BIOL 201: Invertebrate Zoology

Chapter 7: Cnidaria

Rob SwatskiAsst. Prof. Biology

HACC-York

Phylum Cnidaria

Hydra, anemones, stony & soft corals,

hydroids, & jellyfishes

Most marine (10,000 sp); few FW (20 sp);

no terrestrial sp

Colonial or solitary

Corals build reefs (#2 biodiversity)

2

CnidariaStructure &

Function

Cnidocytes!

Body plan resembles gastrula; mouthsurrounded by

tentacles

Coelenteron(Gastrovascular

cavity, GVC)

Radial symmetry around oral-aboral

axis: advantage?3

Body Forms

Polyp

Resembles flower &

stem

Pedal disc & oral

disc

Sessile & benthic; “mouth-

up”

Medusa

Umbrella or bell-shaped

Manubrium: “elephant

trunk”

“Mouth-down”

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5

6

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3 Tissue Layers

Epidermis: epithelium

Gastrodermis: epithelium

Mesoglea (gelatinous ECM): CT

Diploblastic

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Colonial Cnidarians

Reproduce via budding, but buds

don’t separate

Produce zooids that resemble juveniles

Small zooids have large SA:V

Fillter-feeding: many mouths & tentacles, broad

distribution, small size

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Types of Colonies

Stolonate: have stolons (strawberry

runners)

Coenosarc: have coenosarc & solenia

(hollow tubes)

Fruticose: upright & branching; plantlike

or feathery

Budding: fixed-length (Obelia) & axial-polyp

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CnidarianSkeletons

More diverse than sponges

Exoskeletons: chitinous periderm

(hydrozoans); calcium carbonate (stony

corals); shell fragments (anemones)

Endoskeletons: fibers & spicules (soft corals)

or cell columns with turgid vacuoles

(hydrozoans)

Hydrostatic skeletons: Hydra, anemones

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Musculature

Antagonistic sheets of muscle

Circular smooth muscle in

gastrodermis

Longitudinal smooth muscle in epidermis

Coronal muscles around medusa

subumbrella13

Diversity of Movement

Polyps: shorten, extend, & bend

Medusae: constrict bells for

swimming

Some polyps inch-worm &

somersault!

Tentacles move for prey

manipulation

Subumbrellaretracts in polyps

& medusae 14

15

Nervous Systems

Two nerve nets: base of epidermis & gastrodermis; joined

by nerve bridges across mesoglea

Nerve impulses can travel any direction

(radial sensory system)

Medusae: nerve rings, musculature,

ganglia, & sense organs around bell

Statocysts, ocelli, chemoreceptors, & mechanoreceptors

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Cnidocytes

Used for prey capture & defense

Cnidocytes (cells) contain cnida (fluid-filled capsule with

tubule)

Nematocytes contain nematocyst that stings

& releases toxins

Spirocytes contain spirocyst that contains

sticky threads for adhesion

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Cnidocytes, cont.

Cnidocytes: abundant in epidermis, on

tentacles, in gastrodermis

Chemical & mechanical cues from prey trigger

firing of nematocyst

Toxins interfere with Na+/K+ pumps or

degrade cell membranes

Hydra discharges 25% of nematocysts eating

1 brine shrimp (replaced in 24hr) 18

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Nutrition & Internal

TransportCoelenteron (GVC):

blind gut

Septa increase SA for digestion, absorption,

gas exchange, excretion, repro,

hydrostatic skeleton

Radial & ring canals in medusae radiate out from central stomach

Coelenteron often branches into each

tentacle20

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Feeding

Prey is caught, mouth opens, tentacles stuff

prey inside, & enzymes digest extracellulary

Gastrodermis absorbs nutrients & larger

particles are digested intracellularly

Slow process, taking several days

Wastes ejected out through mouth

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Nutrition & Circulation

Many species have PSN endosymbionts

May account for 90% of nutrition in

some species

Often defined patterns of fluid

circulation around coelenteron

Ciliated gastrodermis;

muscular contractions

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Gas & Waste

ExchangeGas exchange occurs across general body surfaces: tentacles,

body wall

Waste exchange (excretion) occurs across body wall

Ammonia is primary waste product

Terrestrial inverts are adapted for

conserving water during excretion

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General Reproduction

Amazing regenerators!

