Marine Invertebrate Zoology
The Hydrostatic Skeleton
Phylum Cnidaria
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Phylum Cnidaria
The Hydrostatic Skeleton
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Phylum Cnidaria
The Hydrostatic Skeleton
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The hydrostatic
skeleton requires:
Phylum Cnidaria
Phylum Cnidaria Overview
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Defining Characteristics
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Formation of complex intracellular organelles called
nematocysts
Planula larvae in the life cycle
The phylum is composed of anemones, corals,
sea whips, hydroids, and jellyfishes
Diploblastic
Phylum Cnidaria
Diploblastic Animals
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Epidermis
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Gastrodermis
Phylum Cnidaria
Nematocysts
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Used for gathering
food or defense
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Produced in
special cells called
cnidoblasts
SEM micrograph of nematocysts penetrating skin
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Phylum Cnidaria
Nematocysts
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Phylum Cnidaria
Cnidarian Nutrition
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A major cnidarian drawback is the absence
of an anus
Carnivorous feeders on zooplankton and
small fish
Reef building corals also depend on
symbiotic photosynthetic dinoflagellates for
energy
Phylum Cnidaria
Reproduction and Nervous System
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Reproduce asexually and sexually
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In the classes Scyphozoa and Hydrozoan the sexual stage
is the medusa
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Larvae - mouthless, ciliated, swimming planula
Alternate between the polyp and medusa
The polyp asexually produces a medusa that sexually
produces a planula larvae, which then settles into a polyp
Polymorphism = more than one stage per species
Phylum Cnidaria
Polyp & Medusa
Polyp and Medusa
Medusa (jellyfish)
 Usually solitary and free
swimming
 The mouth and tentacles
are directed downwards
 Contains thick mesoglea
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Polyp (tube shape)
 Solitary or colonial,
usually sessile attached
by its base
 Mouth and tentacles are
pointed upward
 Mesoglea is usually thin
 May have a fluid
skeleton or calcium
carbonate exoskeleton
Phylum Cnidaria
Cnidarian Classification
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Phylum Cnidaria
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Class Scyphozoa
Class Cubozoa
Class Hydrozoa
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Class Anthozoa
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Order Hydroida
Order Siphonophora
Subclass Alcyonaria
(Octocorallia)
Subclass Zoantharia
(Hexacorallia)
Phylum Cnidaria
Class Scyphozoa
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Defining characteristic
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Asexual replication by strobilation
Includes the true jellies, moon jellies
(Aurelia) and Sea Nettles (Chrysaora)
Phylum Cnidaria
Scyphomedusa Structure
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Contains a large
swimming bell that has
tentacles and sensory
capsules
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Rhopalia
 Statocysts
 Ocelli
Rhopalia
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Phylum Cnidaria
Medusa Movement
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Phylum Cnidaria
Scyphomedusa Structure
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Phylum Cnidaria
Scyphozoa Lifecycle
Class Cubozoa
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Defining
characteristics
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Medusa with box-like
body
Chiropsalmus
quadrumanus (Sea
Wasp)
Phylum Cnidaria
Class Cubozoa
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Known as box jellies or
sea wasps due to shape
and painful sting
Occur in tropical and
warm temperate seas
Cubozoa Eyes
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Phylum Cnidaria
Class Hydrozoa
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Due too small sizes and plant-like appearance
people are unaware of their existence
Display both polyp and medusa in the life cycle
Mesoglea is thin or absent
Phylum Cnidaria
Hydroid Structure
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Polyps can be solitary
or colonial
Very drab in
appearance, no
external skeleton,
colonies are
polymorphic
Hydra
Phylum Cnidaria
Polymorphism
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Hydroid colonies have
polyps of different
functions
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Feeding - gastrozooids
Defense - dactylozooids
Reproduction gonozooids
Phylum Cnidaria
Hydromedusa Structure
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Phylum Cnidaria
Hydrozoan Reproduction
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Hydroid stage can reproduce by asexual
budding
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Eventually the bud detaches and becomes
independent
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Gonozooids will release medusa
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Also has considerable powers of regeneration
All medusa reproduce sexually
Phylum Cnidaria
Hydrozoa Lifecycle
Order Hydroida
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Most species of the
class hydrozoa in our
area are hydroids
Form sessile colonies
of polyps and are
usually mistaken as
seaweed
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Exceptions are Velella and
Porpita = hydroids that float
freely at the surface
Velella
Phylum Cnidaria
Order Siphonophora
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Swimming or
floating hydrozoan
colonies
Portuguese mano-war (Physalia
physalis)
Phylum Cnidaria
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Phylum Cnidaria
Class Anthozoa
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Defining characteristics
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Absence of a medusa stage
Anemones, corals, sea whips, sea pansies
Polyps consists of a column, flattened oral disk
surrounding the mouth which bears tentacles
Some polyps are solitary (anemones) others
are colonial (coral)
Phylum Cnidaria
Class Anthozoa
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Feed using tentacles
that move food to the
mouth
Water is used by some
as a fluid skeleton
Produce planula larvae
that settle and form new
colonies
Can also reproduce
using pedal laceration
and fission
Phylum Cnidaria
Subclass Zoantharia (Hexacorallia)
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Posses 6 septa that separate the
gastrovascular cavity
Many species are solitary and lack a protective
covering (sea anemones)
Separated into two major orders
Phylum Cnidaria
Sea Anemones
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Solitary and lack a rigid
skeleton
Have a broad adherent
pedal disk
Can reproduce asexually
(pedal laceration) and
sexually
Phylum Cnidaria
Stony corals
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Colonial with calcium carbonate skeleton secreted by
the epidermis
May be reef-building (hermatypic) or non reef building
(ahermatypic)
Hermatypic corals live in warm clear waters and can
form chains around islands
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Zooxanthellae forms a symbiotic relationship with coral
Phylum Cnidaria
Subclass Alcyonaria (Octocorallia)
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Distinguishing
characteristics
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Sea whips, sea fans,
and sea pansies are in
this group
Can harbor
zooxanthellae
Phylum Cnidaria