their cartilaginous skeleton.

Characteristics of ClassChondrichthyes

1. Large (average about 2 m), bodyfusiform, or dorsoventrallydepressed, with a heterocercalcaudal fin (diphycercal in chi-maeras) (Figure 26-16); paired pec-toral and pelvic fins, two dorsalmedian fins; pelvic fins in malemodified for “claspers”

2. Mouth ventral; two olfactory sacsthat do not open into the mouth cav-ity in elasmobranchs; nostrils openinto mouth cavity in chimaeras; jawsmodified from pharyngeal arch

3. Skin with placoid scales or nakedin elasmobranchs (Figure 26-18);skin naked in chimaeras; teeth ofmodified placoid scales and seriallyreplaced in elasmobranchs; teethmodified as grinding plates in chi-maeras

4. Endoskeleton entirely cartilagi-nous; notochord persistent butreduced; vertebrae complete andseparate in elasmobranchs; verte-brae present but centra absent inchimaeras; appendicular, girdle, andvisceral skeletons present; craniumsutureless

5. Digestive system with J-shapedstomach (stomach absent in chi-maeras); intestine with spiralvalve; often with large oil-filledliver for buoyancy

6. Circulatory system of several pairsof aortic arches; dorsal and ventralaorta, capillary and venous systems,hepatic portal and renal portal sys-tems; four-chambered heart withsinus venosus, atrium, ventricle, andconus arteriosus

7. Respiration by means of five toseven pairs of gills leading toexposed gill slits in elasmobranchs;four pairs of gills covered by anoperculum in chimaeras

8. No swim bladder or lung9. Opisthonephric kidney and

rectal gland; blood isosmotic or slightly hyperosmotic to seawater; high concentrations ofurea and trimethylamine oxidein blood

10. Brain of two olfactory lobes, two cerebral hemispheres, two optic lobes, cerebellum, medullaoblongata; 10 pairs of cranialnerves; three pairs of semicircu-lar canals

11. Senses of smell, vibration reception(lateral line system), vision, andelectroreception well developed;inner ear opens to outside viaendolymphatic duct

12. Sexes separate; gonads paired;reproductive ducts open into cloaca(separate urogenital and anal open-ings in chimaeras); oviparous, ovovi-viparous, or viviparous; direct devel-opment; fertilization internal

Horny tongue

Pharynx

Buccal funnelwith hornyteeth

Tongue protruded forrasping flesh

Lingualcartilage

Attachment to fish withhorny teeth and suction

Figure 26-6How the lamprey uses its horny tongue to feed.After firmly attaching to a fish by its sucker, theprotrusible tongue rapidly rasps an openingthrough the fish’s integument. Body fluid,abraded skin, and muscle are eaten.


Although calcification may be exten-sive in their skeletons, bone is entirelyabsent throughout the class—a curiousevolutionary feature, since the Chon-drichthyes are derived from ancestorshaving well-developed bone. Almostall chondrichthyans are marine; only28 species live primarily in fresh water.

With the exception of whales,sharks include the largest living verte-brates. The larger sharks may reach 12 m in length. Dogfish sharks sowidely studied in zoological laborato-ries rarely exceed 1 m.

Subclass Elasmobranchii:Sharks, Skates, and RaysThere are nine living orders of elasmo-branchs, numbering about 815 species.

Coastal waters are dominated by therequiem sharks, order Carcharhini-formes, which consist of typical-lookingsharks such as the tiger and bull sharksand more bizarre forms, including thehammerheads (Figure 26-7). The orderLamniformes contains several large,pelagic sharks dangerous to humans,including the great white and makosharks. Dogfish sharks, familiar to gen-erations of comparative anatomy stu-dents, are in the order Squaliformes.Skates and several groups of rays (saw-fish rays, electric rays, stingrays, eaglerays, manta rays, and devil rays) belongto the order Rajiformes.

Much has been written about thepropensities of sharks to attack humans,both by those exaggerating their fero-cious nature and by those seeking to

write them off as harmless. It is true, asthe latter group of writers argues, thatsharks are by nature timid and cautious.But it also is a fact that certain of themare dangerous to humans. There arenumerous authenticated cases of sharkattacks by Carcharodon (Gr. karcharos,sharp, � odous, tooth), the great whiteshark (reaching 6 m); mako sharks Isu-rus (Gr. is, equal, � ouros, tail); the tigershark Galeocerdo (Gr. galeos, shark, �kerd|, fox); and hammerhead sharksSphyrna (Gr. sphyra, hammer). Moreshark casualties have been reportedfrom the tropical and temperate watersof the Australian region than from any other. During World War II therewere several reports of mass sharkattacks on the victims of ship sinkings intropical waters.

Great white shark

Hammerhead shark

Whale shark

Thresher shark

Figure 26-7Diversity in sharks of the subclass Elasmobranchii. The thresher shark Alopias vulpinus, exceptional because of its long upper tail lobe, may exceed 4 m inlength. The great white shark Carcharodon carcharias, largest and most notorious of dangerous sharks, is a heavy-bodied, spindle-shaped shark that mayreach 6 m in length. The nine species of hammerheads (genus Sphyrna) are distinguished from all other sharks by the flattened head with hammerlike lobesbearing eyes and nostrils on the ends. The whale shark Rhincodon typus is the world’s largest fish, reaching 12 m in length. It is a suspension feeder thatfeeds on plankton collected on a sievelike mesh over its gills.


The worldwide shark fishery is experiencingunprecedented pressure,driven by the highprice of shark fins for shark-fin soup,an orien-tal delicacy (which commonly sells for$50.00 per bowl).Coastal shark populationsin general have declined so rapidly that“finning”is to be outlawed in the UnitedStates;other countries, too,are setting quotasto protect threatened shark populations.Evenin the Marine Resources Reserve of the Galá-pagos Islands,one of the world’s exceptionalwild places, tens of thousands of sharks havebeen killed illegally for the Asian shark-finmarket. That illegal fishery continues at thiswriting.Contributing to the threatened col-lapse of shark fisheries worldwide is the lowfecundity of sharks,and the long time re-quired by most sharks to reach sexual maturi-ty; some species take as long as 35 years.

Form and Function

Although to most people sharks have asinister appearance and fearsome rep-utation, they are at the same timeamong the most gracefully streamlinedof all fishes. The body of a dogfishshark (Figure 26-8) is fusiform (spindleshaped). In front of the ventral mouthis a pointed rostrum; at the posteriorend the vertebral column turns up toend in the longer upper lobe of thetail. This type of tail is called hetero-cercal. The fins consist of paired pec-toral and pelvic fins supported by

appendicular skeletons, one or twomedian dorsal fins (each with a spinein Squalus [L. a kind of sea fish]), and amedian caudal fin. A median anal finis present in most sharks, including thesmooth dogfish Mustelus (L. mustela,weasel). In the male, the medial part ofthe pelvic fin is modified to form aclasper, which is used in copulation.Paired nostrils (blind pouches) areventral and anterior to the mouth (Fig-ure 26-9). The lateral eyes are lidless,and behind each eye is a spiracle (rem-nant of the first gill slit). Five gill slitsare found anterior to each pectoral fin.The tough, leathery skin is coveredwith toothlike, dermal placoid scalesarranged to reduce the turbulence ofwater flowing along the body surfaceduring swimming.

Sharks are well equipped for theirpredatory life. They track their preyusing highly sensitive senses in anorderly sequence. Sharks may initiallydetect prey from a kilometer or moreaway with their large olfactory organs,capable of detecting chemicals as lowas 1 part per 10 billion. The laterallyplaced nostrils of hammerhead sharks(Figure 26-7) may enhance odor local-ization by improving stereo-olfaction.Prey also may be located from longdistances by sensing low-frequencyvibrations with mechanoreceptors inthe lateral line system. This system is

composed of special receptor organs(neuromasts) in interconnected tubesand pores extending along the sides ofthe body and over the head (Figure 26-10). At closer range the shark switchesto vision as the primary method oftracking prey. Contrary to popularbelief, most sharks have excellentvision, even in dimly lit waters. Duringthe final stage of attack, sharks areguided to their prey by the bioelectricfields that surround all animals. Elec-troreceptors, the ampullae of Loren-zini (Figure 26-9), are located primar-ily on the shark’s head. In addition,sharks may use electroreception to findprey buried in the sand.

Both the upper and lower jaws ofsharks are provided with many sharp,triangular teeth. The front row of func-tional teeth on the edge of the jaw isbacked by rows of developing teeththat replace worn teeth throughout thelife of the shark (Figure 26-8 and 26-9).The mouth cavity opens into the largepharynx, which contains openings tothe separate gill slits and spiracles. Ashort, wide esophagus runs to the J-shaped stomach. A liver and pan-creas open into the short, straightintestine, which contains the spiralvalve that slows passage of food andincreases the absorptive surface (Fig-ure 26-11). Attached to the short rec-tum is the rectal gland, unique to

Spiracle Spine

Externalgill openings

Nostril

Rostrum

Pectoral fin

Pelvic fin

Caudalfin

Firstdorsal fin

Seconddorsal fin

Toothshed

Figure 26-8Dogfish shark, Squalus acanthias. Inset: section of lower jaw shows new teeth developing inside the jaw. These move forward to replace lost teeth. Rate ofreplacement varies in different species.


chondrichthyans, which secretes a col-orless fluid containing a high concen-tration of sodium chloride. It assists theopisthonephric kidney in regulatingthe salt concentration of the blood.The chambers of the heart arearranged in tandem formation, and theflow pattern of the circulatory system isbasically the same as that of other gill-breathing vertebrates (Figure 26-11).

All chondrichthyans have internalfertilization, but maternal support ofthe embryo is highly variable. Manyelasmobranchs lay large, yolky eggsimmediately after fertilization; thesespecies are termed oviparous. Someoviparous sharks and rays deposit theireggs in a horny capsule called a “mer-maid’s purse,” which often is providedwith tendrils that wrap around the firstfirm object it contacts, much like thetendrils of grape vines. The embryo isnourished from the yolk for a pro-longed period—6 to 9 months in some,as much as 2 years in one species—before hatching as a miniature replicaof the adult. Many sharks, however,retain the embryos in the reproductivetract for prolonged periods. Some areovoviviparous species, which retainthe developing young in the uteruswhile they are nourished by the con-tents of their yolk sac until born. Still other species have true vivipa-rous reproduction. In these, embryosreceive nourishment from the maternalbloodstream through the placenta,or from nutritive secretions, “uterinemilk,” produced by the mother. Somesharks (sand tigers) exhibit a grislytype of reproduction in which embryos

receive additional nutrition by eatingeggs and siblings. The evolution ofprolonged retention of embryos bymany elasmobranchs was an importantinnovation that contributed to the suc-cess of these fish. However, regardlessof the form of maternal support, oncethe eggs are laid, or the young born,all parental care ends.

