Radiate animals Cnidarians and Ctenophores

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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 Cnidaria

• Characteristics 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
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Phylum Cnidaria

• Form 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
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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 Cnidaria

• Life 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 Digestion
• Gastrovascular 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 Cnidaria

• Body 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 Cnidaria

• Nerve 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 Cnidaria

• Class 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 610 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 Cnidaria

• Class 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 Cnidaria

• Class 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
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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 Cnidaria

• Sea 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 Corals
• Soft 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 Cnidaria

• Classification 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 Ctenophora

• Representative Type Pleurobrachia
• Transparent and 1.52 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