The anomurans are superficially crablike, holding the abdomen beneath the body and most often possessing large chelae (pincers) on the first legs. However, unlike true crabs, the anomuran abdomen still possesses uropods on either side of the telson (forming a visible tailfan), and is used either for swimming (galatheoids), burrowing (hippoids), or holding the body inside of a gastropod shell (paguroids). The last pair of legs are reduced and often held out of sight under the carapace giving the impression of four pairs of thoracic legs instead of five.
The land hermits, called soldier crabs in the BVI, can be found several kilometers from the sea, only returning to the shore periodically to release their eggs. At this time they may find new gastropod shells to carry back inland with them. Reduced gills and highly vascular branchial cavities act as lungs, freeing these crabs from the marine environment.
Coenobitids, like their close relatives, the diogenids, have a much larger cheliped on the left side. In the land hermits, this chela, along with the left second walking leg, forms an operculum that seals the opening of the shell when the crab retracts. This protects the crab from predators, and from drying out.
The only species found in the Caribbean, Coenobita clypeatus, can reach the size of a baseball. Larger individuals are most often found inhabiting the shells of the West Indian topshell, Cittarium pica, while smaller individuals are commonly found in the green star shell Astraea tuber, as well as in top shells and a wide variety of other shells.
Hermit crabs in the family Diogenidae may have the larger (major) cheliped on either side, or the chelae may be equal. In the Caribbean, the larger species (species of Petrochirus and Dardanus) belong to this family.
The pagurids generally have the larger chela on the right side. This can range from equal-sized chelae to a large, circular opercula-like chela on the right side.
Gallery 2 Family Galatheidae (Squat Lobsters) Galatheids appear as tail-less lobsters, having large chelipeds, a somewhat elongate carapace, and a weak abdomen held under the body. The abdomen is used for escape swimming. They are most often found on the bottom. Family Porcellanidae (Porcelain crabs) Gallery 2 Porcellanids share the same body plan as Galatheids, but they are more compact and flattened for living under and between rocks. Their large chelae are used for territorial disputes and not for feeding (which is accomplished by combing particles out of the water using long setae on the mouth parts). Porcelain crabs can be distinguished from true crabs by the apparent number of walking legs (three instead of 4 pair), the apparent lack of a wrist (carpal) segment on the chelipeds, and long antennae originating on the front outside of the eye stalks. The most obvious species in the Caribbean are Petrolisthes quadratus, found in large numbers under rocks in the intertidal, and the red and white polka dotted Porcellana sayana, which lives commensally within the shells inhabited by large hermit crabs.
Superfamily Hippoidea (sand crabs and mole crabs) Gallery 2 The hippidoids consist of two families (Albuneidae and Hippidae). Both live in sandy habitats close to shore and are most often found in the swash zone of beaches where they move up and down the beach with the tide. Their spike-like telson and specialized legs allow them to quickly burrow by literally swimming backwards into the sand. The antennae have specialized setae that form either a water channel for breathing or combs for feeding.