Phylum Cnidaria (radially symmetric, 2 cell layers in body) Jellyfish and allies. These alternate 2...

Phylum Cnidaria (radially symmetric, 2 cell layers in body)

Jellyfish and allies. These alternate 2 phases in their life cycle: the free-living medusoid phase (“jellyfish”), and a sessile hydroid phase.

Both feed by capturing planktonic food using tentacles armed with a cnidarian speciality, the class of stinging cell called nematocysts. Some are entangling, some inject barbed points to anchor, some inject toxins.

The main classes are:

• Hydrozoa: various medusoid radiations, often with several body forms fused into one animal ie Physalia physalis, the infamous, portugese man o’war (avoid!).

• Scyphozoa = jellyfish, Aurelia aurita in the common UK moon jelly (harmless to humans)

• Anthozoa: sessile forms: sea anemones, corals, sea fans

• Cubozoa: sea wasp

Cnidirians

• Overview: Life Without a Backbone

• Invertebrates– Are animals that lack a backbone– Account for 95% of known animal species

Figure 33.1

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Evolutionary Relationships of Cnidaria

The Radiate AnimalsTissue level of organization

• No anterior or posterior• Terms of direction defined based on the position of the

mouth• Oral/aboral ends• Biradial symmetry- single plane, passing through a

central axis, divides the organism into mirror images• Advantageous b/c sensory receptors evenly distributed.• Diploblasic tissue level organization- similar cells are

organized into tissues, and all cells are derived from two embryonical layers (ectoderm and endoderm/gastrodermis)

• A review of animal phylogeny

Ancestral colonialchoanoflagellate

Eumetazoa

Bilateria

Deuterostomia

Po

rife

ra

Cn

ida

ria

Oth

er

bila

teria

ns

(incl

ud

ing

Ne

ma

tod

a,

Art

hro

po

da

,M

ollu

sca

, a

nd

An

ne

lida

)

Ech

ino

de

rma

ta

Ch

ord

ata

Figure 33.2

• Exploring invertebrate diversity

PORIFERA (5,500 species)

A sponge

CNIDARIA (10,000 species)

A jelly

PLACOZOA (1 species) KINORHYNCHA (150 species)0.5 mm

A placozoan (LM) A kinorhynch (LM)250 µm

PLATYHELMINTHES (20,000 species) ROTIFERA (1,800 species)

A marine flatworm A rotifer (LM)

ECTOPROCTA (4,500 species) PHORONIDA (20 species)

Ectoprocts PhoronidsFigure 33.3

• Cnidarians have radial symmetry, a gastrovascular cavity, and cnidocytes

• All animals except sponges– Belong to the clade Eumetazoa, the animals

with true tissues

• Phylum Cnidaria– Is one of the oldest groups in this clade

• Cnidarians– Have diversified into a wide range of both

sessile and floating forms including jellies, corals, and hydras

– But still exhibit a relatively simple diploblastic, radial body plan

The Body Wall

• Two cellular layers• Epidermis- epithelio-muscular cells• Mesoglia- noncellular gel and is abundant

in the Medussa• Gastrodermis-

– gland cells for the production and release of enzymes.

– Flagellated nutitive muscular cells that contain food vacuoles

9-7

Fig. 9.9


Cnidarian Body Wall

Phylum Cnidaria- Nematocysts

• Name comes from the presence of specialized cells used in defense, feeding and attachment.

• Cnidosytes contain stinging organelles called Nematocysts.

Tentacle

“Trigger”

Nematocyst

Coiled thread

DischargeOf thread

Cnidocyte

Prey

Figure 33.6

• Cnidarians are carnivores– That use tentacles to capture prey

• The tentacles are armed with cnidocytes– Unique cells that function in defense and the

capture of prey

http://www.youtube.com/watch?v=Kukv0AtIVdU

Cnidocyte Structure and Nematocyst Discharge

9-8


Fig. 9.10

• The basic body plan of a cnidarian– Is a sac with a central digestive compartment,

the gastrovascular cavity

• A single opening– Functions as both mouth and anus

• There are two variations on this body plan– The sessile polyp and the floating medusa

Mouth/anus

TentacleGastrovascularcavity

Gastrodermis

Mesoglea

Epidermis

Tentacle

Bodystalk

Mouth/anus

MedusaPolyp

Figure 33.5

Gastrovascular Cavity

• Receives and digest food

• Single opening serves as the mouth and anus

• Tentacles surrounding the opening aid in feeding.

