CNIDARIA

Instructor: Almonther I. Alhamedi

The Islamic University of GazaDepartment of Biology

E-mail : . .mhamedi@iugaza edu psWeb page: http:// . . . /site iugaza edu ps mhamedi

GENERAL CHARACTERSTICS

Radial Symmetry Diploblastic – 2 embryonic tissue layers. Belong to the subkingdom Metazoa. Tissues – derived from two embryonic germ

layers Cnidocytes – special cells with stinging

organelles called nematocysts. The basic body plan of a cnidarian is a sac with a

central digestive compartment, the gastrovascular cavity.

A single opening functions as both mouth and anus. Cnidarians have an outer tissue layer, the

epidermis, derived from ectoderm, and an inner gastrodermis, derived from endoderm, with jellylike mesoglea in between.

PHYLUM CNIDARIA

Cnidarians have diversified into a wide range of both sessile and floating forms including jellies, corals, and hydras.Polymorphism – some species exist as both polyps and medusae during their life cycles.

REPRODUCTION

Polyps can reproduce asexually by budding, fission, or pedal laceration.

REPRODUCTION Cnidarians, typically medusae, can also reproduce

sexually. A zygote usually develops into a motile planula larva. Some species only exist as polyps, others only as

medusae, others alternate between the two.

FEEDING

Cnidarians are carnivores that use tentacles to capture prey.

The tentacles are armed with cnidocytes – unique cells that function in defense and the capture of prey. Nematocysts contain toxins used for prey capture

and defense.

FEEDING

Nematocysts are used to inject a toxin. Variable in form. May be used for prey

capture or defense.

FEEDING

Extracellular digestion begins in the gastrovascular cavity, but is completed within the cells of the gastrodermis.

Some cnidarians supplement their diet with nutrients collected from algal symbionts (zooxanthellae).

CLASSIFICATION

The phylum Cnidaria is divided into four major classes:

Class Hydrozoa Class Scyphozoa Class Cubozoa Class Anthozoa

CLASSIFICATION

polyps and medusa stage, although polyp stage is dominant.

gut cavity of polyp is simple, lacking a pharynx . tetramerous (four-part) radial symmetry. gonads are ectodermal (found in the epidermis). medusa stage may possess specialized balance organs

called statocysts and photosensitive organs called ocelli.

solitary or colonials; some colonial forms highly polymorphic.

Hydras have nematocysts that: 1) penetrate prey and inject poison; 2) recoil and entangle prey; and 3) secrete an adhesive for locomotion and attachment

includes hydroids (“stinging limu”), fire coral, pink coral, siphonophores and obelia

Class Hydrozoa(Gr. hydra, water serpent)

A- Hydra with developing bud and ovary.B- Hydra catches an unwary water flea with the nematocysts of its tentacles. This hydra already contains one water flea eaten previously.

A B

CLASS HYDROZOA - POLYPS

Typical hydroids have a base, a stalk, and one or more terminal zooids (individual polyp animals). Thecate – presence of a

protective cup around the polyp. Obelia

Athecate – no such protection. Ectopleura

CLASS HYDROZOA - MEDUSAE

Hydroid medusae are usually smaller than those in the class Scyphozoa. Gastrovascular cavity is

continuous from mouth to tentacles and is lined by gastrodermis.

Velum, inward projection of the bell, is present.

Specialized organs: Statocysts – equilibrium Ocelli – light sensitive Bell medusa, Polyorchis

penicillatus, medusa stage of an unknown attached polyp.

Structure of Gonionemus. A, Medusa with typical tetramerous arrangement. C, Portion of a tentacle with its adhesive pad and ridges of nematocysts. D, Tiny polyp, or hydroid stage, that develops from the planula larva. It can produce more polyps by budding (frustules) or produce medusa buds.

Life cycle of Obelia, showing alternation of polyp (asexual) and medusa (sexual) stages. Obelia is a calyptoblastic hydroid; its polyps as well as its stems are protected by continuations of the perisarc.

Hydrozoan colony … Obelia

HYDROZOAN MEDUSA

HYDROZOAN COLONIES

A Portuguese man-of-war colony, Physalia physalis (order Siphonophora, class Hydrozoa). Each colony of medusa and polyp types is integrated to act as one individual. As many as 1000 zooids may be found in one colony. The nematocysts secrete a powerful neurotoxin.