Marine elasmobranchs have devel-oped an interesting solution to thephysiological problem of living in asalty medium. To prevent water frombeing drawn out of the body osmoti-cally, elasmobranchs retain nitroge-nous compounds, especially urea andtrimethylamine oxide, in the blood.These solutes, combined with theblood salts, raise the blood solute con-

centration to exceed slightly that ofseawater, eliminating an osmotic in-equality between their bodies and thesurrounding seawater.

A little more than half of all elas-mobranchs are rays, a group thatincludes skates, electric rays, saw-fishes, stingrays, eagle rays, and mantarays. Most are specialized for bottomdwelling, with a dorsoventrally flat-tened body and greatly enlarged pec-toral fins that are fused to the head andused like wings in swimming (Fig-ure 26-12). The gill openings are onthe underside of the head, but thelarge spiracles are on top. Water forbreathing is taken in through thesespiracles to prevent clogging the gills,for the mouth is often buried in sand.Their teeth are adapted for crushingtheir prey: molluscs, crustaceans, andan occasional small fish.

In the order containing the skates and rays(Rajiformes), we commonly refer to mem-bers of only one family (Rajidae) as skates.Alone among members of the Rajiformes,skates do not bear living young but laylarge, yolky eggs enclosed within a hornycovering (the “mermaid’s purse”) that oftenwashes up on beaches. Although the tail isslender, skates have a somewhat more mus-cular tail than most rays, and they usuallyhave two dorsal fins and sometimes a cau-dal fin.

The stingrays have a slender andwhiplike tail that is armed with one ormore saw-edged spines with venomglands at the base. Wounds from thespines are excruciatingly painful, andmay heal slowly and with complica-tions. Electric rays are sluggish fish withlarge electric organs on each side of thehead (Figure 26-13). Each organ ismade up of numerous vertical stacks ofdisclike cells connected in parallel sothat when all the cells are dischargedsimultaneously, a high-amperage cur-rent is produced that flows out into thesurrounding water. The voltage pro-duced is relatively low (50 volts) but thepower output may be almost one kilo-watt—quite sufficient to stun prey ordiscourage predators. Electric rays wereused by the ancient Egyptians for a

Figure 26-9Head of sand tiger shark Carcharias sp. Notethe series of successional teeth. Also visible in arow below the eye are the ampullae of Lorenzini.

Lateral line canal

Opening to surface

Neuromastcells

Lateralline

Jelly-filledcanal

Ampullary organsof Lorenzini

Pore

Ampulla ofLorenzini

Nerve

Figure 26-10Sensory canals and receptors in a shark. Theampullae of Lorenzini respond to weak electricfields, and possibly to temperature, waterpressure, and salinity. The lateral line sensors,called neuromasts, are sensitive to disturbancesin the water, enabling the shark to detect nearbyobjects by reflected waves in the water.


form of electrotherapy in the treatmentof afflictions such as arthritis and gout.

Subclass Holocephali:ChimaerasMembers of the small subclass Holo-cephali, distinguished by such sugges-tive names as ratfish (Figure 26-14), rabbitfish, spookfish, and ghostfish, areremnants of a line that diverged fromthe earliest shark lineage which origi-

nated at least 360 million years ago(Devonian or Silurian periods of thePaleozoic). Fossil chimaeras (ky-meer�-uz) first occurred in the Carboniferousperiod, reached their zenith in the Creta-ceous and early Tertiary periods (120million to 50 million years ago), andhave declined ever since. Today thereare only about 31 species extant.

Anatomically the chimaeras haveseveral features linking them to elas-mobranchs, but possess a suite of

unique characters, too. Instead of atoothed mouth, their jaws bear largeflat plates. The upper jaw is completelyfused to the cranium, a most unusualdevelopment in fishes. Their food isseaweed, molluscs, echinoderms, crus-taceans, and fishes—a surprisinglymixed diet for such a specializedgrinding dentition. Chimaeras are notcommercial species and are seldomcaught. Despite their grotesque shape,they are beautifully colored with apearly iridescence.

Osteichthyes: Bony Fishes

Origin, Evolution, andDiversityIn the early to middle Silurian, a lin-eage of fishes with bony endoskele-tons gave rise to a clade of vertebratesthat contains 96% of the living fishesand all of the living tetrapods. Thefishes of this clade have traditionallybeen termed “bony fishes” (Oste-ichthyes), because it was originallybelieved these were the only fisheswith bony skeletons. Although it is nowrecognized that bone occurs in manyother early fishes (ostracoderms, placo-derms, and acanthodians), bony fishesand their tetrapod descendants areunited by the presence of endochon-dral bone (bone that replaces cartilage

Forebrain

Midbrain

Hindbrain

Efferentbranchial

artery

Dorsalaorta Testis

Celiacartery Stomach

Kidney Firstdorsal

fin

Rostrum

Afferentbranchial

arteryVentralaorta

Heart

Pectoralfin

Liver

Pancreas

IntestineSpiralvalve

Rectalgland Cloaca Pelvic

fin

Caudalartery

Caudalvein

Spleen

Figure 26-11Internal anatomy of dogfish shark Squalus acanthias.

Figure 26-12Skates and rays are specialized for life on the sea floor. Both the clearnose skate Raja eglanteria(A), and the southern stingray Dasyatis americana (B) are flattened dorsoventrally and move byundulations of winglike pectoral fins. The stingray (B) is followed by a pilot fish.

A B


developmentally), the presence oflungs or a swim bladder derived fromthe gut, and several cranial and dentalcharacters. Because the traditionalusage of Osteichthyes does notdescribe a monophyletic (natural)group (Figure 26-2), most recent classi-fications, including the one presentedat the end of this chapter, do not rec-ognize this term as a valid taxon.Rather, it is used as a term of conve-nience to describe a group of verte-brates with endochondral bone thatare conventionally termed fishes.

Fossils of the earliest bony fishesshow similarities in several craniopha-ryngeal structures, including a bonyoperculum and branchiostegal rays,with acanthodians (p. 505 and Fig-ure 25-17), indicating they likely de-scended from a common ancestor. Bythe middle of the Devonian the bonyfishes already had radiated extensivelyinto two major lin-eages, with adapta-tions that fitted them for every aquatichabitat except the most inhospitable.One of these lineages, the ray-finnedfishes (class Actinopterygii), includesthe modern bony fishes (Figure 26-15),the most speciose of living vertebrates.A second lineage, the lobe-finnedfishes (class Sarcopterygii), is repre-sented today by only seven fishlikevertebrates, the lungfishes and thecoelacanth (Figures 26-22 and 26-23).The evolutionary history of this lineage is of particular interest because it gaverise to the land vertebrates (tetrapods).

Several key adaptations contributedto their radiation. Bony fishes have anoperculum over the gill composed ofbony plates and attached to a series ofmuscles. This feature increased respira-tory efficiency because the outwardrotation of the operculum created anegative pressure so that water wouldbe drawn across the gills, as well aspushed across by the mouth pump. Agas-filled derivative of the esophagusprovided an additional means of gasexchange in hypoxic waters and an effi-cient means for achieving neutral buoy-ancy. Progressive specialization of jawmusculature and skeletal elementsinvolved in feeding is another key fea-ture in their evolution.

Olfactory lobes

Cerebellum

Left electricorgan

Cranial nerves

Right electricorgan

Spinal cord

Figure 26-13Electric ray Torpedo with electric organs exposed. Organs are built up of disclike, multinucleatedcells called electrocytes. When all cells are discharged simultaneously, a high-amperage currentflows into the surrounding water to stun prey or discourage predators. Power output may be up toone kilowatt.

Figure 26-14Spotted ratfish, Hydrolagus collei, of North American west coast. This species is one of the mosthandsome of chimaeras, which tend toward bizarre appearances.


Swim bladder

Kidney(opisthonephros)

Second dorsal fin

Lateral line

Caudalfin

Anal fin

Urinarybladder

Urogenitalopening

Anus

Ovary

Intestine

Stomach

Spleen

Pelvic fin

Pyloric ceca

Liver

Ventricle

Bulbus arteriosus

Afferentbranchial artery

Olfactorybulb

Efferentbranchial artery

Brain

Spinal cord

Vertebra

First dorsal fin

Myomere(muscle segments)

Fin ray support

Figure 26-15Internal anatomy of the yellow perch Percaflavescens, a freshwater teleost fish.

Characteristics of Class Actinopterygii

1. Skeleton with bone of endochon-dral origin; caudal fin heterocercal inancestral forms, usually homocercalin advanced forms (Figure 26-16);skin with mucous glands and embed-ded dermal scales (Figure 26-17);scales ganoid in ancestral forms,scales cycloid, ctenoid or absent inadvanced forms (Figure 26-18)

2. Paired and median fins present, sup-ported by long dermal rays (lepi-

dotrichia); muscles controlling finmovement within body

3. Jaws present; teeth usually presentwith enamaloid covering; olfactorysacs do not open into mouth; spiralvalve present in ancestral forms,absent in advanced forms

4. Respiration primarily by gills support-ed by arches and covered with anoperculum

5. Swim bladder often present with orwithout a duct connecting to esopha-gus, usually functioning in buoyancy

6. Circulation consisting of a heart with a

sinus venosus, an undivided atrium,and an undivided ventricle; single cir-culation; typically four aortic arches;nucleated erythrocytes

7. Excretory system of paired opistho-nephric kidneys; sexes usually separate; fertilization usually external;larval forms may differ greatly fromadults

8. Nervous system of a brain with olfac-tory lobes, small cerebrum, opticlobes, and cerebellum; 10 pairs of cra-nial nerves; three pairs of semicircularcanals

Homocercal(perch)

Diphycercal(lungfish)

Heterocercal(shark)

Figure 26-16Types of caudal fins among fishes.


