RADIATE ANIMALS CNIDARIANS AND CTENOPHORES

Chapter 7

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Phylum Cnidaria

• Over 9,000 species in the phylum Cnidaria• Equipped with specialized cells: cnidocytes – Contain a specialized stinging organelle, the nematocyst

• Fossil specimens dated to over 700 million years ago Species– Most common in shallow marine environments – Some freshwater – None are terrestrial

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Phylum CnidariaCharacteristics of Phylum Cnidaria • All are aquatic and mostly marine • Symmetry– Radial or biradial

• Two body types – Free-swimming medusae – Sessile polyps

• Diploblastic – Epidermis and gastrodermis– Mesoglea: extracellar matrix that lies between

ectodermis and gastrodermis

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Phylum Cnidaria• Four classes of Cnidaria– Hydrozoa-hydroids,Portuguese man of war– Scyphozoa-jellyfish– Cubozoa-cube jellyfish – Anthozoa-sea anemone and coral

• A fifth class, Staurozoa,stalked jellyfish, has been proposed– No medusae in life cycle but polyp topped by medusa-like

region

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Ecological Relationship

• Some ctenophores, molluscs and flatworms eat hydroids and use the stinging nematocysts for their own defense

• Some molluscs and fish feed on cnidarians• Symbiotic relationship with crabs• Coral reefs

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Figure 7_02a

Phylum CnidariaForm and Function • Cnidaria have two basic body plans: polyp and

medusa– Polyp • Hydroid form• Adaptation to a sedentary life• Tubular body with the mouth directed upward and

surrounded by tentacles• Mouth leads into a blind gastrovascular cavity• Attach to substratum by pedal disc• Reproduce asexually by budding, fission, or pedal

laceration

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Phylum Cnidaria

– In colonial forms• Polyps may be specialized for feeding, reproduction, or

defense

– In class Hydrozoa• feeding polyps (hydranths), and defense (dactylozooids)

distinguished from reproductive polyps (gonangia) by absence of tentacles in gonangia

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Figure 7_02b

Phylum Cnidaria– Medusa• Bell or umbrella shaped‑• Usually free-swimming• Mouth directed downward • Tentacles may extend down from rim of umbrella

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Phylum Cnidaria

• Colonial polyps are sessile. • Hydra can move freely across a substrate by

gliding on their pedal disc aided by mucous secretions.

• Sea anemone can move similarly on pedal discs

• Medusae-contract the bell which expels water

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Phylum CnidariaLife Cycles • Polyps and medusae play different roles in the

cnidarian life cycle• Typically, zygote develops into a motile planula larva• Planula settles, and metamorphoses into a polyp– Produce other polyps asexually– Polyps eventually produce a free-swimming medusa by

asexual reproduction• Budding or strobilation

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Phylum Cnidaria

• Medusae – Dioecious-separate sexes in each individual– Reproduce sexually– True jellyfish (class Scyphozoa)

• Medusa is large and conspicuous• Polyps typically very small

– Most colonial hydroids • Feature a polyp stage and a pelagic medusa stage

– Some hydrozoans (Physalia) form floating colonies– In Hydra, only stage is a small freshwater polyp

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Phylum Cnidaria

Feeding and DigestionGastrovascular cavity acts as stomach. – Catch prey with tentacles and pass them to the

gastrovascular cavity– Gland cells discharge enzymes to begin extracellular

digestion• Intracellular digestion continues in the cells of the

gastrodermis

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Phylum CnidariaBody Wall• Cnidarian body– Outer epidermis – Inner gastrodermis – Layers separated by mesoglea– Mesoglea • Gelatinous• Continuous in polyps, extending through body and

tentacles• Supports body• Thicker in medusae• Thinner in hydromedusae• Gastrovascular cavity- Water enters due to

beating cilia–Water serves as a hydrostatic skeleton

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Phylum Cnidaria– Epitheliomuscular cells• Form most of epidermis and cause muscular

contraction – Undifferentiated interstitial cells

Develop into cnidoblasts, sex cells, buds, or nerve cells, but not epitheliomuscular cells.

