Crustaceans

The largest group of marine arthropods are the crustaceans. Some of the crustaceans include a large range of shrimps, water fleas, barnacles, copepods, fish lice, ostracods, sandfleas, beach hoppers, isopods, amphipods, pill bugs, slaters, a range of prawns and shrimp, freshwater yabbies, rock lobsters, hermit crabs, half crabs, spider crabs, swimming crabs, normal crabs and shore crabs. These prawn, lobster and crab-like crustaceans form a class called Malocostraca.

.Some researchers consider that the group Crustacea is so important that it should have its own phylum, called Crustacea.

The name Crustacea comes from their hard "crusty" outer shell.

Crustaceans vary so much in shape, form and behaviour that it is a difficult task to create a definition which lists traits that are common to all of them. This is especially so with the barnacles, which look like no other crustacean in the adult stage. It is the juvenile, planktonic stage which tells that it is a crustacean.

All crustaceans have:

• Two pairs of antennae at some stage in their life cycle.
• The body is divided into three parts, a head, thorax and abdomen.
• There may be between sixteen and sixty body segments. Although the segments look like rings, the outer shell is continuous. The hard tube-like parts are hardened by deposits of calcium and protein. At the joints the shell is thin and flexible, so that the body may bend.
• The Head carries pairs of appendages, usually the antennae, used for sensing its environment and for feeding. The head also carries three pair of appendages that are modified into mouthparts.
• Most crustaceans have a carapace, or shell, which folds outwards from the upper surface of the body and covers all or part of the thorax.
• The thorax and abdomen have pairs of appendages used for movement such as walking, swimming, climbing. In some cases these appendages are used for respiration and mating.
• In advanced crustaceans, such as the prawns, lobsters and crabs, three pair of thoracic appendages help the head mouthparts to handle food.

In primitive crustaceans the appendages have two branches, called rami. This is called the biramous condition.

Advanced crustaceans have single appendages, and this is called the uniramous condition. This s better for walking and climbing.

The ten-legged crabs (decapods) appear to be the most advanced group, having many forms and lifestyles.

References

Bennett, I. (1987) W.J. Dakin's classic study: Australian Seashores. Angus & Robertson, Sydney.

Davey, K. (1998) A Photographic Guide to Seashore Life of Australia. New Holland, Sydney.

Edgar, G. J. (1997) Australian Marine Life: the plants and animals of temperate waters. Reed Books, Kew.

Jones, D. & Morgan, G. (1994) A Field Guide to Crustaceans of Australian Waters. Reed, Chatswood.

Underwood, A. J. & Chapman, M. G. (1993) Seashores: a beachcomber's guide. New South Wales University Press, Sydney.

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