Class Actinopterygii: Ray-Finned FishesThe ray-finned fishes are an enormousassemblage containing all of our famil-iar bony fishes—more than 23,600species. The earliest actinopterygians,known as palaeoniscids (pay�lee-o-nis�ids), were small fishes, with largeeyes, a heterocercal tail (Figure 26-16),and thick, interlocking scales with anouter layer of ganoin (Figure 26-18).These fishes had a single dorsal fin andnumerous bony rays derived fromscales stacked end to end, distinctivelydifferent in appearance from the lobe-finned fishes with which they sharedthe Devonian waters. Palaeoniscids arerepresented by fossil fragments asearly as the late Silurian, and flour-ished throughout the late Paleozoicera, during the same period that ostra-coderms, placoderms, and acanthodi-ans disappeared and sarcopterygiansdeclined in abundance (Figure 26-1).

This suggests the morphological spe-cializations evolving in the actinoptery-gian lineage gave them ecologicalsuperiority over most other fishes.

From these earliest ray-finnedfishes, two major groups appeared.Those with the most primitive charac-teristics are the chondrosteans (Gr.chondros, cartilage, � osteon, bone),represented today by the freshwaterand anadromous sturgeons, paddle-fishes, and bichirs (Figure 26-19). Thechondrosteans show many characterssimilar to their paleoniscid ancestors,including a heterocercal tail andganoid scutes or scales. The bichirPolypterus (Gr. poly, many, � pteros,winged) of African waters is an inter-esting relict with lungs and other prim-itive characters that make it resemblepalaeoniscids more than any other liv-ing fish.

The second major group of ray-finned fishes to emerge from thepalaeoniscid stock were the neoptery-gians (Gr. neos, new, � pteryx, fin).The neopterygians appeared in the latePermian and radiated extensively dur-ing the Mesozoic era (Figure 26-1).During the Mesozoic one lineage gaverise to a secondary radiation that led tothe modern bony fishes, the teleosts.There are two surviving genera of earlyneopterygians, the bowfin Amia (Gr.tunalike fish) of shallow, weedy watersof the Great Lakes and Mississippi Val-ley, and the gars Lepisosteus (Gr. lepi-dos, scale, � osteon, bone) of eastern

and southern North America (Fig-ure 26-20). The seven species of garsare large, ambush predators with elon-gate bodies and jaws filled withneedlelike teeth. Gars and bowfin maygulp air to surface, filling their vascu-larized swim bladder with air to sup-plement oxygen obtained in the gills.

The major lineage of neoptery-gians are the teleosts (Gr. teleos, per-fect, � osteon, bone), the modernbony fishes (Figure 26-15). Teleostdiversity is astounding, with about23,600 described species, representingabout 96% of all living fishes or abouthalf of all vertebrates (Figure 26-21). Inaddition, it has been estimated thereare an additional 5,000 to 10,000 unde-scribed species. Although most of the200 or so new species of teleostsdescribed each year are from poorlysampled areas such as South Americaor deep oceanic waters, several newspecies are described each year fromareas as well known as the freshwaters of North America! Teleostsrange in size from 10 mm adult gobiesto the 17 m oarfish and the 900 kg, 4.5 m blue marlin (Figure 26-21).These fishes occupy almost every con-ceivable habitat, from elevations up to5200 m in Tibet to 8000 m below thesurface of the ocean. Some species livein hot springs at 44° C, while otherslive under the Antarctic ice at �2° C.They may live in lakes with salt con-centrations three times that of sea-water, caves of total darkness, swamps

Dentine

Epidermis

Basal plate Pulp cavity

Placoid scales(cartilaginous fishes)

Ganoid scales(nonteleost bony fishes)

Cycloid scales(teleost fishes)

Ctenoid scales(teleost fishes)

Figure 26-18Types of fish scales. Placoid scales are small, conical toothlike structures characteristic of Chondrichthyes. Diamond-shaped ganoid scales, present in earlybony fishes such as the gar, are composed of layers of silvery enamel (ganoin) on the upper surface and bone on the lower. Teleosts have either cycloid orctenoid scales. These are thin and flexible and are arranged in overlapping rows.

Bony partof scale Epidermis

Mucousglands

Figure 26-17Section through the skin of a bony fish, showingthe overlapping scales (red). The scales lie in thedermis and are covered by epidermis.


devoid of oxygen, or even makeextended excursions onto land, as dothe mudskippers (Figure 26-21).

Several morphological trends inthe teleost lineage allowed them todiversify into this truly incredible vari-ety of habitats and forms. The heavydermal armor of primitive ray-finnedfishes was replaced by light, thin, flex-ible cycloid and ctenoid scales (Fig-ure 26-18). Some teleosts, such as mosteels and catfishes, completely lackscales. The increased mobility andspeed that resulted from the loss of theheavy armor improved predator avoid-ance and food getting. Changes in thefins of teleosts increased maneuver-ability and speed and allowed fins toserve a variety of other functions. Thesymmetrical shape of the homocercaltail (Figure 26-16) of most teleostsfocused musculature contractions onthe tail, resulting in greater speed. Thedorsal fin shifted from a fixed keel thatprimarily prevented rolling, to a flexi-ble and highly specialized structure inadvanced teleosts (Figure 26-15).

Atlantic sturgeon

Bichir

American paddlefish

Figure 26-19Chondrostean ray-finned fishes of the classActinopterygii. A, Atlantic sturgeon, Acipenseroxyrhynchus (now uncommon), of Atlanticcoastal rivers. B, Bichir Polypterus bichir ofequatorial west Africa. It is a nocturnal predator.C, Paddlefish Polyodon spathula of theMississippi River reaches a length of 2 m and a weight of 90 kg.

A B

Figure 26-20Nonteleost neopterygian fishes. A, Bowfin Amia calva. B, Longnose gar Lepisosteus osseus. Thebowfin lives in the Great Lakes region and Mississippi basin. Gars are common fishes of eastern andsouthern North America. They frequent slow-moving streams where they may hang motionless in thewater, ready to snatch passing fish.

A

B

C D

Figure 26-21Diversity among teleosts. A, Blue marlin, Makaira nigricans, one of the largest teleosts. B, Mudskippers, Periophthalmus sp., make extensive excursions on land to graze on algae and captureinsects; they build nests in which the young hatch and are guarded by the mother. C, Protectivecoloration of the flamboyant lionfish, Pterois sp., advises caution; the dorsal spines are poisonous.D, The sucking disk on the sharksucker, Echeneis naucrates, is a modification of the dorsal fin.

A

B

C


These changes in the morphology ofthe fins were useful for camouflage,braking and other complex movements,streamlining, and social communica-tion. Bizarre modifications of the dorsalfin include the lure of anglerfishes,venom-delivering spines of scorpion-fishes, and the suctorial disc of shark-suckers (Figure 26-21). In addition, theswim bladder shifted from primarily re-spiratory to buoyancy in function. Theteleost lineage demonstrated an increas-ingly fine control of gas resorption andsecretion in the swim bladder. Controlof buoyancy likely coevolved with finmodifications to improve locomotion.Finally, greater feeding efficiency wasbrought about by several anatomicalmodifications. Changes in the jaw sus-pension enabled the orobranchial cavityto expand rapidly, creating a highlysophisticated suction device. Rapid jawprotrusion was made possible by slid-ing the upper jaw forward, increasingfinal attack velocity by 39% to 89%. Thegill arches of many teleosts diversifiedinto powerful pharyngeal jaws forchewing, grinding, and crushing. Withso many separate innovations to workwith, it is not surprising the teleostshave become the most diverse of fishes.

Class Sarcopterygii: Lobe-Finned FishesThe lobe-finned fishes are today repre-sented by only seven species: sixspecies (three genera) of lungfishesand the coelacanth—survivors of agroup once abundant during theDevonian period of the Paleozoic (Fig-ures 26-22 and 26-23).

All early sarcopterygians had lungsas well as gills, and a tail of the hetero-cercal type. However, during the Pa-leozoic the orientation of the vertebralcolumn changed so that the tail becamesymmetrical, with the median dorsaland ventral fins displaced posteriorly toform one continuous, flexible finaround the tail. This type of tail is calleddiphycercal (Figure 26-16). Thestrong, fleshy, paired lobed fins of thesarcopterygians (pectoral and pelvic)may have been used much like fourlegs to scuttle along the bottom. They

had powerful jaws and their skin wascovered with heavy scales that con-sisted of a dentine-like material calledcosmine overlaid by a thin enamel.

Of the three surviving genera oflungfishes, most similar to early formsis Neoceratodus (Gr. neos, new, � ker-atos, horn, � odes, form), the livingAustralian lungfish, which may attain alength of 1.5 m (Figure 26-22). Thislungfish, unlike its relatives, normallyrelies on gill respiration, and cannotsurvive long out of water. The SouthAmerican lungfish Lepidosiren (L. lep-idus, pretty, � siren, mythical mer-

Characteristics of Class Sarcopterygii

1. Skeleton with bone of endo-chondral origin; caudal findiphycercal in living representa-tives, heterocercal in ancestralforms; skin with embedded dermalscales (Figure 26-17) with a layerof dentine-like material, cosmine,in ancestral forms

2. Paired and median fins present;paired fins with a single basalskeletal element and short dermalrays; muscles that move pairedfins located on limb

3. Jaws present; teeth are coveredwith true enamel and typically arecrushing plates restricted to palate;olfactory sacs paired, may or maynot open into mouth; intestinewith spiral valve

4. Gills supported by bony archesand covered with an operculum

5. Swim bladder vascularized andused for respiration and buoyancy(fat-filled in the coelacanth)

6. Circulation consisting of heart witha sinus venosus, two atria, a partlydivided ventricle, and a conus arte-riosus; double circulation withpulmonary and systemic circuits;characteristically five aortic arches

7. Nervous system with olfactorylobes, a cerebrum, a cerebellum,and optic lobes; 10 pairs of cranialnerves; three pairs of semicircularcanals

8. Sexes separate; fertilization exter-nal or internal

maid) and the African lungfish Pro-topterus (Gr. pr|tos, first, � pteron,wing) can live out of water for longperiods of time. Protopterus lives inAfrican streams and ponds that maydry during the dry season, with theirmud beds baked hard by the hot tropi-cal sun. The fish burrows down at theapproach of the dry season andsecretes a copious slime that is mixedwith mud to form a hard cocoon inwhich it estivates until the rains return.Surprisingly little is known about theecology of the South American lung-fish Lepidosiren.

