The Astacidea is the large group containing the marine clawed lobsters and freshwater crayfish. The most obvious feature of the astacideans, differentiating them from the palinurid lobsters, is the possession of at least one pair of enlarged, powerful chelipeds. In almost all cases, the large, first pair of chelipeds is followed by two pairs of smaller chelipeds. Freshwater crayfish, such as the yabby and marron (Cherax) are common on the mainland, and marine counterparts, such as scampi (Metanephrops, Nephropsis and Thaumastochelopsis) occur in deep, outer shelf waters. However, only a few species of astacideans live on the reef itself: the dwarf reef lobsters of the genus Enoplometopus (Fig. 23.3D) and family Enoplometopidae. Enoplometopidae, containing about 20 species worldwide, possibly represents the last remnant of a lineage of lobsters that otherwise went extinct in the Lower Jurassic Period. Species of Enoplometopus do not exceed 20 cm in length (more often less than 10 cm) but have vivid orange, red and purple colouration. Enoplometopids are very distinctive, but they are seldom seen because of their shy, nocturnal habit and lower reef slope habitat.
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