CV cavity continue

• Simplicity of body plan

• Hydras and other cnidirians don not require a true circulatory systems

• Body wall two cell thick encloses CV cavity

• Serves both in digestion and distribution of substances throughout the body

• Extend to tentacles

• The phylum Cnidaria is divided into four major classes

Cladogram of Cnidarian Taxonomy

9-17


Fig. 9.23

– Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa

(a) These colonial polyps are members of class Hydrozoa.

(b) Many species of jellies (classScyphozoa), including thespecies pictured here, are bioluminescent. The largest scyphozoans have tentaclesmore than 100 m long dangling from a bell-shaped body up to 2 m in diameter.

(c) The sea wasp (Chironex fleckeri) is a member of class Cubozoa. Its poison,which can subdue fish andother large prey, is more potent than cobra venom.

(d) Sea anemones and othermembers of class Anthozoaexist only as polyps.

Figure 33.7a–d

Alterations of Generations

• Typical cnidarian alternates b/w a hydroid stage (polyp) and a swimming medusa (jellyfish) stage.

• Polyp – assexual stage

• Medusa- sexual stage

• In some cnidarian classes either the polyp or the medussa is reduced or missing.

Generalized Cnidarian Life Cycle

Hydrozoans• Most hydrozoans

– Alternate between polyp and medusa forms

Feeding polyp

Reproductivepolyp

Medusabud

ASEXUALREPRODUCTION(BUDDING)

GonadMedusa

MEIOSIS

FERTILIZATION

SEXUALREPRODUCTION Egg Sperm

Developingpolyp

Portion ofa colonyof polyps

Maturepolyp

Planula(larva) Key

Haploid (n)Diploid (2n)1 mm

Zygote

Figure 33.8

A colony ofinterconnected

polyps (inset,LM) results

from asexualreproductionby budding.

1

Some of the colony’s polyps, equipped with tentacles, are specialized for feeding.

2 Other polyps, specialized for reproduction, lack tentacles and produce tiny medusae by asexual budding.

3

The medusae swim off, grow, and reproduce sexually.

4

The zygote develops into a solid ciliated larva called a planula.5 The planula eventually settles

and develops into a new polyp.6

Hydrazoa Obelia Structure and Life Cycle

9-10


Fig. 9.12

Structure of Gonionemus Medusa

9-11


Fig. 9.13b

CNIDARIA Class Hydrozoa

• Feather hydroid (Halocordyle disticha, formerly called Pennaria tiarella), from Bermuda

• Tiny, white tufts along lateral branches of the colony are individual polyps

Copyright ©The McGraw-Hill Companies, Inc. Permission required for reproduction or display

Photo Copyright © Diane R. Nelson


• Fire coral (Millepora complanata), common in the Caribbean

• Polyps on surface and along edges (“fuzz”) are loaded with toxic stinging nematocysts




• Portuguese Man-o-War (Physalia physalia)

• This is a colony with several types of polyps: gastrozooids, gonozooids, and dactylozooids

• Highly toxic



Scyphozoans

• In the class Scyphozoa– Jellies (medusae) are the prevalent form of the life

cycle– Polyp reduced or absent– Gametes produced gastrodermally– Cnidocytes present in gastrodermis and epidermis– Marine

– Examples- aurelia

Structure of Scyphozoan Medusa

9-12


Fig. 9.15

Source: After L. H. Hyman, Biology of the Invertebrates, Copyright © 1940 McGraw-Hill Publishing Co.