Australian lungfish

African lungfish

South American lungfish

Figure 26-22Lungfishes are lobe-finned fishes of the classSarcopterygii. The Australian lungfishNeoceratodus forsteri is the least specialized ofthree lungfish genera. The African lungfishProtopterus sp. is best adapted of the three forremaining dormant in mucous-lined cocoonsbreathing air during prolonged periods of drought.


Coelacanths and rhipidistians col-lectively have been termed cros-sopterygians, but this group is consid-ered polyphytetic and no longer isrecognized by most classifications.The rhipidistians flourished in thelate Paleozoic era and then becameextinct. Rhipidistians are of specialimportance because they include theancestors of the tetrapods (and, incladistic terms, are therefore a para-phyletic group). The coelacanthsalso arose in the Devonian period,radiated somewhat, and reached theirevolutionary peak in the Mesozoicera. At the end of the Mesozoic era theynearly disappeared but left one re-markable surviving species, Latimeriachalumnae (named for M. Courtenay-Latimer, South African museum direc-tor) (Figure 26-23). Since the lastcoelacanths were believed to havebecome extinct 70 million years ago,the astonishment of the scientificworld may be imagined when theremains of a coelacanth were foundon a dredge off the coast of SouthAfrica in 1938. An intensive search to

locate more specimens was successfuloff the coast of the Comoro Islands.There fishermen occasionally catchthem at great depths with hand lines,providing specimens for research.This was believed to be the only pop-ulation of Latimeria until 1998, whenthe scientific world was again sur-prised by the capture of a coelacanth,possibly representing a new species,in Indonesia, 10,000 km from theComoros!

The “modern” marine coelacanthis a descendant of the Devonianfreshwater stock. The tail is of thediphycercal type (Figure 26-16) butpossesses a small lobe between theupper and lower caudal lobes, pro-ducing a three-pronged structure(Figure 26-23).

Coelacanths are a deep metallicblue with irregular white or brassyflecks, providing camouflage againstthe dark lava-cave reefs they inhabit.Young are born fully formed afterhatching internally from eggs 9 cm indiameter—the largest among bonyfishes.

Structural andFunctionalAdaptations of Fishes

Locomotion in WaterTo the human eye, some fishes appearcapable of swimming at extremelyhigh speeds. But our judgment isunconsciously tempered by our ownexperience that water is a highly resis-tant medium through which to move.Most fishes, such as a trout or a min-now, can swim maximally about 10body lengths per second, obviously animpressive performance by humanstandards. Yet when these speeds aretranslated into kilometers per hour itmeans that a 30 cm (1 foot) trout canswim only about 10.4 km (6.5 miles)per hour. As a general rule, the largerthe fish the faster it can swim.

Measuring fish cruising speeds accurately isbest done in a “fish wheel,” a large ring-shaped channel filled with water that isturned at a speed equal and opposite tothat of the fish. Much more difficult to mea-sure are the sudden bursts of speed thatmost fish can make to capture prey or toavoid being captured. A hooked bluefintuna was once “clocked”at 66 km per hour(41 mph); swordfish and marlin are thoughtto be capable of incredible bursts of speedapproaching, or even exceeding, 110 kmper hour (68 mph). Such high speeds can be sustained for no more than 1 to 5 seconds.

The propulsive mechanism of afish is its trunk and tail musculature.The axial, locomotory musculature is composed of zigzag bands, calledmyomeres. Muscle fibers in eachmyomere are relatively short and con-nect the tough connective tissue parti-tions that separate each myomere fromthe next. On the surface the myomerestake the shape of a W lying on its side(Figure 26-24) but internally the bandsare complexly folded and nested sothat the pull of each myomere extendsover several vertebrae. This arrange-ment produces more power and finer

ComoroIslands

Madagascar

AFRICA

AUSTRALIA

PhilippineIslands

Figure 26-23The coelacanth Latimeria chalumnae is a surviving marine relict of a group of lobe-finned fishes thatflourished some 350 million years ago.


control of movement since manymyomeres are involved in bending agiven segment of the body.

Understanding how fishes swimcan be approached by studying themotion of a very flexible fish such as aneel (Figure 26-25). The movement isserpentine, not unlike that of a snake,with waves of contraction movingbackward along the body by alternatecontraction of the myomeres on eitherside. The anterior end of the bodybends less than the posterior end, sothat each undulation increases in ampli-tude as it travels along the body. Whileundulations move backward, the bend-ing of the body pushes laterally againstthe water, producing a reactive forcethat is directed forward, but at an angle.It can be analyzed as having two com-ponents: thrust, which is used to over-come drag and propels the fish for-ward, and lateral force, which tendsto make the fish’s head “yaw,” or devi-ate from the course in the same direc-tion as the tail. This side-to-side headmovement is very obvious in a swim-ming eel or shark, but many fishes havea large, rigid head with enough surfaceresistance to minimize yaw.

The movement of an eel is reason-ably efficient at low speed, but its bodyshape generates too much frictionaldrag for rapid swimming. Fishes thatswim rapidly, such as trout, are lessflexible and limit body undulationsmostly to the caudal region (Fig-ure 26-25). Muscle force generated inthe large anterior muscle mass is trans-

ferred through tendons to the relativelynonmuscular caudal peduncle and tailwhere thrust is generated. This form ofswimming reaches its highest develop-ment in the tunas, whose bodies donot flex at all. Virtually all thrust isderived from powerful beats of the tailfin (Figure 26-26). Many fast oceanicfishes such as marlin, swordfish,amberjacks, and wahoo have swept-back tail fins shaped much like asickle. Such fins are the aquatic coun-terpart of the high-aspect ratio wingsof the swiftest birds (p. 597).

Swimming is the most economicalform of animal locomotion, largelybecause aquatic animals are almostperfectly supported by their mediumand need expend little energy to over-come the force of gravity. If we com-pare the energy cost per kilogram ofbody weight of traveling 1 km by dif-ferent forms of locomotion, we findswimming costs only 0.39 kcal (salmon)as compared with 1.45 kcal for flying(gull) and 5.43 for walking (groundsquirrel). However, part of the unfin-ished business of biology is understand-ing how fish and aquatic mammals areable to move through the water whilecreating almost no turbulence. Thesecret lies in the way aquatic animalsbend their bodies and fins (or flukes) toswim and in the friction-reducing prop-erties of the body surface.

Neutral Buoyancy and theSwim BladderAll fishes are slightly heavier thanwater because their skeletons andother tissues contain heavy elements

Figure 26-24Trunk musculature of a teleost fish, partly dissected to show internal arrangement of the musclebands (myomeres). The myomeres are folded into a complex, nested grouping, an arrangement thatfavors stronger and more controlled swimming.

Eel

Trout

Thrust Reactiveforce

Lateralforce

Reactiveforce

Push

Push

90˚

90˚Lateral

force

Thrust

Figure 26-25Movements of swimming fishes, showing theforces developed by an eel-shaped and spindle-shaped fish.

Pedunclenarrow, stiff

Tail stiff

Figure 26-26Bluefin tuna, showing adaptations for fastswimming. Powerful trunk muscles pull on theslender tail stalk. Since the body does not bend,all of the thrust comes from beats of the stiffsickle-shaped tail.


that are present only in trace amountsin natural waters. To keep from sink-ing, sharks must always keep movingforward in the water. The asymmetrical(heterocercal) tail of a shark providesthe necessary tail lift as it sweeps toand fro in the water, and the broadhead and flat pectoral fins (Fig-ure 26-8) act as angled planes to pro-vide head lift. Sharks are also aided inbuoyancy by having very large liverscontaining a special fatty hydrocarboncalled squalene with a density of only0.86. The liver thus acts like a largesack of buoyant oil that helps to com-pensate for the shark’s heavy body.

By far the most efficient flotationdevice is a gas-filled space. The swimbladder serves this purpose in the

bony fishes (Figure 26-27). It arosefrom the paired lungs of the primitiveDevonian bony fishes. Lungs wereprobably a ubiquitous feature of theDevonian freshwater bony fisheswhen, as we have seen, warm,swampy habitats would have madesuch an accessory respiratory structureadvantageous. Swim bladders are pres-ent in most pelagic bony fishes but areabsent in tunas, most abyssal fishes,and most bottom dwellers, such asflounders and sculpins.

By adjusting the volume of gas inthe swim bladder, a fish can achieveneutral buoyancy and remain sus-pended indefinitely at any depth withno muscular effort. There are severetechnical problems, however. If thefish descends to a greater depth, theswim bladder gas is compressed sothat the fish becomes heavier andtends to sink. Gas must be added tothe bladder to establish a new equilib-rium buoyancy. If the fish swimsupward, the gas in the bladderexpands, making the fish lighter.Unless gas is removed, the fish will risewith ever-increasing speed while thebladder continues to expand.

Gas may be removed from theswim bladder in one of two ways. Themore primitive phystostomous (Gr.,phys, bladder, stoma, mouth) fishes(trout, for example) have a pneumaticduct that connects the swim bladder tothe esophagus. These fishes may sim-ply expel air out through the pneu-matic duct. More advanced teleostsexhibit the physoclistous (Gr., phys,bladder, clist, closed) condition inwhich the pneumatic duct is lost inadults. In physoclistous fishes, gasmust be secreted into the blood fromthe ovale, a vascularized area (Fig-ure 26-27). Both types of fishes requiregas to be secreted into the swim blad-der from the blood, although a fewshallow-water-inhabiting phystostomesmay gulp air to fill their swim bladder.

Gas is secreted into the swim blad-der at the highly specialized gas gland.The gas gland is supplied by a remark-able network of blood capillaries,called the rete mirabile (“marvelousnet”) that functions as a countercurrent

exchange system to trap gases, espe-cially oxygen, and prevent their loss tothe circulation (Figure 26-27)

The amazing effectiveness of thisdevice is exemplified by a fish living ata depth of 2400 m (8000 feet). To keepthe bladder inflated at that depth, thegas inside (mostly oxygen, but alsovariable amounts of nitrogen, carbondioxide, argon, and even some carbonmonoxide) must have a pressureexceeding 240 atmospheres, which ismuch greater than the pressure in afully charged steel gas cylinder. Yet theoxygen pressure in the fish’s blood can-not exceed 0.2 atmosphere—equal tothe oxygen pressure at the sea surface.