– Gland cells • On the adhesive disc secrete an adhesive and

sometimes a gas bubble for floating

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Phylum Cnidaria

– In Hydra• Epidermis contains epitheliomuscular, interstitial,

gland, cnidocytes, sensory, and nerve cells

– Cnidarian bodies extend contract, bend, and pulse• No mesodermally derived muscle cells• Have epitheliomuscular cells

– Form most of epidermis– Cover organism and function in muscle contraction

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Phylum Cnidaria

Cnidocytes• Many cnidarians are effective predators• Possible due to presence of a unique cell type, the

cnidocyte• Cnidoctyes – Located in invaginations of ectodermal cells and some

gastrodermal cells– Produces one of over 20 types of cnidae – One type of cnida (tiny capsule) is the nematocyst– End of cnida is covered by a little lid-operculum

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Phylum Cnidaria• Nematocysts – Tiny capsules made of chitin-like material and containing a

coiled filament– Filament may have tiny barbs or spines– Except in Anthozoa, cnidocytes equipped with trigger-like

cnidocil (modified cilium)– Tactile stimulation cause nematocyst to discharge– After Cnidae discharge, cnidocyte is absorbed and another

develops

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Phylum Cnidaria

• Mechanism of Nematocyst Discharge – When stimulated, water to rush into the capsule– The operculum opens and rapidly launches the

filament– Barbs inject poison into prey– Only a few jellyfish and the Portuguese man-of-war

can seriously harm humans

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Phylum CnidariaNerve Net• Nerve net of cnidarians one of the best examples of

diffuse nervous system– Two nerve nets, one at the base of epidermis and one at

the base of gastrodermis, interconnect• Nerve action potentials transmitted across synapses

by neurotransmitters• Unlike higher animals, – Nerve nets have neurotransmitters on both sides of the

synapses – Allowing transmission in either direction– No myelin sheath on axons

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Phylum Cnidaria

• Nerve cells synapse with both slender sensory cells and epitheliomuscular – Association often called a neuromuscular system

• The nerve net pattern is also found in annelid and human (nerve plexus) digestive systems

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Phylum CnidariaClass Hydrozoa

• Most marine and colonial with both polyp and medusa forms– Freshwater Hydra is not typical

• Colonial Obelia is more exemplary– Typical hydroid has a base, a stalk, and one or more

terminal zooids (individual polyp animals)– Base is a rootlike stolon, or hydrorhiza• Gives rise to stalks called hydrocauli

– Living part of the hydrocaulus is a tubular coenosarc– Hydrocaulus covered by a non-living chitinous sheath, the

perisarc

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Phylum Cnidaria• Hydra– Found on the underside of aquatic leaves and lily

pads in clean fresh water– Worldwide distribution (16 species in North

America)– Body is a cylindrical tube– Bottom end has a basal or pedal disc for

attachment– The mouth (oral end) on a conical elevation, the

hypostome– Ring of 6–10 hollow tentacles encircles mouth

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Phylum Cnidaria– The mouth opens to a gastrovascular cavity– Buds may project from the side, each develop a

mouth and tentacles– Hydras feed on a variety of small crustaceans,

insect larvae, and worms• Mouth is located on a raised hypostome, and

opens into the gastrovascular cavity• Food organisms brush against the tentacles are

captured by nematocysts

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Phylum Cnidaria– Hydras reproduce sexually and asexually• Asexual reproduction –Budding

• Most hydra are dioecious-separate sexes–Temporary gonads appear in autumn,

stimulated by lower temperatures or stagnation–Eggs and sperm shed externally–Cyst forms around embryo–Encysted form endures the winter, then

young hydras hatch in the spring

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Phylum Cnidaria– Hydroid Colonies– Individual zooids are attached to the base– Hydranths (gastrozooids) are feeding polyps with

circle of tentacles surrounding mouth– Colonial hydroids bud off new individuals• Individuals may be new hydranths or medusae

buds– In Obelia, the medusae buds are formed by a

reproductive polyp called a gonangium

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Phylum Cnidaria– Hydroid medusae • Usually smaller than schyphozoan medusae• Margin of the bell projects inward as a shelf-like velum• Mouth opens at the end of a suspended manubrium• Mouth connects to a stomach and four radial canals• Radial canals connect to a ring canal that runs around

the margin of the bell and connects with the hollow tentacles• Bell margin has many sensory cells• Typically also bears statocysts, specialized sense organs

that function in equilibrium, and light-sensitive ocelli

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Hydroid-Gonionemus

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Phylum Cnidaria• Other Hydrozoans – Orders Siphonophora form floating colonies– Contain several types of polyp individuals-feeding polyps,

reproductive polyps, nematocysts, jelly polyps• In Physalia, the float, pneumatophore– Thought to have expanded from the original larval

polyp– tentacles can stretch up to 165 feet below the

surface of the water– it is able to deflate itself and descend to escape the

threat

– Other hydrozoans secrete calcareous skeletons resembling true corals and are the hydrocorals

