shrimps, prawns, crays and crabs

The Decapoda are a well known order comprising shrimps, prawns, crayfish and crabs. The terms 'shrimp' and 'prawn' tend to be used interchangeably. Some species of the larger crays have their own common names, e. g., marron, yabbie, gilgie, koonac. Several species are of economic importance either in fisheries or for their destructive burrowing habits.

In all Decapoda a very prominent carapace encloses the head and thorax, forming a cephalothorax. The name "Decopoda" refers to the five pairs of walking legs. There are as well two pairs of antennae -- the first with two very long rami, the second with one long and one very short ramus (except in crabs where all are short) -- three pairs of mouthpart appendages, five pairs of abdominal 'swimming' legs and a pair of uropods held as a tail fan (again, except in the crabs).

There are three freshwater families of shrimps/ prawns/ crays. In Atyidae the first two pairs of legs are similar in size and tipped with tufts of hairs. Atyids feed by scraping algae and organic detritus with these legs and sweeping it forward to the mouth. Palaemonidae are the long-armed shrimps, in which the second walking legs are longer than the first and are used for grasping prey. Family Parastacidae comprises the crays, etc. In this family the first three pairs of walking legs are clawed and of the first are exceptionally robust.

Crabs have a very broad and strongly build carapace, and the abdomen is reduced in size and permanently curled forward under the thorax. Three families are included in this key. The small spider crabs (Hymenosomatidae) are a marine family uncommonly found in waterbodies without direct sea connection in southern Australia. Sundathelphusidae are the true freshwater crabs of tropical rivers and lakes. The shore crabs (Grapsidae) are the common crabs of seashores, dune slack-pools, mangroves, etc. They can be distinguished from freshwater crabs by the toothed margins of the carapace.