Physiologists who were at first baf-fled by the secretion mechanism nowunderstand how it operates. In brief,the gas gland secretes lactic acid,which enters the blood, causing alocalized high acidity in the retemirabile that forces hemoglobin torelease its load of oxygen. The capillar-ies in the rete are arranged so that thereleased oxygen accumulates in therete, eventually reaching such a highpressure that the oxygen diffuses intothe swim bladder. The final gas pres-sure attained in the swim bladderdepends on the length of the rete cap-illaries; they are relatively short infishes living near the surface, but areextremely long in deep-sea fishes.

RespirationFish gills are composed of thin fila-ments, each covered with a thin epider-mal membrane that is folded repeatedlyinto platelike lamellae (Figure 26-28).These are richly supplied with bloodvessels. The gills are located inside thepharyngeal cavity and are covered witha movable flap, the operculum. Thisarrangement provides excellent protec-tion to the delicate gill filaments,streamlines the body, and makes possi-ble a pumping system for moving waterthrough the mouth, across the gills, andout the operculum. Instead of opercularflaps as in bony fishes, the elasmo-branchs have a series of gill slits (Fig-ure 26-8) out of which the water flows.In both elasmobranchs and bony fishes

Swim bladder

Dorsal aorta

Toheart

Retemirabile

Gas gland

Constrictormuscles

Ovale

A

B

C

Figure 26-27A, Swim bladder of a teleost fish. The swimbladder lies in the coelom just beneath thevertebral column. B, Gas is secreted into theswim bladder by the gas gland. Gas from theblood is moved into the gas gland by the retemirabile, a complex array of tightly-packedcapillaries that act as a countercurrent multiplierto build up the oxygen concentration. Thearrangement of venous and arterial capillaries inthe rete is shown in C. To release gas duringascent, a muscular valve opens, allowing gas toenter the ovale from which the gas is removedby the circulation.


the branchial mechanism is arranged topump water continuously and smoothlyover the gills, although to an observer itappears that fish breathing is pulsatile.The flow of water is opposite to thedirection of blood flow (countercurrentflow), the best arrangement for extract-ing the greatest possible amount of oxy-gen from the water. Some bony fishescan remove as much as 85% of the oxy-gen from water passing over their gills.Very active fishes, such as herring andmackerel, can obtain sufficient water fortheir high oxygen demands only byswimming forward continuously toforce water into the open mouth andacross the gills. This process is calledram ventilation. Such fish will beasphyxiated if placed in an aquariumthat restricts free swimming move-ments, even if the water is saturatedwith oxygen.

A surprising number of fishes canlive out of water for varying lengths oftime by breathing air. Several devicesare employed by different fishes. Wealready have described the lungs of thelungfishes, Polypterus, and the extinctrhipidistians. Freshwater eels oftenmake overland excursions during rainyweather, using the skin as a major re-spiratory surface. The bowfin, Amia,has both gills and a lunglike swimbladder. At low temperatures it usesonly its gills, but as the temperatureand the fish’s activity increase, itbreathes mostly air with its swim blad-der. The electric eel, Electrophorus (Gr.e-lektron, something bright, � phoros,to bear), has degenerate gills and mustsupplement gill respiration by gulpingair through its vascular mouth cavity.One of the best air breathers of all isthe Indian climbing perch Anabas (Gr.anabaino-, to go up), which spendsmost of its time on land near thewater’s edge, breathing air throughspecial air chambers above much-reduced gills.

Osmotic RegulationFresh water is an extremely dilutemedium with a salt concentration(0.001 to 0.005 gram moles per liter[M]) much below that of the blood of

freshwater fishes (0.2 to 0.3 M). Watertherefore tends to enter their bodiesosmotically, and salt is lost by diffusionoutward. Although the scaled andmucous-covered body surface is al-most totally impermeable to water,water gain and salt loss do occuracross thin membranes of the gills.Freshwater fishes are hyperosmoticregulators that have several defensesagainst these problems (Figure 26-29).First, the excess water is pumped outby the opisthonephric kidney (p. 670), which is capable of formingvery dilute urine. Second, special salt-absorbing cells located in the gillepithelium actively move salt ions,principally sodium and chloride, fromthe water to the blood. This, togetherwith salt present in the fish’s food,replaces diffusive salt loss. Thesemechanisms are so efficient that afreshwater fish devotes only a smallpart of its total energy expenditure tokeeping itself in osmotic balance.

Perhaps 90% of all bony fishes are restrictedto either a freshwater or a seawater habitatbecause they are incapable of osmotic regu-lation in the “wrong”habitat. Most fresh-water fishes quickly die if placed in sea-water, as will marine fishes placed in freshwater. However, some 10% of all teleostscan pass back and forth with ease betweenboth habitats. These euryhaline fishes(Gr. eurys, broad, � hals, salt) are of twotypes: those such as many flounders,sculpins, and killifish that live in estuariesor certain intertidal areas where the salinityfluctuates throughout the day; and thosesuch as salmon, shad, and eels, that spendpart of their life cycle in fresh water andpart in seawater.

Marine bony fishes are hypoos-motic regulators that encounter acompletely different set of problems.Having a much lower blood salt concen-tration (0.3 to 0.4 M) than the seawateraround them (about 1 M), they tend tolose water and gain salt. The marine

Filament

Waterflow

Bloodflow

Lamella

Gill arch

Gill arch

Filament withlamellae

Gill raker

Gill rakers

Gill filaments

Figure 26-28Gills of fish. Bony, protective flap covering the gills (operculum) has been removed, A, to revealbranchial chamber containing the gills. There are four gill arches on each side, each bearing numerousfilaments. A portion of gill arch (B) shows gill rakers that project forward to strain out food and debris,and gill filaments that project to the rear. A single gill filament (C) is dissected to show the bloodcapillaries within the platelike lamellae. Direction of water flow (large arrows) is opposite the directionof blood flow.

A

B

C


teleost fish quite literally risks dryingout, much like a desert mammal de-prived of water. Again, marine bonyfishes, like their freshwater counterparts,have evolved an appropriate set of de-fenses (Figure 26-29). To compensatefor water loss, the marine teleost drinksseawater. Although this behavior obvi-ously brings needed water into the body,it is unfortunately accompanied by a

great deal of unneeded salt. Unwantedsalt is disposed in two ways: (1) themajor sea salt ions (sodium, chloride,and potassium) are carried by the bloodto the gills where they are secreted out-ward by special salt-secretory cells;and (2) the remaining ions, mostly thedivalent ions (magnesium, sulfate, andcalcium), are left in the intestine andvoided with the feces. However, a small

but significant fraction of these residualdivalent salts in the intestine, some 10%to 40% of the total, penetrates the intesti-nal mucosa and enters the bloodstream.These ions are excreted by the kidney.Unlike the freshwater fish kidney, whichforms its urine by the usual filtration-resorption sequence typical of mostvertebrate kidneys (pp. 672 to 674),the marine fish’s kidney excretes diva-lent ions by tubular secretion. Since verylittle if any filtrate is formed, the glo-meruli have lost their importanceand disappeared altogether in somemarine teleosts. The pipefishes, and thegoosefish shown in Figure 26-31, are ex-amples of “aglomerular” marine fishes.

Feeding BehaviorFor any fish, feeding is one of themain concerns of day-to-day living.Although many a luckless anglerwould swear otherwise, the fact isthat a fish devotes more time andenergy to eating, or searching forfood to eat, than to anything else.Throughout the long evolution offishes, there has been unrelentingselective pressure for those adapta-tions that enable a fish to come outon the better end of the eat-or-be-eaten contest. Certainly the most far-reaching single event was the evolu-tion of jaws. Their possessors werefreed from a largely passive filter-feeding existence and could adopt apredatory mode of life. Improvedmeans of capturing larger preydemanded stronger muscles, moreagile movement, better balance, andimproved special senses. More thanany other aspect of its life habit, feed-ing behavior shapes the fish.

Most fishes are carnivores thatprey on a myriad of animal foods fromzooplankton and insect larvae to largevertebrates. Some deep-sea fishes arecapable of eating victims nearly twicetheir own size—an adaptation for life ina world where meals are necessarilyinfrequent. Most advanced ray-finnedfishes cannot masticate their food as wecan because doing so would block thecurrent of water across the gills. Some,however, such as the wolf eel (Fig-ure 26-30), have molarlike teeth in the

Largeglomerulus

Active tubularreabsorption

of NaCl

Kidney: Excretionof dilute urine

Active tubularsecretionof MgSO4

NaClNaCl

FRESHWATERFISH

MARINE FISH

Food,fresh water

Food,seawater

Gills:Active absorption ofNaCl, water entersosmotically

Stomach:Passive reabsorptionof NaCl and water

Gills:Active secretion ofNaCl, water loss Intestinal wastes:

MgSO4 voidedwith feces

Kidney:Excretion of MgSO4,

urea, little water

UrineIntestinalwastes

MgSO4

MgSO4

Glomerulusreduced orabsent

Kidneytubule

Figure 26-29Osmotic regulation in freshwater and marine bony fishes. A freshwater fish maintains osmotic andionic balance in its dilute environment by actively absorbing sodium chloride across the gills (some saltis gained with food). To flush out excess that constantly enters the body, the glomerular kidneyproduces a dilute urine by reabsorbing sodium chloride. A marine fish must drink seawater to replacewater lost osmotically to its salty environment. Sodium chloride and water are absorbed from thestomach. Excess sodium chloride is actively transported outward by the gills. Divalent sea salts,mostly magnesium sulfate, are eliminated with feces and secreted by the tubular kidney.


jaws for crushing their prey, which mayinclude hard-bodied crustaceans. Othersthat do grind their food use powerful pharyngeal teeth in the throat. Most car-nivorous fish almost invariably swal-low their prey whole, using sharp-pointed teeth in the jaws and on theroof of the mouth to seize their prey.The incompressibility of water makesthe task even easier for many large-mouthed predators. When the mouth issuddenly opened, a negative pressure iscreated that sweeps the victim inside(Figure 26-31).

A second group of fishes are her-bivores that eat plants and algae.Although plant eaters are relatively fewin number, they are crucial intermedi-ates in the food chain, especially infreshwater rivers, lakes, and ponds thatcontain very little plankton.

