Biology 320 Invertebrate Zoology Fall 2005

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Biology 320Invertebrate ZoologyFall 2005

• Chapter 7 Phylum Cnidaria

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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)

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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

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• 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
  elephants trunk with a mouth at the end
• Mouth-down orientation

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• In general, the body wall is composed of three
  tissue layers
• Epidermis - epithelium
• Gastrodermis epithelium
• Mesoglea (a gelatinous ECM) connective tissue
• Cnidarians are diploblastic

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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 SAVol because zooids are small and
  thus have a large SAVol
• Predisposition to filter feeding
• Multiple mouths and feeding appendages
• Broad distribution
• Small size

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• 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

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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

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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

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• 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

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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
• Statocysts
• Ocelli
• Chemoreceptors
• Mechanoreceptors

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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

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• 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

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Nutrition and Internal Transport

• Blind gut called coelenteron or gastrovascular
  cavity
• Coelenteron may posses septa to increase SA for
• 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

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• 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

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• 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

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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)

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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

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• 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

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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

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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

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• Musculature
• Mostly epidermal and gastrodermal
  epitheliomuscular cells
• Epidermal musculature controls tentacles and oral
  disc
• Gastrodermal musculature controls body column

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• 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
  draw-string
• Must remove coelenteric water to retract.
  Siphonoglyph cilia beat inwards to pump water
  back in

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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

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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

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• 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

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• Interesting ecological relationships
• Hermit crabs will wear anemone on shell will
  transfer to new shell if anemone doesnt 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

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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

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• 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 1C)

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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

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Subclass Alcyonaria, Order Gorgonacea

• Plantlike sea whips, fans, and plumes
• Highly branched
• Endoskeleton is an axial rod made of gorgonin
  (highly cross-linked collagen)

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Subclass Alcyonaria, Order Pennatulacea

• Sea pens, feathers and pansies

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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

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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

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• 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

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• 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

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• Lappets rounded lobes of umbrellar margin
• Rhopalia sensory organs found in grooves
  between lappets
• Statocyst
• General mechanoreceptor
• Possibly a chemoreceptor
• Sometimes a photoreceptor
• Jellies have a nerve net and nerve ring

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• 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

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Diversity of Class Scyphozoa

• Five orders

Semaeostomeae Aurelia
Rhizostomeae - Stomolophus
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Order Coronatae - Linuche
Order Stauromedusae - Haliclystus
Order Cubomedusae - Chironex
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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

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Hydrozoan Body Plans

• Two forms of polyps
• Athecate (A form)
• Thecate (L form)
• Zooids are usually 1mm, or less, in length
• Large SAVol
• Lack gastrodermal septa

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• 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)

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• 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

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• 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 fixed-length budding

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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

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Typical Life Cycle
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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 mouth-down

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Male and female Hydra. Life cycle lacks medusa
phase
Millepora
Velella
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• Order Siphonophora
• Polymorphic A-form hydrozoans
• Pedal end of polyp is a gas-filled 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

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• Order Leptothecatae
• L-form hydroids
• Feather-shaped colonies
• Obelia is the most famous representative

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• 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