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Molluscs

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clam
The spectacular colours of a giant clam

The Great Barrier Reef is home to between 5000 and 8000 molluscs, a significant proportion of all the molluscs found throughout the world.

The greatest number of species is found in shallow parts of the Reef. Although most of the molluscs on the Reef are widely distributed throughout the seas of the tropical Indo-Pacific region, a number of bivalves and other shellfish are restricted to the southern end of the Reef and northern New South Wales.

Molluscs are soft-bodied animals and most have a hard shell for protection. Coral reef shells show a huge array of shapes, colours and patterns. Some molluscs, such as sea slugs (nudibranchs) do not have a shell, but use poisons for defence. The more familiar molluscs include snails and slugs (gastropods), oysters and mussels (bivalves), and octopuses and squids (cephalopods).

Mollusc physiology

Although molluscs show an incredible range of shapes and forms, they all share the same basic body plan: a head, and a body supported on a singular muscular 'foot' (so called because it is generally used for movement).

Despite differences in appearance there is a basic plan to the bodies of all molluscs. They are generally made up of three regions: a head, a muscular 'foot' and a mass of internal organs covered by a skin-like membrane called the mantle. The mantle secretes the shell. The shell is generally the most conspicuous part of a mollusc but it is sometimes hidden within the body (as in the squid and cuttlefish) or it is absent altogether (for example, slugs and octopuses).

All aquatic molluscs absorb oxygen from the water by means of feathery gills which sit in a cavity created by the folded mantle tissue.

The main function of a hard shell is to provide its owner with protection against predators and to prevent drying out in intertidal and terrestrial environments. Those molluscs without shells have other ways of defending themselves.

Molluscs without shells

Octopus, squid, cuttlefish and nudibranchs (sea slugs) are examples of shell-less molluscs. Their bodies are supported by internal muscles and the water that surrounds them. Without the protection of a hard shell these animals must rely on other defence mechanisms. Many warn off predators by being brightly coloured and unpleasant tasting. In fact, nudibranchs are among the most colourful and patterned marine animals. They also secrete a nasty-tasting acidic fluid when attacked.

Many octopus, squid and cuttlefish (cephalopods) defend themselves by rapid colour changes or by spraying ink or a jet of water at an aggressor. For example, the Australian giant cuttlefish emits a cloud of black ink behind which it hides or flees to safety if it is attacked.

Bivalves

Bivalves are the only group of molluscs that do not have some form of head. They are also the only group of molluscs that lack a radula, a special feeding organ. This is a ribbon-like structure with regular rows of tiny, horny teeth which lies on the floor of the mouth and throat. The number of teeth varies from a few to many thousands, according to the type of mollusc. The radula is pulled in and out of the mouth with a rasping motion, enabling the mollusc to feed on fragments of animal and plant tissue.

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