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Phylum CnidariaClass Scyphozoa

• Most of the larger jellyfishes belong to this class• Nearly all float in open sea – One order is sessile, attached to seaweeds by a stalk

• Bells vary in shape and size– Composed mostly of mesoglea

• Mesoglea contains ameboid cells and fibers• Lack shelf-like velum found in hydrozoan medusae• Margin of the umbrella has indentations, each

bearing a pair of lappets

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Phylum Cnidaria• Between lappets is a equilibrium sense organ called a

rhopalium• Mouth located beneath the umbrella• Manubrium forms four oral arms– Capture and ingest prey

• Tentacles, manubrium, and often entire body may have nematocysts

• Feed on small organisms from protozoa to fish• Cilia on gastrodermis circulates food and oxygen

throughout the animal

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Phylum Cnidaria• Sexes are separate – Fertilization is internal in the gastric pouch of the

female-sperm is carried to gastric pouches– Zygote develops into a ciliated planula larva• Attaches and develops into a scyphistoma

– Scyphistoma undergoes strobilation – Form buds called ephyrae that break loose to

form jellyfish medusae

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Phylum Cnidaria

Class Staurozoa• Commonly called stauromedusans• No medusa stage• Solitary polyp body that is stalked • Uses adhesive disk to attach to seaweeds, and

objects on sea bottom• Polyp top resembles a medusa with eight extensions

(“arms”) ending in tentacle clusters surrounding mouth

• Reproduce sexually

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Phylum Cnidaria

Class Cubozoa• Medusa form is dominant• Polyp is inconspicuous or unknown• Umbrella is square– One or more tentacles extend from each corner

• At base of each tentacle is a flat blade called a pedalium

• Rhopalia has 6 eyes and other sense organs• Umbrella edge turns inward to form a velarium– Increases swimming efficiency

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Phylum Cnidaria• Strong swimmers • Feed mostly on fish in nearshore areas• Polyp stage is tiny– New polyps bud laterally– Do not produce ephyrae but directly change into medusae

• The sea wasp – Potentially fatal cubomedusan from Australia

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Phylum CnidariaClass Anthozoa-flower animals

• Lack a medusa stage• All marine, in both deep and shallow water, and vary

in size

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Phylum Cnidaria

• Three subclasses: • Zoantharia (aka Hexacorallia) -sea anemones

and hard coral• Cerianthpatharia- black coral, tube anemone• Octocorallia (aka Alcyonaria) –soft, flexible

and horny coral(sea fans)• Zoantharia and Cerianthpatharia are

hexamerous(6); Alcyonaria are octomerous(8)

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Sea anemone-rose anemone

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Figure 7_21a

Figure 7_21b

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Phylum Cnidaria• Gastrovascular Cavity– Large and partitioned by septa or mesenteries, inward

extensions of body wall– Septa may be coupled or paired

• The mesoglea contains ameboid cells• No special organs for respiration or excretion

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Phylum CnidariaSea Anemones• Polyps larger and heavier than hydrozoan polyps• Attach to shells, rocks, timber, etc. by pedal discs Some

burrow in mud or sand• Carnivorous• Crown of tentacles surrounds the flat oral disc• Slit-shaped mouth leads into a pharynx• Siphonoglyph (ciliated groove) creates a water current

directed into the pharynx• Transports oxygen, removes wastes, and maintains fluid

pressure for a hydrostatic skeleton

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Phylum Cnidaria• Gastrovascular cavity divided into six pairs of primary

septa or mesenteries-tissues– Smaller incomplete septa subdivide the large chambers

increasing surface area– Free edge of each incomplete septum• Forms a septal filament with nematocysts and gland

cells for digestion• Acontia threads at lower ends of septal filaments – Equipped with nematocysts– May protrude through mouth to help secure prey

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Phylum Cnidaria• When in danger, water rapidly expelled through pores as the

anemone contracts to a small size

• Most anemones can glide slowly on pedal discs– Some can swim with limited ability

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Phylum Cnidaria• When in danger, they contract and withdraw

tentacles and oral discs• Most harbor symbiotic algae• Some have a mutualistic relationship with hermit

crabs

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Phylum Cnidaria• Reproduction– Some dioecious, some monoecious– Monoecious species are protandrous • Produce sperm first and eggs later

