- Cnidaria -


The Cnidaria include a wide variety of familiar organisms

such as the corals, anemones and jellyfish. A generalized

cnidarian body plan is radially symmetrical and consists

of a ring of tentacles surrounding a central mouth. The

mouth leads to a gastrovascular cavity, the only opening

to the gut. The gastrovascular cavity’s function includes

both digestion and circulation. This structure is repeated

over a diverse range of organisms across two body forms:

the polyp and medusa. Polyps are familiar as the sessile

animals that live attached to rocks, like anemones, or

form large colonies, such as corals. Jellyfish demonstrate

the other body form as free living medusae. Some cnidarians,

such as certain colonial hydrozoans, have both polyp

and medusa stages during their life cycle.

Cnidarians possess stinging cells called cnidocytes. They

are located throughout the epidermis but are most abundant

in the tentacles. They contain hollow coiled threads

called nematocysts. The cnidocytes are sensitive to chemical

stimulus or mechanical pressure (or a combination)

and eject when another organism, enemy or prey, is close.

When discharged the nematocysts eject and entangle or

pierce their prey, immobilizing it by injecting toxins.

There are several forms of nematocyst and they often have

barbs to ensure they pierce the epidermis of their target.

The effectiveness of the stinging cells is demonstrated by

ROV observations showing that deep-sea anemones can

catch fish almost as large as the anemone itself. If seabed

videos of anemones are viewed at increased speed

they can often be seen drawing their arms through their

mouths to remove and digest tiny food particles.

At some of the sites off Tanzania, most notably Pweza-3,

areas of hard substratum dramatically increased the

available habitat for anemones and corals. Other groups,

such as seapens and burrowing anemones, inhabit soft

sediment and were more common at the deeper sites

like Mzia, Lavani and Zafarani. Some groups, such as

the zoanthids, make use of the hard substratum available

on other living organisms and encrust structures such as

sponge stalks and hermit crab shells.

Benthocodon sp.

Actinernus sp.

Poralia rufescens

Paragorgia sp.


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