Chapter 18

The Fishes: Vertebrate Success in Water

Evolutionary Perspective

• Phylogenetic Relationships– Subphylum Craniata• Skull surrounds brain, olfactory organs, eyes,

and inner ear.• Infraphylum Hyperotreti• Infraphylum Vertebrata

– Fossils record• Craniates and bone date earlier than 500

mya.

Survey of Fishes

• Infraphylum Hyperotreti—Class Myxini– Hagfishes– Head supported by cartilaginous bars.– Lack vertebrae and retain notochord– 4 pairs of sensory tentacles around mouth– Ventrolateral slime glands– Marine– Scavenge dead and dying fish

Figure 18.4 Class Myxini.

Survey of Fishes

• Infraphylum Vertebrata– Vertebrae surround

nerve cord.– Ostracoderms

• Extinct agnathans• Bony armor• Bottom dwellers

Figure 18.5 An ancient Silurian seafloor with two ostracoderms (Pteraspis and Anglaspis).

Survey of Fishes

• Class Petromyzontida–Marine and freshwater–Most are predators as adults, filter-

feeders as larvae– Brook lampreys• Adults do not feed.

– Life cycles involve open water adult stages and stream or river larval stages (figure 18.7).

Figure 18.6 Class Petromyzontida (Petromyzon marinus).

Figure 18.7 Life history of a sea lamprey.

Survey of Fishes• Superclass Gnathostomata

– Jaws developed from anterior pharyngeal arches.– Paired appendages– Classes

• Chondrichthyes• Actinopterygii• Sarcopterygii

Figure 18.8 Paired pectoral and pelvic appendages of a member of the Gnathostomata.

Survey of Fishes

• Class Chondrichthyes– Placoid scales, cartilaginous skeleton– Subclass Elasmobranchii• Sharks, skates, rays

– Subclass Holocephali• Ratfish• Operculum present

(a)

(b)

(c)

Figure 18.9 Class Chondrichthyes. (a and b) Subclass Elasmobranchii. (a) Reef shark (Carcharhinus perezi). (b) A bullseye stingray (Urolophus concentricus). (c) Subclass Holocephali. The ratfish (Hydrolagus colliei).

(a)

(b)

Figure 18.10 Scales and teeth of sharks.

Survey of Fishes

• Class Sarcopterygii– Lobe-finned fishes

• Fins with muscular lobes

– Lungs used in gas exchange.– Lungfish

• 3 genera• Australia, Africa, South America

– Coelacanths• 2 species• African and Indonesian coasts

– Tetrapodomorpha• Extinct ancestors of ancient amphibians and all tetrapods

Figure 18.11 Class Sarcopterygii. The lungfish, Lepidosiren paradoxa.

Figure 18.12 Class Sarcopterygii. The coelacanth Latimeria.

Survey of Fishes

• Class Actinopterygii– Ray-finned fishes• Fins lack muscular lobes

– Swim bladders– Chondrosteans• Sturgeons and paddlefish

– Neopterygii• Garpike (Lepisosteus) and dogfish or bowfin

(Amia)• Modern bony fish—the teleosts

Figure 18.13 Class Actinopterygii, the chondrosteans. (a) Shovelnose sturgeon (Scaphirhynchus platorynchus). (b) Paddlefish (Polydon spathula).

Figure 18.14 Class Actinopterygii, the teleosts. (a) A flounder (Pseudopleuronectes americanus). (b) Yellowtail snappers (Ocyurus chrysurus).

Evolutionary Pressures

• Locomotion– Streamlined shape, mucoid secretions, buoyancy of water,

body-wall muscles, fin shape all promote efficient locomotion.

• Nutrition and the digestive system– Filter feeders and scavengers

• Modern filterers use gill rakers.

– Predators (most modern fish)• Swallow food whole

– External parasites (lampreys)– Herbivores– Digestive tract

• Specializations include spiral valve (sharks) and pyloric cecae (bony fishes).

Evolutionary Pressures

• Circulation– Closed– Heart

• 4 embryological enlargements of ventral aorta– Sinus venosus– Ventricle– Atrium– Conus arteriosus

– Most fish have single circuit.– Lungfish

• Pulmonary circulation• Pulmonary and systemic circuits

Figure 18.15 Circulatory system of fishes.