Can lose oral end & regrow it

Anemones can fully recover from dissections

Clonal reproduction is common in

polyps, but less common in medusae

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Sexual Reproduction

Sexual repro in most: some monoecious

(hermaphroditic); most dioecious

Germ cells develop in gastrodermis &

gametes released into coelenteron

Most have external fertilization; some

internal fertilization

Zygote develops into planula larva, settles

with aboral end down, & develops into juvenile polyp

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Class Anthozoa

“Flower animals”

Sea anemones, corals, sea fans, & sea pens

Largest class: 6000 sp; solitary or colonial

No medusa phase in lifecycle

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AnthozoanBody Form

Long pharynxattached to coelenteron

Many septa, 6-192 depending on

species size

Some have acontia: stringy filaments

attached near septa base

Heavily armed: spew from mouth as

anemone deflates29

Musculature

Mostly epidermal & gastrodermal

epitheliomuscular cells

Epidermal musculature: controls tentacles & oral

disc

Gastrodermalmusculature: controls

body column

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Retraction

Controlled by retractors

(longitudinal septalmuscles)

Tentacles & oral disc deflate & pulled into

body

Mesogleal sphincter muscle closes

opening (draw-string)

Must remove pump water out to retract;

siphonoglyph cilia beat inwards to

pump water back in31

Diversity of Class Anthozoa

Subclass Zoantharia

(Hexacorallia)

Order Actiniaria: anemones

Order Scleractinia: stony corals

Subclass Alcyonaria(Octocorallia)

Order Stolonifera: organ pipe

corals

Order Gorgonacea:

sea fans, whips, & plumes

Order Pennatulacea:

sea pens, feathers, &

pansies

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Subclass Zoantharia:

Order Actiniaria

Hexamarous symmetry: septa & tentacles in

multiples of 6

Sea anemones: 1350 sp; brightly colored; most

<10 cm

Some have unique methods of locomotion

Burrowing via peristalsis; walking on

tentacles; swimming by thrashing tentacles

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Order Actiniaria:

TraitsCarnivorous suspension

feeders: mucus traps particles on tentacles,

moved to mouth

Photosynthate: may have 2 sets of tentacles: false for PSN (exposed

during day) & true(exposed during night)

Clonal repro via pedal laceration

Gonads located in septa 34

Order Actiniaria:

EcologyHermit crabs wear

anemone on shell & will transfer to new

shells

Anemone gets substrate, food,

protection, access to mates; Crab gets

camouflage & protection

Clownfish has protective surface

mucus

Anemone gets food, removal of sediment

& necrotic tissue; Clownfish gets

protection & food scraps 35

Subclass Zoantharia:

Order Scleractinia

Stony corals (3600 sp); close relation to

anemones

Secrete CaCO3

exoskeleton that can weigh tons

Produce cups (corallites) that they

retract into

Most colonial polyps, 1-3mm

diameter36

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Subclass Zoantharia:

Order Scleractinia

Corals are often very colorful due to PSN

endosymbionts

Algae often released in conjunction with

gametes

Coral bleaching can occur under stressful

environmental conditions

Incorrect light/UV intensity, salinity, temp (even 1°C)

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Subclass Alcyonaria:

Order Stolonifera

Octomeroussymmetry:

septa/tentacles in multiples of 8

Soft corals: don’t produce CaCO3;

organ pipe corals

Most lack nematocysts;

secrete noxious chemicals to deter

predators

More tolerant of environmental

fluctuations 40

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Subclass Alcyonaria:

Order Gorgonacea

Plant-like sea whips, fans, &

plumes

Highly branched

Endoskeleton: axial rod made of

gorgonin (highly cross-linked

collagen)42

Subclass Alcyonaria:

