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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”
4
5
6
7
3 Tissue Layers
Epidermis: epithelium
Gastrodermis: epithelium
Mesoglea (gelatinous ECM): CT
Diploblastic
8
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
9
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
10
11
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
12
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
16
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
17
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
19
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
21
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
22
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
23
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
24
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
25
26
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
27
Class Anthozoa
“Flower animals”
Sea anemones, corals, sea fans, & sea pens
Largest class: 6000 sp; solitary or colonial
No medusa phase in lifecycle
28
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
30
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
32
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
33
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
37
38
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)
39
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
41
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
43
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
46
Class Scyphozoa: Body Form
Medusa bells: 2-40 cm diameter
Manubrium divided into 4 oral arms
Tentacles located around periphery of
bell47
48
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
49
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
50
Scyphozoan Reproduction
Clonal: differentiate into strobila
Stacked mini medusae separate
via transverse fission (strobilation)
Juvenile medusae are called ephyra
Adult medusaereproduce sexually
51
52
Scyphozoan Diversity: 5 Orders
Semaeostomeae
Aurelia
Rhizostomeae
Stomolophus
Coronatae
Linuche
Cubomedusae
Chironex
Stauromedusae
Haliclystus
53
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
55
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
56
HydrozoanA-form
Colonies
Athecate: lack theca(protective cup)
Periderm ends at attachment point of
zooids
Typically grow via axial-polyp budding
57
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
58
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
59
60
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
62
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
64
65
Physalia
Class Hydrozoa:
Order Leptothecatae
L-form hydroids: Obelia
Feather-shaped colonies
66
Class Hydrozoa:
Order Limnomedusae
L-form hydroids with both polyp & medusa
phase
Gonionemus
67
68
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