Suspension-feeders that crop theabundant microorganisms of the seaform a third and diverse group of fishesranging from fish larvae to baskingsharks. However, the most characteristicgroup of plankton feeders are herringlikefishes (menhaden, herring, anchovies,capelin, pilchards, and others), mostlypelagic (open-sea dwellers) fishes thattravel in large schools. Both phytoplank-ton and the smaller zooplankton are

strained from the water with the sieve-like gill rakers (Figure 26-28). Becauseplankton feeders are the most abundantof all marine fishes, they are importantfood for numerous larger but less abun-dant carnivores. Many freshwater fishesalso depend on plankton for food.

A fourth group of fishes containsomnivores that feed on both plantand animal food. Finally there are scav-engers that feed on organic debris(detritus) and parasites that suck thebody fluids of other fishes.

Digestion in most fishes followsthe vertebrate plan. Except in severalfishes that lack stomachs altogether,the food proceeds from stomach totubular intestine, which tends to beshort in carnivores (Figure 26-15) butmay be extremely long and coiled inherbivorous forms. In the herbivorousgrass carp, for example, the intestinemay be nine times the body length, anadaptation for the lengthy digestionrequired for plant carbohydrates. Incarnivores, some protein digestionmay be initiated in the acid mediumof the stomach, but the principal func-tion of the stomach is to store theoften large and infrequent mealswhile awaiting their reception by theintestine.

Digestion and absorption proceedsimultaneously in the intestine. A curi-ous feature of ray-finned fishes, espe-cially the teleosts, is the presence ofnumerous pyloric ceca (Figure 26-15)found in no other vertebrate group.Their primary function appears to be fatabsorption, although all classes of diges-tive enzymes (protein-, carbohydrate-,and fat-splitting) are secreted there.

Migration

Eel

For centuries naturalists had been puz-zled about the life history of the fresh-water eel Anguilla (an-gwil�la) (L. eel),a common and commercially impor-tant species of coastal streams of theNorth Atlantic. Eels are catadromous(Gr. kata, down, � dromos, running),meaning that they spend most of theirlives in fresh water but migrate to thesea to spawn. Each fall, large numbersof eels were seen swimming down therivers toward the sea, but no adultsever returned. Each spring countlessnumbers of young eels, called “elvers”(Figure 26-32), each about the size of awooden matchstick, appeared in thecoastal rivers and began swimmingupstream. Beyond the assumption thateels must spawn somewhere at sea,location of their breeding grounds wascompletely unknown.

The first clue was provided by twoItalian scientists, Grassi and Calandruc-cio, who in 1896 reported that elverswere not larval eels but rather were rel-atively advanced juveniles. True larvaleels, they discovered, were tiny, leaf-shaped, completely transparent crea-tures that bore absolutely no resem-blance to an eel. They had been calledleptocephali (Gr. leptos, slender, �kephalƒ, head) by early naturalists, whonever suspected their true identity. In1905 Johann Schmidt, supported by theDanish government, began a systematicstudy of eel biology that he continueduntil his death in 1933. With cooperationof captains of commercial vessels plyingthe Atlantic, thousands of leptocephaliwere caught in different areas of theAtlantic with the plankton nets Schmidtsupplied them. By noting where larvae

Figure 26-30Wolf eel, Anarrhichthys ocellatus, feeding on asea cucumber it has captured and pulled to theopening of its den.

Figure 26-31Goosefish Lophius piscatorius awaits its meal.Above its head swings a modified dorsal finspine ending in a fleshy tentacle that contractsand expands in a convincing wormlike manner.When a fish approaches the alluring bait, thehuge mouth opens suddenly, creating a stronginward current that sweeps the prey inside. In asplit second all is over.


in different stages of development werecaptured, Schmidt and his colleagueseventually reconstructed the spawningmigrations.

When adult eels leave the coastalrivers of Europe and North America,they swim steadily and apparently atgreat depth for 1 to 2 months until theyreach the Sargasso Sea, a vast area ofwarm oceanic water southeast ofBermuda (Figure 26-32). Here, at depthsof 300 m or more, the eels spawn anddie. The minute larvae then begin anincredible journey back to the coastalrivers of Europe. Drifting with the GulfStream and preyed on constantly bynumerous predators, they reach themiddle of the Atlantic after 2 years. Bythe end of the third year they arrive inthe coastal waters of Europe where theleptocephali metamorphose into elvers,with an unmistakable eel-like body form(Figure 26-32). Here the males and

females part company; males remain inthe brackish waters of coastal rivers andestuaries while females continue up therivers, often traveling hundreds of milesupstream. After 8 to 15 years of growth,the females, now 1 m or more long,return to the sea to join the smallermales; both return to the ancestralbreeding grounds thousands of milesaway to complete the life cycle.

Recent enzyme electrophoresis analysis ofeel larvae confirmed not only the existenceof separate European and American speciesbut also Schmidt’s belief that the Europeanand American eels spawn in partially over-lapping areas of the Sargasso Sea.

Schmidt found that the Americaneel (Anguilla rostrata) could be distin-guished from the European eel (A. vul-garis) because it had fewer vertebrae—

an average of 107 in the American eel ascompared with an average 114 in the European species. Since the Ameri-can eel is much closer to the NorthAmerican coastline, it requires onlyabout 8 months to make the journey.

Homing Salmon

The life history of salmon is nearly asremarkable as that of the eel and cer-tainly has received far more popularattention. Salmon are anadromous (Gr.anadromos, running upward); that is,they spend their adult lives at sea butreturn to fresh water to spawn. TheAtlantic salmon (Salmo salar) (L. salmo,salmon, sal, salt) and the Pacific salmon(six species in the genus Oncorhynchus[on-ko-rink�us] [Gr. onkos, hook, �rhynchos, snout]) have this practice, butthere are important differences amongthe seven species. The Atlantic salmon

Elver

53

6

45

21

Eel speciesand agesLarval stages

Migration patterns

American2 monthsEuropean2 months

Adult eel

2

3

AmericanJust hatched

EuropeanJust hatched

4



American1 year

European3 years

European8–15 years

5

6 American6–10 years

1Leptocephalus

2

3

14

45

Europe

Africa

SouthAmerica

North America

Greenland

Europe

Africa

SouthAmerica

North America

Greenland

Figure 26-32Life histories of the European eel, Anguilla anguilla, and American eel, Anguilla rostrata. Migration patterns of European species are shown in pink. Migrationpatterns of American species are shown in black. Boxed numbers refer to stages of development. Note that the American eel completes its larvalmetamorphosis and sea journey in one year. It requires nearly three years for the European eel to complete its much longer journey.


may make repeated upstream spawningruns. The six Pacific salmon species(king, sockeye, silver, humpback, chum,and Japanese masu) each make a singlespawning run (Figure 26-33), afterwhich they die.

The virtually infallible hominginstinct of the Pacific species is leg-endary: after migrating downstream as asmolt, a sockeye salmon ranges manyhundreds of miles over the Pacific fornearly 4 years, grows to 2 to 5 kg inweight, and then returns almost unerr-ingly to spawn in the headwaters of itsparent stream. Some straying does occurand is an important means of increasinggene flow and populating new streams.

Experiments by A. D. Hasler andothers have shown that homingsalmon are guided upstream by thecharacteristic odor of their parentstream. When the salmon finally reachthe spawning beds of their parents(where they themselves werehatched), they spawn and die. The fol-lowing spring, newly hatched fry trans-form into smolts before and during thedownstream migration. At this timethey are imprinted (p. 789) with thedistinctive odor of the stream, which isapparently a mosaic of compounds

released by the characteristic vegeta-tion and soil in the watershed of theparent stream. They also seem toimprint on the odors of other streamsthey pass while migrating downriverand use these odors in reversesequence as a map during the uprivermigration as returning adults.

Salmon runs in the Pacific Northwest havebeen devasted by a lethal combination ofspawning stream degradation by logging,pollution and, especially, by more than 50 hydroelectric dams which obstructupstream migration of adult salmon and killdownstream migrants as they pass throughthe dams’ power-generating turbines. Inaddition, the chain of reservoirs behind thedams, which has converted the Columbiaand Snake Rivers into a series of lakes,increases mortality of young salmon migrat-ing downstream by slowing their passageto the sea. The result is that the annual runof wild salmon is today only about 3% ofthe 10 to 16 million fish that ascended therivers 150 years ago. While recovery planshave been delayed by the power industry,environmental groups argue that in thelong run losing the salmon will be moreexpensive to the regional economy thanmaking the changes now that will allowsalmon stocks to recover.

How do salmon find their way tothe mouth of the coastal river from thetrackless miles of the open ocean?Salmon move hundreds of miles awayfrom the coast, much too far to be ableto detect the odor of their parentstream. Experiments suggest that somemigrating fish, like birds, can navigateby orienting to the position of the sun.However, migrant salmon can navigateon cloudy days and at night, indicatingthat sun navigation, if used at all, can-not be the salmon’s only navigationalcue. Fish also (again, like birds) appearable to detect and navigate to theearth’s magnetic field. Finally, fisherybiologists concede that salmon maynot require precise navigational abili-ties at all, but instead may use oceancurrents, temperature gradients, andfood availability to reach the generalcoastal area where “their” river islocated. From this point, they wouldnavigate by their imprinted odor map,making correct turns at each streamjunction until they reach their natalstream.

Reproduction and GrowthIn a group as diverse as the fishes, itis no surprise to find extraordinaryvariations on the basic theme of sex-ual reproduction. Most fishes favor asimple theme: they are dioecious,with external fertilization andexternal development of the eggsand embryos (oviparity). How-ever, as tropical fish enthusiasts arewell aware, the ever-popular ovovi-viparous guppies and mollies of homeaquaria bear their young alive afterdevelopment in the ovarian cavity of the mother (Figure 26-34). Asdescribed earlier in this chapter (p. 517), some viviparous sharksdevelop a kind of placental attach-ment through which the young arenourished during gestation.

Let us return to the much morecommon oviparous mode of reproduc-tion. Many marine fishes are extraordi-narily profligate egg producers. Malesand females come together in greatschools and release vast numbers ofgametes into the water to drift with the

Figure 26-33Migrating Pacific sockeye salmon (Oncorhynchus nerka).


Figure 26-35Male banded jawfish Opistognathusmacrognathus orally brooding its eggs. Themale retrieves the female’s spawn and incubatesthe eggs until they hatch. During brief periodswhen the jawfish is feeding, the eggs are left inthe burrow.

