Download Biology 320 Invertebrate Zoology Fall 2005 Chapter 7

Biology 320
Invertebrate Zoology
Fall 2005
Chapter 7 – Phylum Cnidaria
Phylum Cnidaria
 Hydra, anemones, stony corals,
soft corals, hydroids, and
jellyfishes
 All are marine (approx. 10,000
spp.) or freshwater (approx. 20
spp.), with no terrestrial spp.
 May be colonial or solitary
 Responsible for the building of
coral reefs, which are rivaled in
diversity only by tropical
rainforests
 Posses cnidocytes (cnid = nettle)
General Form and Function
 General body structure resembles a gastrula
 Posses a cavity known as a coelenteron
(gastrovascular cavity)
 Have a mouth surrounded by tentacles
 Radially symmetrical around an oral-aboral axis
 Useful because food, predators, etc. may approach
from any angle
 Display two body forms, often
in one life-cycle
 Polyp
 Resembles a flower and stem
 Has a pedal disc and an oral
disc
 Sessile and benthic, with a
“mouth-up” orientation
 Medusa
 Umbrella or bell-shaped
 Manubrium is more defined
and resembles an elephant’s
trunk with a mouth at the end
 “Mouth-down” orientation
 In general, the body wall
is composed of three
tissue layers
 Epidermis - epithelium
 Gastrodermis – epithelium
 Mesoglea (a gelatinous
ECM) – connective tissue
 Cnidarians are
diploblastic
Colonial Cnidarians
 Occurs when a juvenile
replicates via budding,
however, buds do not separate
 Produces zooids
 Means “tiny animal”
 Pronounced “ZOE-oid”
 Resemble the juvenile
 Preserves SA:Vol because
zooids are small and thus have
a large SA:Vol
 Predisposition to filter feeding
 Multiple mouths and feeding
appendages
 Broad distribution
 Small size
 Three main types of colonies, varying in
complexity
 Stolonate – posses stolons
 Coenosarc – posses a coenosarc and solenia
 Fruticose – typically upright and branching
with a plantlike / feathery appearance
 Two types of budding
 Fixed-length – as found in Obelia
 Axial-polyp
Cnidarian Skeletons
 Exceeds the diversity of poriferan skeletons
 Exoskeletons of:
 Chitinous periderm – some hydrozoans
 Calcium carbonate – stony corals
 Shell fragments covering the epidermis – some anemones
 Endoskeletons of:
 Fibers and spicules, similar to those of poriferans – soft corals
 Columns of cells containing turgid vacuoles – some hydrozoans
 Hydrostatic skeletons – Hydra, many anemones
Musculature and Movement
 Posses antagonistic sheets of muscle
 Circular smooth muscle – gastrodermis
 Longitudinal smooth muscle - epidermis
 Medusae posses coronal muscles,
encircling the subumbrella. These
muscles are antagonized by the elastic
mesoglea
 Cnidarians perform a wide variety
of movements
 Shortening, extending and bending in
polyps
 Constriction of bells in medusae
(facilitates swimming)
 Inch-worming and somersaulting in
polyps
 Movement of feeding appendages for
prey manipulation
 Retraction of the subumbrella in polyps
and medusae
Cnidarian Nervous Systems
 Two nerve nets
 Base of epidermis
 Base of gastrodermis
 Nets are joined by nerve
bridges that span mesoglea
 Nerve impulses can travel any
direction
 Important because of radial
sensory structures
 Medusa posses nerve rings,
musculature, ganglia, and
sense organs around bell
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Statocysts
Ocelli
Chemoreceptors
Mechanoreceptors
Cnidocytes
 For prey capture and defense
 Grasp terminology first
 Cnidocytes (cells) posses cnida (fluid-filled capsule with tubule)
Cnidocyte
Cnida
Function
Nematocyte
Nematocyst
Sting / release toxins
Spirocyte
Spirocyst
Tubule contains sticky
threads used for adhesion
 Cnidocytes are fairly ubiquitous
 Throughout epidermis
 On tentacles
 Often in gastrodermis
 Nematocyst firing
 Tubule coiled in capsule
 Triggered by a combination of
chemical and mechanical cues
from prey (rarely fires on
accident)
 Cytoplasmic water rushes in and
ejects tubule
 Toxins (proteins) may interfere
with Na+/K+ pumps or degrade
cell membranes
 Hydra discharges 25% of
nematocysts eating one brine
shrimp
 Replaced in 24hr
Nutrition and Internal Transport
 Blind gut called coelenteron or
“gastrovascular cavity”
 Coelenteron may posses septa to
increase SA for:
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Digestion
Absorption
Gas exchange
Excretion
Reproduction
Hydrostatic skeletal support
 Various canals in medusae, that
radiate out from central stomach
 Radial canals
 Ring canals
 In polyps and medusae, coelenteron
typically branches into each tentacle
 When prey is caught, mouth
opens and tentacles stuff prey
in
 Enzymes digest extracellulary
 Gastrodermal cells absorb
monomers
 Larger particles are digested
intracellularly
 Slow process; may take several
days
 Wastes ejected through mouth
 Many spp. posses
photosynthetic
endosymbionts
 May account for 90% of
nutrition in some
 Often defined patterns of
fluid circulation around
coelenteron
 Ciliated gastrodermis
 Muscular contractions
Gas and Waste Exchange
 All gas exchange occurs
across general body
surfaces
 Tentacles
 Body wall
 Waste exchange (excretion)
occurs across body wall
 Ammonia is primary waste
product (aqueous)
 Marine animal physiology
does not require water
conservation
 Terrestrial invertebrates
have many structural
adaptations for conserving
water during excretion (as
we will see later)
General Reproduction
 Amazing regenerators
 Can lose oral end and regrow it
 Living anemones can fully recover from dissections
 Clonal reproduction is common among polyps, but is less common
in medusae
 Sexual reproduction in most
 Some are monoecious
(hermaphroditic)
 Most are dioecious (separate
sexes)
 Germ cells usually develop in
gastrodermis
 Gametes are exocytosed into
coelenteron
 Typically extruded = external
fertilization
 Retained in some spp. =
internal fertilization
 Zygote typically develops into
planula (swimming larva)
 Settles (aboral end down)
 Develops into a juvenile polyp
Class Anthozoa
 “Flower animals”
 Sea anemones, corals,
sea fans, sea pens, etc.