By-the-Wind-SailorA Floating Colony of Polyps

These hydrozoans form calcareous skeletons that resemble true coral. A, Stylaster roseus (order Stylasterina) occurs commonly in caves and crevices in coral reefs. These fragile colonies branch in only a single plane and may be white, pink, purple, red, or red with white tips. B, Species of Millepora (order Milleporina) form branching or platelike colonies and often grow over the horny skeleton of gorgonians, as is shown here. They have a generous supply of powerful nematocysts that produce a burning sensation on human skin, justly earning the common name fire coral.

A B

Fire Corals• NOT TRUE CORAL - Hydrozoan colony• TRUE CORAL - Anthozoan colony

• life cycle with both polyps and medusae, but medusae dominate with polyp stage reduced or absent.

• polyp stage (scyphistoma) goes through strobilization (transvers fission ) to produce young medusa.

• bell margin lacks a velum(amemmbrane on the subumbrella surface of the jelly fish).

• tetramerous (= four-part) radial symmetry.

• gut divided into a complex system of radial canals.

• some with a simple single mouth, but many with thousands of microscopic “mouths” at the ends of oral arms.

• gonads endodermal (found in the gastrodermis).

• specialized sense organs called rhopalia(club-shaped sense organ) with ocelli & statocysts .

• includes some 200 marine species.

• "true" sea jellies.

Class Scyphozoa(Gr. skyphos, cup)

Sea Jellies

Life cycle of Aurelia, a marine scyphozoan medusa.

A- Thaumatoscyphus hexaradiatus (order Stauromedusae, class Scyphozoa). Members of this order are unusual scyphozoans in that the medusae are sessile and attached to seaweed or other objects.

B- Moon jellyfish Aurelia aurita (class Scyphozoa) is cosmopolitan in distribution. It feeds on planktonicorganisms caught in mucus on its umbrella.

A B

• polyps and medusae stages, but medusae dominate with polyp stage reduced.

• polyp stage develops directly into medusa.• bell margin with a velarium(shelf-like extension of

the sub umbrella edge that increase swimming efficiency).

• tetramerous (= four-part) radial symmetry; bell cube-shaped with tentacles arising from each corner.

• gonads endodermal (found in the gastrodermis).• specialized sense organs called rhopalia with ocelli

& statocysts .• includes some 15 marine species.• includes box jellies and sea wasps.

Class Cubozoa(Gr. kybos, a cube)

• lack medusa stage entirely (polyp forms only)• mouth with a tubular pharynx that projects

inward into the gut• large gut cavity divided by mesenteries that

radiate inwards from the body wall• hexamerous (6-part) or octamerous (8-part)

radial symmetry or biradial (modified radial symmetry that limits the number of planes that can divide the body into equal halves

• includes sea anemones, “true” stony corals, sea fans, sea pens, organ pipe coral, precious black coral.

• Sea anemones usually move by gliding slowly along on their pedal discs.

(Gr. anthos, flower)Class Anthozoa

sea pens

sea fans

sea fans

sea anemonesprecious black coral

Sea anemone attach to rocks using their pedal disc . usually move by gliding slowly along on their pedal discs.Feed on fish or any other food of suitable size.

MUTUALISMS Sea anemones sometimes harbor zooxanthellae

(photosynthetic protists) like hard corals do.  The algae gives off oxygen and other nutrients that the coral polyp

needs to live and in return the polyp gives the algae carbon dioxide and other substances the algae needs. That is why coral reefs grow so near the surface of the water where it is the sunniest--the algae need sunshine for photosynthesis.

Some crabs will decorate their shells with anemones.

MUTUALISMS

Some damselfish (anemone fishes) form associations with large anemones. In a symbiotic mutualistic relationship, the clownfish

 feeds on small invertebrates that otherwise have potential to harm the sea anemone, and the fecal matter from the clownfish provides nutrients to the sea anemone.

The clownfish is additionally protected from predators by the anemone's stinging cells, to which the clownfish is immune.

CLASS ANTHOZOA – HARD CORAL

Hexacorallia – 6-fold symmetry

Precipitate Ca3(CO3)2

from sea water to produce skeletal structures that become coral reefs

Contain zooxanthella

CLASS ANTHOZOA – SOFT CORALS

Octocorallia – 8-fold symmetry

Form complex tube-like skeletal structures

Lack zooxanthella

CLASS ANTHOZOA – SOFT CORALS

Class Anthozoa – Soft Corals

REEFS

Shallow tropical waters Hermatypic corals (Class Anthozoa,

subclass hexacorallia, order scleractinia) Contain symbiotic dinoflagellates Precipitate CaCO3 to form “exoskeleton”

ZOOANTHELLA

Green algae

ATOLL