Figure 26-34Rainbow surfperch Hypsurus caryi giving birth.All of the West Coast surfperches (familyEmbiotocidae) are ovoviviparous.

current. Large female cod may release4 to 6 million eggs at a single spawn-ing. Less than one in a million will sur-vive the numerous perils of the oceanto reach reproductive maturity.

Unlike the minute, buoyant, trans-parent eggs of pelagic marine teleosts,those of many near-shore bottom-dwelling (benthic) species are larger,typically yolky, nonbuoyant, and adhe-sive. Some bury their eggs, many attachthem to vegetation, some deposit themin nests, and some even incubate themin their mouths (Figure 26-35). Manybenthic spawners guard their eggs.Intruders expecting an easy meal ofeggs may be met with a vivid and oftenbelligerent display by the guard, whichis almost always the male.

Freshwater fishes almost invariablyproduce nonbuoyant eggs. Those,such as perch, that provide no parentalcare simply scatter their myriads ofeggs among weeds or along the bot-tom. Freshwater fishes that do providesome form of egg care, such as bull-head catfishes and some darters, pro-duce fewer, larger eggs that enjoy abetter chance for survival.

Elaborate preliminaries to matingare the rule for freshwater fishes. Thefemale Pacific salmon, for example,performs a ritualized mating “dance”with her breeding partner after arrivingat the spawning bed in a fast-flowing,gravel-bottomed stream (Figure 26-36).She then turns on her side and scoopsout a nest with her tail. As the eggs are laid by the female, they are fertil-ized by the male (Figure 26-36). Afterthe female covers the eggs with gravel,the exhausted fish dies and driftsdownstream.

Soon after the egg of an oviparousspecies is laid and fertilized, it takes upwater and the outer layer hardens.Cleavage follows, and the blastodermforms, sitting astride a relatively enor-mous yolk mass. Soon the yolk mass isenclosed by the developing blastoderm,which then begins to assume a fishlikeshape. The fish hatches as a larva carry-ing a semitransparent sac of yolk, whichprovides its food supply until the mouthand digestive tract have developed. Thelarva then begins searching for its own

food. After a period of growth the larvaundergoes a metamorphosis, especiallydramatic in many marine species suchas the freshwater eel described previ-ously (Figure 26-32). Body shape isrefashioned, fin and color patternschange, and the animal becomes a juve-nile bearing the unmistakable definitivebody form of its species.

Growth is temperature depen-dent. Consequently, fish living intemperate regions grow rapidly insummer when temperatures are highand food is abundant but nearly stopgrowing in winter. Annual rings inthe scales, otoliths, and other bonyparts reflect this seasonal growth(Figure 26-37), a distinctive record ofconvenience to fishery biologists whowish to determine a fish’s age. Unlikebirds and mammals, which stopgrowing after reaching adult size,most fishes after attaining reproduc-tive maturity continue to grow for aslong as they live. This may be a selec-tive advantage, since the larger thefish, the more gametes it producesand the greater its contribution tofuture generations.

Figure 26-37Scale growth. Fish scales disclose seasonalchanges in growth rate. Growth is interruptedduring winter, producing year marks (annuli).Each year’s increment in scale growth is a ratioto the annual increase in body length. Otoliths(ear stones) and certain bones can also be used in some species to determine age andgrowth rate.


Feeding for3–4 years inocean

Adults:

Dying salmonSpawning salmon

Asia

Oceanic distribution

Alevins hatching

Fry

Parr

Smolts

Adultmigratingupstream

Alaska

Figure 26-36Spawning Pacific salmon and development of the eggs and young.


Classification of Living FishesThe following Linnaean classification ofmajor fish taxa follows that of Nelson(1994). The probable relationships ofthese traditional groupings together withthe major extinct groups of fishes areshown in a cladogram in Figure 26-2.Other schemes of classification have beenproposed. Because of the difficulty ofdetermining relationships among thenumerous living and fossil species, we canappreciate why fish classification hasundergone, and will continue to undergo,continuous revision.Phylum Chordata

Subphylum Vertebrata (Craniata)Superclass Agnatha (ag�na-tha)(Gr. a, not � gnathos, jaw). Nojaws; cartilaginous skeleton; pairedlimbs absent; one or two semicir-cular canals; notochord persistent.Not a monophyletic taxon.

Class Myxini (mik-sy�ny) (Gr.myxa, slime): hagfishes. Fourpairs of tentacles aroundmouth; nasal sac with duct topharynx; 5 to 15 pairs of gillpouches, accessory hearts andslime glands present; poorlydeveloped eyes. Examples.Myxine, Bdellostoma; 43 species, marine.Class Cephalaspidomorphi(sef-a-lass�pe-do-morf�e) (Gr.kephalƒ, head, � aspidos, shield,� morphƒ, form): lampreys.Buccal funnel with keratinizedteeth; nasal sac not connected tomouth; seven pairs of gill pouch-es; well-developed eyes. Exam-ples: Petromyzon, Ichthyomy-zon, Lampetra; 41 species,freshwater and anadromous.

Superclass Gnathostomata(na�tho-sto�-ma-ta) (Gr. gnathos,jaw, � stoma, mouth). Jaws pres-

ent; paired limbs present (secon-darily lost in a few forms); threepairs of semicircular canals; noto-chord partly or completelyreplaced by centra.

Class Chondrichthyes (kon-drik�thee-eez) (Gr. chondros,cartilage � ichthys, fish): carti-laginous fishes. Cartilaginousskeleton; teeth not fused to jawsand usually replaced; no swimbladder; intestine with spiralvalve; claspers present in males.

Subclass Elasmobranchii(e-laz�mo-bran�kee�i) (Gr.elasmos, plated, � branchia,gills): sharks, skates, andrays. Placoid scales or deriv-atives (scutes and spines)usually present; five to sevengill arches and gills in sepa-rate clefts along pharynx;upper jaw not fused to cra-nium. Examples: Squalus,Raja, Charcarodon, Sphyr-na. About 815 species, mostly marine.Subclass Holocephali(hol�o-sef�a-li) (Gr. holos,entire, � kephalƒ, head): chi-maeras, ratfishes. Scalesabsent; four gill slits coveredby operculum; jaws withtooth plates; accessory clasp-ing organ (tentaculum) inmales; upper jaw fused tocranium. Examples: Chi-maera, Hydrolagus; 31species, marine.

Class Actinopterygii (ak�ti-nop-te-rij�ee-i) (Gr. aktis, ray, �pteryx, fin, wing): ray-finnedfishes. Skeleton ossified; singlegill opening covered by opercu-lum; paired fins supported pri-marily by dermal rays; limbmusculature within body;swimbladder mainly a hydrostatic

organ, if present; atrium andventricle not divided; teeth withenamaloid covering.

Subclass Chrondrostei(kon-dros�tee-i) (Gr. chon-dros, cartilage, � osteon,bone): bichirs, paddle-fishes, sturgeons. Skeletonprimarily cartilage; caudal finheterocercal; scales ganoid, ifpresent; spiral valve present;spiracle usually present; morefin rays than ray supports.Examples: Polypterus, Polyo-don, Acipenser. 34 species,freshwater and anadromous.Subclass Neopterygii(nee�op-te-rij�ee-i) (Gr. neo,new, � pteryx, fin, wing):gars, bowfin, teleosts.Skeleton primarily bone; cau-dal fin usually homocercal;scales cycloid, ctenoid,absent, or rarely, ganoid. Finray number equal to theirsupports in dorsal and analfins. Examples: Amia, Lepisos-teus, Anguilla, Oncorhynchus,Perca. About 23,600 species,nearly all aquatic habitats.

Class Sarcopterygii (sar-cop-te-rij�ee-i) (Gr. sarkos, flesh, �pteryx, fin, wing): lobe-finnedfishes. Skeleton ossified; singlegill opening covered by opercu-lum; paired fins with sturdyinternal skeleton and muscula-ture within limb; diphycercaltail; intestine with spiral valve;usually with lunglike swim blad-der; atrium and ventricle at leastpartly divided; teeth with enam-el covering. Examples: Latime-ria (coelacanth); Neoceratodus,Lepidosiren, Protopterus (lung-fishes). 7 species, marine andfreshwater. Not monophyleticunless tetrapods are included.

Summary

Fishes are poikilothermic, gill-breathingaquatic vertebrates with fins for limbs.They include the oldest vertebrategroups, having originated from anunknown chordate ancestor in the

Cambrian period or possibly earlier. Five classes of fishes are recognized.Least derived are the jawless hagfishes(class Myxini) and lampreys (classCephalaspidomorphi), remnant groups

having an eel-like body form withoutpaired fins; a cartilaginous skeleton(although their ancestors, the ostra-coderms, had bony skeletons); anotochord that persists throughout life;


and a disclike mouth adapted for suckingor biting. All other vertebrates havejaws, a major development in vertebrateevolution. Members of the classChondrichthyes (sharks, rays, skates,and chimaeras) are a secure grouphaving a cartilaginous skeleton (adegenerative feature), paired fins,excellent sensory equipment, and anactive, characteristically predaceoushabit. Bony fishes (Osteichthyes), maybe divided into two classes of fishes.One is a relic group, the lobe-finnedfishes of class Sarcopterygii, representedtoday by lungfishes and the coelacanth.The terrestrial vertebrates arose fromwithin one lineage of this group. Thesecond is the ray-finned fishes (classActinopterygii), a huge and diversemodern assemblage containing nearlyall familiar freshwater and marine fishes.

Modern bony fishes (teleosts) haveradiated into approximately 23,600 speciesthat reveal an enormous diversity of adap-tations, body form, behavior, and habitatpreference. Fishes swim by undulatorycontractions of the body muscles, whichgenerate thrust (propulsive force) and lat-eral force. Flexible fishes oscillate thewhole body, but in more rapid swimmersthe undulations are limited to the caudalregion or tail fin alone.

Most pelagic bony fishes achieve neutral buoyancy in water using a gas-filledswim bladder, the most effective gas-secreting device known in the animal king-dom. The gills of fishes, having efficientcountercurrent flow between water andblood, facilitate high rates of oxygenexchange. All fishes show well-developedosmotic and ionic regulation, achievedprincipally by the kidneys and gills.

With the exception of the jawlessagnathans, all fishes have jaws that are var-iously modified for carnivorous, herbivo-rous, planktivorous, and omnivorous feed-ing modes.

