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Marine Invertebrate Zoology

The Hydrostatic Skeleton

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The Hydrostatic Skeleton

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The Hydrostatic Skeleton

The hydrostatic skeleton requires:

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Phylum Cnidaria Overview

Defining Characteristics– 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

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Diploblastic Animals

Epidermis

Gastrodermis

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Nematocysts

Used for gathering food or defense

Produced in special cells called cnidoblasts

SEM micrograph of nematocysts penetrating skin

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Nematocysts

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Cnidarian Nutrition

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

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Reproduction and Nervous System

Reproduce asexually and sexually– Larvae - mouthless, ciliated, swimming planula

In the classes Scyphozoa and Hydrozoan the sexual stage is the medusa

– 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

Polyp & Medusa

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Polyp and Medusa

Medusa (jellyfish) Usually solitary and free

swimming The mouth and tentacles

are directed downwards Contains thick mesoglea

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

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Cnidarian Classification

Phylum Cnidaria– Class Scyphozoa– Class Cubozoa– Class Hydrozoa

Order Hydroida Order Siphonophora

– Class Anthozoa Subclass Alcyonaria

(Octocorallia) Subclass Zoantharia

(Hexacorallia)

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Class Scyphozoa

Defining characteristic– Asexual replication by strobilation– Includes the true jellies, moon jellies

(Aurelia) and Sea Nettles (Chrysaora)

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Scyphomedusa Structure

Contains a large swimming bell that has tentacles and sensory capsules

– Rhopalia Statocysts Ocelli

Rhopalia

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Medusa Movement

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Scyphomedusa Structure

Scyphozoa Lifecycle

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Class Cubozoa

Defining characteristics– Medusa with box-like

body Chiropsalmus

quadrumanus (Sea Wasp)

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Class Cubozoa

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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Class Hydrozoa

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

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Hydroid Structure

Polyps can be solitary or colonial

Very drab in appearance, no external skeleton, colonies are polymorphic

Hydra

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Polymorphism

Hydroid colonies have polyps of different functions– Feeding - gastrozooids– Defense - dactylozooids– Reproduction -

gonozooids

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Hydromedusa Structure

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Hydrozoan Reproduction

Hydroid stage can reproduce by asexual budding– Eventually the bud detaches and becomes

independent Also has considerable powers of regeneration

Gonozooids will release medusa– All medusa reproduce sexually

Hydrozoa Lifecycle

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Order Hydroida

Most species of the class hydrozoa in our area are hydroids

Form sessile colonies of polyps and are usually mistaken as seaweed

– Exceptions are Velella and Porpita = hydroids that float freely at the surface

Velella

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Order Siphonophora

Swimming or floating hydrozoan colonies

Portuguese man-o-war (Physalia physalis)

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Class Anthozoa

Defining characteristics– 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)

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Class Anthozoa

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

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Subclass Zoantharia (Hexacorallia)

Posses 6 septa that separate the gastrovascular cavity

Many species are solitary and lack a protective covering (sea anemones)

Separated into two major orders

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Sea Anemones

Solitary and lack a rigid skeleton

Have a broad adherent pedal disk

Can reproduce asexually (pedal laceration) and sexually

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Stony corals

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

– Zooxanthellae forms a symbiotic relationship with coral

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Subclass Alcyonaria (Octocorallia)

Distinguishing characteristics– Sea whips, sea fans,

and sea pansies are in this group

– Can harbor zooxanthellae