Order Pennatulacea

Sea pens, feathers, & pansies

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Medusozoa

Medusa phase in life cycle: planula

polyp medusa

Tetramerous radial symmetry:

multiples of 4

Cnidae are all nematocysts

2 major classes: Scyphozoa (large

jellies) & Hydrozoa(small jellies - Hydra,

& hydroids) 44

Class Scyphozoa

200 sp of large jellies

Polyps: small & funnel-shaped (scyphistomae)

Coelenteron divided by 4 septa

Have 4 septal funnels that circulate water to

gonads in adults45

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Class Scyphozoa: Body Form

Medusa bells: 2-40 cm diameter

Manubrium divided into 4 oral arms

Tentacles located around periphery of

bell47

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Class Scyphozoa: Body Form

Coelenteron divided by septa into 4 gastric

pockets

4 pairs of gonads in septa & 4 septal

funnels

Many have radial canals & marginal

canals

Gastrodermal cilia circulate water

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Class Scyphozoa: Body Form,

cont.Lappets: rounded lobes

on umbrella margin

Rhopalia: sensory organs in grooves b/w

lappets

Statocysts: mechanoreceptors

(also chemoreceptors& photoreceptors)

Nerve net & nerve ring

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Scyphozoan Reproduction

Clonal: differentiate into strobila

Stacked mini medusae separate

via transverse fission (strobilation)

Juvenile medusae are called ephyra

Adult medusaereproduce sexually

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Scyphozoan Diversity: 5 Orders

Semaeostomeae

Aurelia

Rhizostomeae

Stomolophus

Coronatae

Linuche

Cubomedusae

Chironex

Stauromedusae

Haliclystus

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Class Hydrozoa

Hydra & hydroids, fire corals, & Portuguese man-of-war; 3000 sp

Most are colonies of polyp & medusa

zooids

Medusa often form as buds, but are not

released from colony

Hydra life cycle does not include medusa

phase 54

HydrozoanBody Plans

Two types of polyps: Athecate (A-form) &

Thecate (L-form)

Zooids 1mm or smaller in length

Large SA:Vol

No gastrodermalsepta

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HydrozoanBody Plans

Medusae arise from colony as

lateral buds (not via strobilation)

Velum: iris diaphragm on subumbrellar

margin for swimming

Hydroid colonies are sessile &

benthic (seaweed-like)

3 colony forms: stolonate,

coenosarcal, or fruticose

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HydrozoanA-form

Colonies

Athecate: lack theca(protective cup)

Periderm ends at attachment point of

zooids

Typically grow via axial-polyp budding

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HydrozoanL-form

ColoniesPeriderm forms a wine-glass shape

theca

Hydranth can retract into theca

Theca may have a hinged lid

(operculum)

Grow via fixed-length budding

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Zooid Terminology

Monomorphiccolonies: 1 type of

zooid

Gastrozooids: feedinghydranths

Polymorphic colonies: several types of zooids

Gonozooids: reproductive polyps

Dactylozooids: have nematocysts for

protection & food capture

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Class Hydrozoa:

Order Anthoathecatae

Hydra: both clonal & sexual repro, but no

medusa stage

Fire corals: reef builders with nasty sting

Velella: by-the-wind sailor with a float & sail; extreme polymorphism, with zooids suspended

mouth-down61 Hydra

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Fire coral

63 Vellela: By-the-wind sailor

Class Hydrozoa: Order

Siphonophora

Physalia: Portuguese man-of-war

Pedal end of polyp bears a gas-filled float

(pneumatophore)

Buds arise from a column of polyp sets called

cormidia (oldest near float)

Possess diversity of zooids

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Physalia

Class Hydrozoa:

Order Leptothecatae

L-form hydroids: Obelia

Feather-shaped colonies

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Class Hydrozoa:

Order Limnomedusae

L-form hydroids with both polyp & medusa

phase

Gonionemus

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