Many fishes are migratory to someextent, and some, such as catadromousfreshwater eels and anadromous salmon,make remarkable migrations of great lengthand precision. Fishes reveal an extraordi-nary range of sexual reproductive strate-gies. Most fishes are oviparous, but ovovi-viparous and viviparous fishes are notuncommon. Reproductive investment maybe in large numbers of eggs with low sur-vival (many marine fishes) or in fewer eggswith greater parental care for better sur-vival (freshwater fishes).

Review Questions

1. Provide a brief description of the fish-es citing characteristics that would dis-tinguish them from all other animals.

2. What characteristics distinguish hagfish-es and lampreys from all other fishes?

3. Describe feeding behavior in hagfishesand lampreys. How do they differ?

4. Describe the life cycle of the sea lam-prey, Petromyzon marinus, and the his-tory of its invasion of the Great Lakes.

5. In what ways are sharks wellequipped for the predatory life habit?

6. The lateral line system has beendescribed as a “distant touch” systemfor sharks. What function does the lat-eral line system serve? Where are thereceptors located?

7. Explain how bony fishes differ fromsharks and rays in the following sys-tems or features: skeleton, scales,buoyancy, respiration, reproduction.

8. Match the ray-finned fishes in the rightcolumn with the group to which eachbelongs in the left column:

Chondrosteans a. PerchNonteleost b. Sturgeonneopterygians c. GarTeleosts d. Salmon

e. Paddlefishf. Bowfin

9. Although the chondrosteans are todaya relic group, they were one of two

major lineages that emerged fromearly ray-finned fishes of the Devonianperiod. Give examples of living chon-drosteans. What does the term“Actinopterygii”, the class to which thechondrosteans belong, literally mean(refer to the Classification of LivingFishes on p. 534)?

10. List four characteristics of teleosts thatcontributed to their incredible diversityand success.

11. Only seven species of lobe-finned fish-es are alive today, remnants of agroup that flourished in the Devonianperiod of the Paleozoic. What mor-phological characteristics distinguishthe lobe-finned fishes? What is the lit-eral meaning of Sarcopterygii, the classto which the lobe-finned fishesbelong?

12. Give the geographical locations of thethree surviving genera of lungfishesand explain how they differ in theirability to survive out of water. Whichof the three is the least specialized?

13. Describe the discovery of the livingcoelacanth. What is the evolutionarysignificance of the group to which itbelongs?

14. Compare the swimming movements ofthe eel with that of the trout, andexplain why the latter is more efficientfor rapid locomotion.

15. Sharks and bony fishes approach orachieve neutral buoyancy in differentways. Describe the methods evolvedin each group. Why must a teleost fishadjust the gas volume in its swim blad-der when it swims upward or down-ward? How is gas volume adjusted?

16. What is meant by “countercurrentflow” as it applies to fish gills?

17. Compare the osmotic problem and themechanism of osmotic regulation infreshwater and marine bony fishes.

18. Two principal groups of fishes, withrespect to feeding behavior, are thecarnivores and the suspension-feeders.How are these two groups adapted fortheir feeding behavior?

19. Describe the life cycle of the Europeaneel. How does the life cycle of theAmerican eel differ from that of theEuropean?

20. How do adult Pacific salmon find theirway back to their parent stream tospawn?

21. What mode of reproduction in fishesis described by each of the followingterms: oviparous, ovoviviparous, vivip-arous?

22. Reproduction in marine pelagic fishesand in freshwater fishes is distinctivelydifferent. How and why do they differ?


Selected References

Bond, C. E. 1996. Biology of fishes, ed. 2. FortWorth, Harcourt College Publishers.Although somewhat less readable thanother ichthyology texts, it has a superiortreatment of fish biology, anatomy, andgenetics.

Bone, Q., and N. B. Marshall. 1982. Biology of fishes. New York, Chapman & Hall.Concise, well-written, and well-illustratedprimer on the functional process of fishes.

Conniff, R. 1991. The most disgusting fish in thesea. Audubon 93(2):100–108 (March).Recent discoveries shed light on the life his-tory of the enigmatic hagfish that fisher-men loathe.

Helfman, G. J., B. B. Collette, and D. E. Facey.1997. The diversity of fishes. Malden,Massachusetts, Blackwell Science. Thisdelightful and information-packed text-

book focuses on adaptation and diversityand is particularly strong in evolution, sys-tematics, and history of fishes.

Horn, M. H., and R. N. Gibson. 1988. Intertidalfishes. Sci. Am. 258:64–70 (Jan.). Describesthe special adaptations of fishes living in ademanding environment.

Long, J. A. 1995. The rise of fishes: 500 millionyears of evolution. Baltimore, The JohnsHopkins University Press. A lavishly illus-trated evolutionary history of fishes.

Moyle, P. B., and J. J. Cech, Jr. 1996. Fishes: anintroduction to ichthyology, ed. 3. Engle-wood Cliffs, New Jersey, Prentice-Hall, Inc.Textbook written in a lively style and stress-ing ecology rather than morphology.

Nelson, J. S. 1994. Fishes of the world, ed. 3.New York, John Wiley & Sons, Inc.Authoritative classification of all majorgroups of fishes.

Page, L. M., and B. M. Burr. 1991. A field guideto freshwater fishes: North America northof Mexico. Boston, Houghton Mifflin.Range maps and color illustrations for mostspecies.

Paxton, J. R., and W. N. Eschmeyer. 1998. Ency-clopedia of fishes, ed. 2. San Diego, Aca-demic Press. Excellent authoritative refer-ence that focuses on diversity and isspectacularly illustrated.

Stevens, J. D. (ed.) 1987. Sharks. New York,Facts on File Publications. Evolution, biology, and behavior of sharks, hand-somely illustrated.

Thomson, K. S. 1991. Living fossil. The story ofthe coelacanth. New York, W.W. Norton.

Webb, P. W. 1984. Form and function in fishswimming. Sci. Am. 251:72–82 (July). Spe-cializations of fish for swimming andanalysis of thrust generation.

Zoology Links to the Internet

Visit the textbook’s web site atto find live Inter-

net links for each of the references below.

Arizona’s Tree ofLife Web Page. An introduction, pictures,characteristics, discussion of the skull,phylogenetic relationships, and referenceson lampreys.

University ofCalifornia at Berkeley Museum of Paleon-tology site contains images, photos, sys-tematics, more information, and links.

Arizona’s Tree ofLife Web Page. An introduction, pictures,characteristics, discussion of the skull,phylogenetic relationships, and referenceson hagfishes.

University of Michi-gan site on chondrichthyean fish. Pictures,much information on the morphology, dis-tribution, and ecology of a large numberof sharks. Each fish is linked to webpages. Images may not be available fordisplay depending on your server.

Uni-versity of California at Berkeley Museumof Paleontology site contains images, pho-tos, and systematics.

University of Cali-fornia at Berkeley Museum of Paleontol-ogy site contains images, photos, systemat-ics, more information, and links.

The Great White Shark.

Introduction to the Chondrichthyes.

Class Chondrichthyes.

Hyperotreti (Hagfishes).

Introduction to the Myxini.

Hyperoartia (Lampreys).

www.mhhe.com/zoologySea World Edu-

cation Department information on sharks.

University of California at Berke-ley Museum of Paleontology site containsimages, photos, systematics, more informa-tion, and links.

Shark research, shark myths,information on sharks and cancer, sharkattacks, and more.

University of Michi-gan site on actinopterygiian fish. Pictures,much information on the morphology, dis-tribution, and ecology of a large numberof fish. Each fish is linked to additionalweb pages. Images may not be availablefor display depending on your server.

Arizona’s Tree of Life Web Page.Pictures, characteristics, phylogenetic rela-tionships, references on teleost fishes. Acladogram and references on teleosts.Some links to various groups of teleosts.

Interest-ing chart tells of recent trends in popula-tions of seafood that humans commonlyconsume, and gives recommendations ofwhat to eat, and what to avoid to be eco-logically responsible.

Terrific photos ofexternal and internal organs (loads slow-ly); some pictures are labeled, others arenot. Separate pictures of various aspects of

Dissection of the Shark.

The Audubon Guide to Seafood.

Teleostei.

Class Actinopterygii.

Research.Mote Marine Laboratory, Center for Shark

Teleostei.

Sharks and Their Relatives. external anatomy, individual pictures ofvarious internal organs or organ systems.

Squalus acanthius, the spiny dogfish.Information on physical characteristics,natural history, etc.

Containssome information, but of greater utility arethe many links to other sites with informa-tion on the coelocanth.

Arizona’s Tree of Life WebPage. Pictures, characteristics, phylogeneticrelationships, references on sarcopterygianfishes.

Univer-sity of California at Berkeley Museum ofPaleontology site contains images, photos,systematics, more information, and links.

University of California atBerkeley Museum of Paleontology sitecontains images, photos, systematics, moreinformation, and links.

A pictorial guideto the families of marine fishes found inthe waters surrounding the HawaiianIslands.

Several pictures ofinternal organs.

from the University of Minnesota.Subphylum Vertebrata, Class Osteichthyes,

Dissection of the Perch.

Marine Fishes of Hawaii.

Neopterygii.

Introduction to the Actinopterygii.

Sarcopterygii.

The Coelacanth: Living Fossil.

gan.Animal Diversity Web, University of Michi-


This organization publish-es Copeia, and includes related societies,links, and publications. Many links toother sites focusing on ichthyology andherpetology.

Many links and resources on fisheries.

University of Cali-fornia at Berkeley Museum of Paleontol-Vertebrate Systematics.

FAO Fisheries Department Homepage.

Herpetologists.American Society of Ichthyologists and ogy site contains information on the sys-

tematics and natural history of each of themajor groups of fishes and links to otherfish sites. Click on photographs for moreinformation about each group.

Informa-tion on fishery management in the GreatLakes. This site contains recent informa-tion on the lamprey problem in the GreatLakes, as well as information on sport and

Great Lakes Fishery Commission.

commercial fishing. It also includes anewsletter and information of fisheriesresearch.

Site provided by the Fish InformationService is an archive of information aboutaquariums.

Entertaining site pro-vides a variety of facts on sharks.Fiona’s Shark Mania.

Fins.

Phylum Chordata Class Myxini Class … · What Is a Fish? In common (and especially older) usage,...

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