The anomurans are the nearest relatives of the Brach-yura, the true crabs, and include the hermit crabs (Paguroidea), squat lobsters and porcelain crabs (Galatheoidea) and mole crabs (Hippoidea). Unifying features of the Anomura include the very small, reduced last walking legs, and presence of an uncalci-fied groove along the sides of the carapace (the linea anomurica). The last pair of walking legs is so reduced, that at first sight, most anomurans appear to have only six instead of eight walking legs. The last pair of walking legs are usually folded under the abdomen or edges of the carapace, and used for grooming. Anomu-rans are abundant on the GBR, with the most obvious being the hermit crabs. Most hermit crabs have a soft, vulnerable abdomen, and usually use a gastropod shell for protection. Hermit crabs (Fig. 23.3G) are especially well adapted to occupying gastropod shells; the asymmetrical, coiled abdomen and asymmetry of walking legs and claws enable the animal to fit snugly inside. Species range from less than 10 mm to almost 300 mm in length. Common hermit crabs on the GBR fall into one of three major groups: the coenobitids, or terrestrial hermit crabs; the diogenids (the left handed hermit crabs), and pagurids (the right handed hermit crabs). Coenobitids, represented by Coenobita on the GBR, live on beaches above the tide line where they forage among flotsam and jetsam. To avoid desiccation, Coenobita carries water in its gastropod home and shelters in the shade or under driftwood during the hottest part of the day. Though they spend their entire adult life out of water, coenobitids must return to the sea to breed. Diogenids, with the left cheliped generally larger than the right, and the pagurids, with the opposite pattern, are common on all parts of the reef, from the intertidal beach rock to the reef flat, slope, and inter-reef areas. Hermit crabs are generally scavengers or herbivores, and large numbers can often be seen at night foraging on the reef flat or on the reef slope.
Other anomurans, such as the colourful squat lobsters and porcelain crabs, are common in most parts of the reef. Squat lobsters (Galatheidae) resemble a small, flattened lobster with numerous transverse groves on the carapace, the tail tucked under the body and cheli-peds pointing forwards. Common galatheids include species of Munida that live in deeper, inter-reefal waters, species of Galathea, that live among corals, on bry-ozoans and sponges, and the conspicuously striped Allogalathea elegans (Fig. 23.3H) that lives in pairs on cri-noids. Porcelain crabs (Porcellanidae) have a short, smooth or spiny carapace and chelipeds protruding sideways. Common porcellanid genera include the free living Petrolisthes and Polyonyx (Fig. 23.3I), and anemone associated Neopetrolisthes. Not more than 30 species each of Porcellanidae and Galatheidae are presently known from GBR waters.
The mole crabs, Hippoidea, are highly specialised for burrowing in sand. Their elongate bodies are flattened or oval in shape, and the legs are flattened into digging spades. They can burrow extremely rapidly and are difficult to detect, let alone capture. Most hip-poids live in shallow water, but some live at depths beyond 200 m.
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