 Largest class, at 6000
marine spp.
 Solitary or colonial
 All lack medusa phase of
lifecycle
Anthozoan Body Form
 Long pharynx attaching to
coelenteron
 Many septa, 6-192
depending on size of
species
 Some posses acontia
 Stringy filaments that are
attached near base of
septa
 Heavily armed
 May spew from mouth in
anemone deflates
 Stony corals lack these
structures
 Musculature
 Mostly epidermal and
gastrodermal epitheliomuscular
cells
 Epidermal musculature controls
tentacles and oral disc
 Gastrodermal musculature
controls body column
 Retraction
 Controlled by longitudinal
septal muscles called
retractors
 Tentacles and oral disc are
deflated of coelenteric fluid
 Pulled inside body column
 Mesogleal sphincter muscle
closes opening like a drawstring
 Must remove coelenteric
water to retract.
Siphonoglyph cilia beat
inwards to pump water back
in
Diversity of Class Anthozoa
 Subclass Zoantharia (Hexacorallia)
 Hexamarous symmetry (septa and
tentacles in multiples of 6)
 Order Actiniaria - anemones
 Order Scleractinia – stony corals
 Subclass Alcyonaria (Octocorallia)
 Octomerous symmetry (septa and
tentacles in multiples of 8)
 Order Stolonifera – organ - pipe corals
 Order Gorgonacea – sea fans, whips,
and plumes
 Order Pennatulacea – sea pens,
feathers, and pansies
Subclass Zoantharia, Order Actiniaria
 Sea anemones
 1350spp.
 Average size
 1.5cm – 10cm long
 1cm – 5cm diameter
 Largest are over a meter wide and
a meter tall
 Often brightly colored
 Some have unique methods of
locomotion
 Burrowing via peristalsis
 Walking on tentacles
 Swimming by thrashing tentacles
 Methods of nutrition
 Carnivores
 Suspension feeding secrete mucus, trap particles
on tentacles, and move to
mouth
 Photosynthate - may posses
two sets of tentacles: false
for photosynthesizing
(exposed during day) and
true (exposed during night)
 Can reproduce asexually
via pedal laceration
 Gonads are located in
septa
 Interesting ecological
relationships
 Hermit crabs will wear anemone
on shell; will transfer to new
shell if anemone doesn’t
transfer itself
 Anemone gets substrate,
transportation to food,
protection from predators, and
access to mates
 Crab gets camouflage,
nematocyst protection
 Clown fish has surface mucus
that lacks nematocyte-triggering
compounds
 Anemone gets food attracted by
fish, and removal of sediment
and necrotic tissue
 Clownfish gets protection and
food scraps
Subclass Zoantharia, Order Scleractinia
 Stony corals
 3600 spp., closely related to
anemones
 Secrete a CaCO3
exoskeleton
 Can weigh tons
 Produce cups known as
corallites that they can
retract into
 Puffer fish are coral
specialists
 Most are colonial with polyps
1mm to 3mm in diameter
 Coral can be very colorful
due to photosynthetic
endosymbionts
 Algae are often released in
conjunction with gametes
 Coral reefs are in danger
 Coral bleaching (algae
partially or completely
expelled) may occur under
stressful environmental
conditions
 Incorrect light intensity
(including UV)
 Salinity
 Temperature (even 1°C)
Subclass Alcyonaria, Order Stolonifera
 Most octocorallians are soft corals, and do not produce a CaCO3 shell
 Most lack nematocysts and produce noxious chemicals to deter predators
 Most are more tolerant of environmental fluctuations
 Organ – pipe corals are included in Order Stolonifera
Subclass Alcyonaria, Order Gorgonacea
 Plantlike sea whips, fans, and plumes
 Highly branched
 Endoskeleton is an axial rod made of gorgonin (highly cross-linked
collagen)
Subclass Alcyonaria, Order Pennatulacea
 Sea pens, feathers
and pansies
Medusozoa
 Medusa phase present in life cycle
 Planula
polyp
medusa
 Tetramerous (multiples of four) radial symmetry
 Cnidae are all nematocysts
 Two major classes
 Scyphozoa – large jellies
 Hydrozoa – small jellies, Hydra, and hydroids
Class Scyphozoa, Body Form
 200 spp. of large jellies
 Polyps are small and
funnel shaped
 Known as scyphistomae
 Coelenteron is divided by
four septa
 Also have four septal