Evolutionary Pressures

• Gas exchange–Water movement over gills

• Opercular and pharyngeal muscles pump water in most fishes.

• Ram ventilation in elasmobranchs and open-ocean bony fish

– Gas exchange surfaces• Visceral arches support gills.• Gill filaments and pharyngeal lamellae

– Countercurrent exchange mechanism (figure 18.16)

Figure 18.16 Gas exchange at pharyngeal lamellae. (a) Gill arches. (b) Trout lamellae. (c and d) Comparison of countercurrent and parallel current exchanges.

Evolutionary Pressures

• Swim bladders and lungs– Pneumatic sacs connect to digestive tract in

nonteleost fish.• Function as lungs in lung fish, climbing perch and ancient

rhipidistians• Function as swim bladders in other bony fish

• Buoyancy Regulation– Low density compounds– Fins provide lift.– Reduction of heavy tissues– Swim bladders

• Pneumatic duct (gulp air)• Counter current exchange at rete mirabile

Figure 18.17 Possible sequence in the evolution of pneumatic sacs. (a) Origin as ventral outgrowths of esophagus. (b) Primitive lungs. (c) Swim bladders move dorsal in position and lose connection to gut tract.

Evolutionary Pressures

• Nervous and sensory functions– Brain and spinal cord– Sensory receptors

• External nares• Eyes

– Lidless and round lens

• Inner ears– Equilibrium, balance, and hearing

• Lateral line system– Sensory pits in skin detect water movements.

• Electroreception– Prey detection by chondrichthyians– Gymnarchus (figure 18.18)– Electrophorus (electric eel)

Figure 18.18 Electric fishes. (a) Electrical fields are used to detect the presence of prey and other objects in a murky environment. (b) The electric fish (Gymnarchus niloticus).

Evolutionary Pressures

• Excretion and Osmoregulation– Kidneys

• Filter nitrogenous wastes, ions, water, and small organic compounds at nephrons– Glomerulus is filtering capillary network.– Tubule system promotes reabsorption.

• Freshwater fishes– Excess water must be excreted.– Ions and organic compounds are selectively

reabsorbed.

• Marine fishes– Water must be conserved.– Excess ions excreted.

Evolutionary Pressures

• Excretion and Osmoregulation– Elasmobranchs

• Sequester urea in body tissues• Rectal gland

– Diadromous fishes• Gills cope with both uptake and excretion of

ions.

– Nitrogen wastes• 90% ammonia (diffusion across gill surfaces)• 10 % urea, creatine or creatinine (kidneys)

Evolutionary Pressures

• Reproduction and development– Most oviparous

• Some ovoviviparous (some elasmobranchs) or viviparous (other elasmobranchs)

– Fertilization• Most external• Copulatory structures

– Claspers in elasmobranch males

– Development• Usually little or no parental care• Some tend nests or brood young

Figure 18.21 The male garibaldi (Hypsypops rubicundus) cultivates a nest of red algae, entices a female to lay eggs in the nest, and defends the nest.

Further Phylogenetic Considerations

• Two series of evolutionary events– Radiation of teleost fishes– Evolution of terrestrialism

• Terrestrialism– Tetrapodomorpha

• Osteolepiform sarcopterygians– Common features with amphibians

» Jaws, teeth, vertebrae, limbs

• Tiktaalik (the “fishapod”)– Fins, gills, scales– Dorsoventrally compressed and widened skull– Tetrapod-like forelimbs– Lacked opercular supports and dorsal and anal fins– Pectoral girdle and freely moveable neck

Figure 18.22 The fishapod Tiktaalik. This 375-million-year-old fossil helps us understand the transition between sarcopterygian fish and tetrapods. Its tetrapod-like features were probably used in foraging the water’s edge for prey.

Box Figure 18.3 The fossil record provides clear evidence of the evolution of the tetrapod limb. (a) The sarcopterygian Eusthenopteron. (b) The sarcopterygian Sauripterus. (c) The forelimb of the tetrapod Acanthostega. (d) The hindlimb of the tetrapod Ichthyostega.