Aurelia Life History

9-13


Fig. 9.16

CNIDARIAClass Scyphozoa

• Sea thimble jellyfish (Linuche unguiculata), Honduras

• Tiny jellyfish (< 1 inch) that swarm in the spring

• Adults and larvae (“sea lice”) may cause a severe skin reaction in humans



CNIDARIA Class Scyphozoa

• Upside down jellyfish (Cassiopea xamachana) from Bermuda, with zooxanthellae

• This specimen is swimming up to the surface, where its reflection is visible



CNIDARIA Class Scyphozoa

• Jellyfish (Mastigias sp., family Rhizostomeae) from Jellyfish Lake in Palau, western Pacific

• These jellyfish have lost their ability to sting and depend on zooxanthellae for nutrition



Anthozoans

• Class Anthozoa includes the corals and sea anemones– Which occur only as polyps

Longitudinal Section of a Stony Coral Polyp

9-15


Fig. 9.20

Structure of an Anemone

9-14


Fig. 9.19

Charles Lytle and J. E. Wodsedalek, General Zoology Laboratory Guide, 11th ed., Complete Version, New York, McGraw-Hill. Reprinted by permission of The McGraw-Hill Companies.

CNIDARIA Class Anthozoa

• Sea fan (Subergorgia sp.) from the Solomon Islands

• This is a colony, with reticulated branches in one plane, at right angles to the prevailing current




• Gorgonian sea fan (Plexuaridae) on a reef wall in Fiji

• Individual polyps in the colony filter plankton from the water column




• Polyp of a tree fern or palm coral (Clavularia sp.) with pinnate tentacles, in the Solomon Islands

• Large sheets of these polyps are attached by a common stolon to the substrate




• Divaricate tree coral (Dendronephthya sp.), from Fiji

• These bushy or tree-like soft corals (alcyonaceans) are found only in the Pacific




• Close-up of an alcyonacean tree coral (Dendronephthya sp.)

• Spicules, the small white rod-shaped structures, are embedded in the tissue and aid in supporting the colony




• Corky sea finger (Briareum asbestinum), common in the Caribbean

• Sea rod in front has polyps extended, while rod in back has polyps retracted, revealing purple color




• Sea anemone (Heteractis sp.) from Fiji

• Note column of anemone and tentacles along margin

• This anemone may be host to symbiotic anemonefish




• The giant anemone or purple-tipped anemone (Condylactis gigantea) often harbors cleaning shrimp among its tentacles

• Common in Bermuda and the Caribbean




• Aerial view of coral reefs in Fiji

• Dark blue on right is deep water

• White is the edge of the reef

• Lighter color is the reef flat




• Extended slender polyps of the stony coral Goniopora sp., found in Fiji and the western Pacific

• Polyps are extended during the day




• Orange cup coral (Tubastraea coccinea) from Bonaire in the Caribbean

• These ahermatypic corals extend their polyps at night to feed on plankton




• Coral bleaching in star coral (Montastrea annularis) in the Caribbean

• Golden brown color indicates healthy tissue

• White area has lost zooxanthellae




• Coral bleaching in star coral (Montastrea annularis) in the Caribbean

• Loss of zooxanthellae due to higher water temperatures results in lighter color




• Close-up of the raised corallites of Caribbean elliptical star coral (Dichocoenia stokesii) in the daytime

• Polyps are extended at night to feed on plankton




• Close-up of giant star coral (Montastrea cavernosa) with polyps extended at night to feed on plankton

• Common in the Caribbean




• Sea pen (Pteroeides sp.) from the Solomon Islands

• This is a colony that lives anchored in the sand, turning at right angles to the prevailing current



Class Cubozoa

A few a lethal to humans - NEVER EVER swim with box jellies (sea wasps)

http://www.youtube.com/watch?v=hUTr0ufJGgo

Characteristics of Ctenophora

1)Radially or biradial Symmetrical. 2)Body multicellular, few tissues, some organs and organelles. 3)Body contains an internal cavity and a mouth and anal pores. 4)Swims by means of plates of cilia (the combs) 5)Reproduction mostly sexual as hermaphrodites, occasionally asexual. 6)Has a well developed subepidermal nerve net. 7)Has a distinct larval stage which is planktonic. 8)Lives in marine environments. 9)All are carnivorous.

Ctenophore Mnemiopsis

9-16


Fig. 9.22

Ctenophora Anatomy

• Comb Jellies

Ctenophora

http://www.youtube.com/watch?v=zi8zXLPMTX4

http://www.arkive.org/species/ARK/invertebrates_marine/Pleurobrachia_pileus/Pleurobrachia_pileus_00.html

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