Brachyurans are the most advanced of the decapods in that they have the body most modified from the primitive shrimp-like decapod ancestor. The crab body is short, wide, and flat. The abdomen, once a muscular organ used for swimming, is now simply a flap used to cover reproductive appendages and hold eggs. The uropods, which along with the telson form the tailfan in other decapods, are totally absent in the crabs. In all but a few small groups all five pairs of walking legs are large. The first pair are always chelipeds. And the antennae and antennules are greatly reduced, and originate before the eyestalks (unlike anomurans such as porcelain crabs).
Families in this section are the most primitive crabs, having...
In the BVI these crabs are generally found carrying either a sponge (Cryptodromiopsis antillensis) or a bivalve shell (Hypoconcha spp.) covering their backs, which they hold in place using the fourth and fifth pair of legs.
The raninids, also called frog crabs, resemble the anomuran hippids by having an elongate body adapted to backing into soft sediment. Although they are generally found by dredging in deeper waters, we have found them in shallow water reef areas of the BVI living in patches of sand.
The box or shamefaced crabs have distinctive chelipeds adapted for peeling open snail shells. The chela also fit together in front of the crab's face to form a water channel (leading to openings in front of the chelipeds) for breathing when the crab buries itself in the sand.
The purse crabs get their name from the female's abdomen, which is rigid and cup shaped, completely enclosing the egg mass while brooding. The third maxillipeds tightly enclose the buchal cavity (mouth area), and are channeled to allow water to flow to and from the gill (branchial) cavity. The opening to these channels can be seen at the top of the mouthparts just below the small eyes. Species are either globular (resembling marbles with thin legs) or spectacularly eroded (resembling rubble).
These species are generally delicate, having very long thin legs. There are generally no orbital cups for the eyes to retract into, and the first segment (basal article) of the antenna is unusually long and thin. The common arrow crab, Stenorhynchus seticornis, is the most visible member of this family in the BVI. Other species are common, but they are smaller and camouflage themselves well by sticking bits of algae to specialized hooked setae on the legs and carapace.
Members of this family have a wide, square, produced front, followed by a rounded body with moderately short stout legs. The shape of the front is due mainly to the carapace features forming the orbits of the eyes. Members of the genus Pitho are commonly encountered in the shallow waters of the BVI.
These crabs have small eyes without true orbits, and a large beak-like rostrum. The body is smooth and usually angular in some way. The last three pair of legs are prehensile and may have a "thumb." These legs are much shorter than the second pair. In the BVI most species encountered have been found living in masses of green algae (mainly Halimeda), but also rarely in the brown alga Sargassum.
These crabs are usually more or less teardrop-shaped, with a small eye that can incompletely retract into an orbit formed by the postocular spine. The rostrum usually has two spines (except in Neodoclea). Legs are moderate to long. Some species can be moderately large, but the most common species in the BVI, Pelia mutica, is a smaller species (about 1-2 cm long) that hides during the day but can be seen at night on sponges and corals.
These crabs are distinguished by the flattened paddle-like dactyl of the last leg, which allows them to swim in open water. Often long carapace spines are present to protect these active crabs who are generally out in the open hunting. Members of the group (i.e blue crabs) are very important economically in some regions of the world. Many similar species are present in the BVI, occurring in nearly every habitat from the swash zone of sandy beaches to masses of floating Sargassum in open water. One species now found in the BVI, Charybdis helleri, is native to the Indo-west Pacific, but has made its way from the Red Sea through the Suez canal to the Mediterranean, and over to the Caribbean, possibly as larvae in ship's ballast water.
Pinnotherids are almost never found free living. They usually live commensally on or in the body, or in the burrow of another invertebrate species. Because of this their bodies are often pale and shaped to best suit their particular living situation.
Gecarcinids appear as over inflated ocypodids because they have inflated, highly vascular branchial cavities that act as lungs. Eyes are set close together on short stalks, and mature males of some species have a disproportionately large major chela. They are not fully terrestrial, in that they need to return to the sea to release their larvae. Three species of Gecarcinids are comon in the BVI. Cardisoma guanhumi, the giant land crab, is light in color (blue-gray to lavender), and is found in low lying wooded areas where it lives in deep burrows up to 20 cm in diam.Young of this species live just behind the beach and resemble ghost crabs. The other two species Gecarcinus quadratus, and G. ruricola are darker purple or red, and live inland. They can be found roaming after rains.
These crabs are flat with a square, or round carapace, the front is wide, and the large eyes are situated on short stalks and set far apart. All but a few grapsid species live in intertidal habitats, a notable exception are species of Percnon, which can be commonly found (but rarely caught) subtidally on rocks and coral heads in tropical reef areas. the most conspicuous grapsid in the BVI is the large Sally Lightfoot, Grapsus grapsus, usually seen gleaning algae off of rocks just above the waterline. Other grapsids can readily be found under rocks, on dock timbers, under washed up boards, and in mangroves.
Ocypodids resemble grapsids and gecarcinids in shape, but their eyes are set close together and at the end of long stalks. The ghost crab, Ocypode quadrata, is common on sandy beaches in the BVI, where it lives in burrows. Muddy areas are inhabited by species of Uca, the fiddler crabs whose males use a greatly inflated major chela to attract females. Uca burgersi inhabits the harsh salt ponds, and can be found in muddy back water areas as well.