funnels
 Circulate water to gonads
in adults
 Medusa bells
 Typically 2-40cm in
diameter
 Some greater than 2m
 Some brightly colored
 Manubrium is divided
into four oral arms
 Tentacles are located
around the periphery
of the bell
 Coelenteron is divided
into four gastric pockets
by septa
 Four pairs of gonads in
septa
 Four septal funnels
 Many have radial
canals and marginal
canals
 Gastrodermal cilia
circulates water
 Lappets – rounded lobes
of umbrellar margin
 Rhopalia – sensory
organs found in grooves
between lappets
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Statocyst
General mechanoreceptor
Possibly a chemoreceptor
Sometimes a
photoreceptor
 Jellies have a nerve net
and nerve ring
 Scyphozoan reproduction
 Scyphistomae undergo
asexual reproduction
 Differentiate into a strobila
from which stacked
miniature medusae separate
via transverse fission
 Process is known as
strobilation
 Juvenile medusa are known
as ephyra
 Adult medusae sexually
reproduce
Diversity of Class Scyphozoa
 Five orders
Semaeostomeae – Aurelia
Rhizostomeae - Stomolophus
Order Coronatae - Linuche
Order Cubomedusae - Chironex
Order Stauromedusae - Haliclystus
Class Hydrozoa
 Hydra and hydroids
 Other examples: fire coral and
Portuguese man of war
 3000 spp.
 Mainly colonies consisting of
polyp zooids and medusa
zooids
 Oftentimes, medusa form as
buds, but fail to be liberated
from colony
 Hydra lifecycle does not
include medusa phase
Hydrozoan Body Plans
 Two forms of polyps
 Athecate (A form)
 Thecate (L form)
 Zooids are usually 1mm,
or less, in length
 Large SA:Vol
 Lack gastrodermal septa
 Medusae
 Arise as lateral buds from
colony, rather than via
strobilation
 Many posses a velum, an
iris diaphragm on
subumbrellar margin that
aids in swimming
 Colonies
 Sessile, benthic, colonies
are called hydroids
 Typically resemble plants
or seaweeds
 May be any of the three
colony forms (stolonate,
coenosarcal, or fruticose)
 A-form colonies
 Athecate – lack
a theca
(extension of
the periderm
that acts as
protective cup)
 Periderm ends
at attachment
point of zooids
 Typically grow
via axial-polyp
budding
 L-form colonies
 Periderm forms a
wine-glass shape
theca
 Hydranth (water –
flower) may
retract into theca
 Theca may have
a hinged lid
(operculum)
 Grow via fixedlength budding
Zooid Terminology
 Can be a bit confusing, so spend some time with this
 Monomorphic colonies
 Only consist of gastrozooids (feeding hydranths)
 Reproduce by releasing medusa buds
 Polymorphic colonies
 Posses other types of zooids, in conjunction with gastrozooids
 Gonozooids – modified gastrozooids that bud gonophores
(permanently attached medusae that produce gonads)
 Dactylozooids – have nematocysts for protection. May capture
food and transfer to gastrozooids
Typical Life Cycle
Diversity of Class Hydrozoa
 Order Anthoathecatae
 Athecate (A-form)
 Examples:
 Hydra – gonochoric, also reproduce asexually, but lack medusa
phase
 Millepora – fire corals. Colonize axial rods of dead gorgonians.
Reef builders that have a nasty sting that burns like fire
 Velella – by-the-wind sailor has a float with a sail, displays
extreme polymorphism, with zooids that are suspended mouthdown
Male and female
Hydra. Life
cycle lacks
medusa phase
Millepora
Velella
 Order Siphonophora
 Polymorphic A-form
hydrozoans
 Pedal end of polyp is a gasfilled float known as a
pneumatophore
 Buds arise from column of
polyp in sets called cormidia.
Oldest is near float, and consist
of:
 Gastrozooid
 Gonozooid
 Dactylozooid – long tentacle with
nematocysts
 Bract – fleshy overhang that
protects zooids
 Prime example is Physalia, the
Portuguese man of war
 Order Leptothecatae
 L-form hydroids
 Feather-shaped
colonies
 Obelia is the most
famous representative
 Order Limnomedusae
 L – form
 Polyp and medusa phase
 Gonionemus is prime
example
 One additional order,
Trachylina
 Life cycles are devoid of
the polyp phase in all
members of this order
 Therefore, neither A-form
or L-form