– Gonads on margins of septa– Fertilization is external or in gastrovascular cavity– Zygote becomes a ciliated larva– Pedal laceration, small pieces of pedal disc break off

and regenerate a small anemone– Transverse fission, and budding occur

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Phylum Cnidaria

Zoantharian Corals • Also called true or stony corals• Described as miniature sea anemones that live in calcareous

cups they have secreted • Gastrovascular cavity is hexamerous – No siphonoglyph

• No pedal disc– Secrete a limey skeletal cup with sclerosepta projecting up

into the polyp• Sheet of living tissue forms over the coral surface– Connects all gastrovascular cavities

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Phylum Cnidaria

Octocorallian CoralsSoft coral, sea pens, sea fans• Octomerous symmetry, – Eight tentacles – Eight septa

• All are colonial • Show great variation in form of colony

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Polyp of Octocorallian

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Octocorallian-soft coral

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Phylum Cnidaria• Coral Reefs• Great diversity of organisms • Rivaled only by tropical rainforests• Plants and animals are limited to top layer – Above the calcium carbonate deposits

• Reef building corals and coralline algae form most coral reefs– Require warmth, light, and salinity of undiluted sea water– Limited to shallow waters between 30 degrees north and

30 degrees south latitude– Photosynthetic zooxanthellae( red algae) live in their

tissues– Provide food for corals and recycle phosphorus and

nitrogenous wastes

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Phylum Cnidaria• Threatened Reefs• Nutrients from fertilizer and sewage threaten coral

reefs with excessive algal growth• Global warming coral tissues turn white and brittle,

this is called coral bleaching• Higher atmospheric concentrations of carbon dioxide

(from burning hydrocarbon fuels) tends to acidify ocean water, which makes precipitation of CaCO3 by corals more difficult metabolically

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Phylum CnidariaClassification of Cnidaria

• Class Hydrozoa• Class Scyphozoa• Class Staurozoa• Class Cubozoa• Class Anthozoa– Subclass Hexacorallia (Zoantharia)– Subclass Ceriantipatharia– Subclass Octocorrallia (Alcyonaria)

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Phylum Ctenophora• Phylum composed of about 150 species-comb jellies• All marine, most prefer warm waters• Ctenophores– Eight rows of comb-like plates of cilia used for locomotion

• Nearly all free-swimming, few creep or are sessile• Body structure (ellipsoid or spherical shape)– Biradial symmetry due to presence of two tentacles.– Tissue level of oganization– No head,– Translucent body with a gelatinous layer

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Phylum Ctenophora

• Feeding Habits– 2 tentacles capture planktonic organisms by means of

epidermal glue cells called colloblasts-no nematocysts– Short tentacles collect food on the ciliated body surface– Ctenophores without tentacles feed on other gelatinous

animals– Structuring classes within the Ctenophores still being

developed– Luminesce

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Phylum CtenophoraRepresentative Type: Pleurobrachia • Transparent and 1.5–2 cm in diameter• Oral pole bears the mouth opening • Aboral pole equipped with statocyst-sense organ for

equilibrium and is associated with the beating of the cilia

• Eight equally-spaced plates called comb plates or ctenes extend from aboral to oral pole

• All eight rows beat in unison• Drives the animal forward mouth-first• Colloblast-stick cells on tentacles

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Phylum Ctenophora

• Digestion and Respiration – Gastrovascular system comprises a mouth,

pharynx, stomach and canals that run to the comb plates, tentacular sheaths, and elsewhere

– Digestion both extracellular and intracellular

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Phylum Ctenophora• Respiration and excretion occur by diffusion across

the body surface• Nervous and Sensory Systems – Resembles cnidarians – Statocyst is a bell-like chamber• Tufts of cilia sense changes in pressure from statolith as

animal changes position– Epidermis bears sensory cells sensitive to chemical

and other stimuli

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Phylum Ctenophora

• Reproduction and Development – Monoecious– Fertilized eggs discharged through epidermis into

water– Free-swimming larva somewhat resembles adult

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Phylogeny and Adaptive Diversification

• Cnidarian Phylogeny – Relationships among cnidarian classes are still

controversial: Which came first, the polyp or the medusa?

– One hypothesis postulates that the ancestral cnidarian was a trachyline-like hydrozoan with a medusa stage.

– Another hypothesis suggests that the ancestral cnidarian was an anthozoan polyp without a medusa in the life cycle.

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Phylogeny and Adaptive Diversification• Adaptive Diversification– Neither phylum has deviated far from basic structural

plan– Cnidarians • Achieved large numbers of individuals and species• Demonstrate large diversity considering the simplicity of

body plan• Efficient predators

– Some feeding on prey larger than themselves– Some feeding on small particles

• Some colonial forms grow to great size among corals• Others show polymorphism and specialization of individuals

within a colony

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