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Crustaceans are a subphylum of a vast army of animals with jointed appendages in the phylum Arthropoda. Within this phylum is the order Decapoda, which includes almost all of the larger crustaceans. They usually have five pairs of legs serving diverse functions, claw-bearing limbs (chelipeds), two pairs of antennae, and a number of small appendages around the mouth that have various feeding functions. This group is further subdivided into two infraorders of crabs. The hermit crab families belong to Anomura, the irregular tails. The true crabs are members of the other infraorder, Brachyura, the short tails. Anomura Hermit crab tails are asymmetrical and usually coiled into empty snail shells. The last pair of legs are much reduced and often point upward to help the animal attach to the shell. The tails are shorter than lobsters and longer than the true crabs. The first pair of legs is enlarged and bears strong pincers, often of unequal size. Family Diogenidae The tail is protected within its shell and the entire animal can completely withdraw into it, and even seal the shells entrance with oversized claws. When the shell is outgrown, the crab will seek a larger one. If it is occupied the hermit crab will tap rapidly shell on shell, and a ritualized fight will ensue. These animals have five pair of legs evolved to fit all shapes and sizes of shells. Only the second and third pairs of legs are used for walking. The first pair bears claws (chelipeds) and the fourth pair has developed to grasp the edge of the shell when the animal extends forward. The tiny fifth pair helps to wedge the coiled tail into the inner part of the shell. Eyes are placed on stalks, which allow them to see above the shells edge. Hairy Yellow Hermit Crab This is a huge, magnificent, hairy, colorful crab. Its legs are golden or orange-yellow trimmed in crimson and ringed with white scales, giving the legs its segmented look. Dense tufts of long bristles grow from under these scales. It is found under ledges or in caves at 50 or more feet. The large Triton's Trumpet shell is its shell of choice. The legs can grip the substrate with such tenacity that it is impossible to pick them up for a better look. Jeweled Anemone Crab The Jeweled Anemone Crab is brown to purple-orange with red eyestalks surrounded with white bands like a barber's pole. On top of the eyestalk are place a pair of greenish or silver eyes. The surface of its left claw is covered with small rounded bumps (tubercles). This nocturnal animal camouflages itself by carrying anemones around attached to its shell. In Hawaii it usually uses only two species on anemones. Up to six large Calliactus polypus will completely cover the shell. Tiny white anemones of the genus Anthothoe are usually found on the upper lip of the shell directly above the crab. They have a mutual relationship. They help to protect the crab by providing camouflage and receive mobility and food in return. Brachyura These are the true crabs, the short tails. They have four pair of walking legs and one pair of pincer-bearing chelipeds. Most move in a sideways fashion. They have also evolved a broad, flat carapace. Their tails are tucked beneath their bodies. Males have narrow, pointed tails. Female tails are broader and can hold her eggs. Family Dromiidae These are the slow moving sponge crabs, the most primitive of the true crabs. They cut sponges from the reef to carry them around on their backs for camouflage. These sponges are held in place by the last two hindmost legs. They are tipped with short needle-like pincers and are permanently bent up over the carapace. These animals will use bivalve shells, sea stars or colonial tunicates in areas where sponges are not found. Sleepy Sponge Crab This slow moving, awkward Sleepy Sponge Crab is the worlds largest sponge crab. Its carapace and limbs are covered with brown, muddy-looking fuzz. It has massive claws with white tips and small beady eyes. This is a nocturnal animal that lurks under crevices and in caves during the day. When sponges are hard to find, it will substitute wood, rope, metal, even old tennis shoes. This animal always strikes me as having a permanent nasty temper. It certainly doesn't like being disturbed. Family Portunidae These are the swimming crabs, swift and aggressive predators that hunt by night using long and powerful pincers. By day they are usually found hiding in crevices or caves. The last two segments of the fifth pair of legs are usually flattened into swimming paddles. Hawaiian Swimming Crab This is Hawaii's most common swimming crab. By day this nocturnal animal is usually found hiding in crevices or between branches of coral (Pocillopora). Its carapace and legs are a patchy-orange and red with white spots ad marks. The swimming legs, the fifth pair, are yellowish. It has white-tipped claws with a broad black band and finely grooved pincers. Its beautiful eyes have blue and black vertical stripes. Rainbow Swimming Crab This is a beautiful, multi-colored swimming crab with blue marks on a yellow-orange carapace and legs. It has large claw-bearing limbs with numerous spines. It is found resting in caves, usually on ledges near the ceiling. Red Swimming Crab This animal is similar to the Hawaiian Swimming Crab in size, shape and habits. It is almost all bright red with black claw tips and black, non-striped eyes. Family Xanthidae Members of this family are sometimes called stone crabs, dark-fingered crabs or mud crabs. This is the largest crab family and is represented by more than one hundred species in Hawaiian waters. Their carapaces are almost oval. The tips of their claws are usually dark and spoon like. The smaller species are slow and not likely to pinch, but one should be careful handling the larger ones. Seven-eleven Crab This is probably the best-known xanthid due to its large size and unmistakable pattern. It is slow moving and found mostly at night, although all the photos I have taken were shot during day dives. It has massive claw-bearing limbs, which are of unequal size. The shell is smooth and heavy. There a seven prominent red spots. Two are located near each eye, three are in the center of the carapace and four, counting the eyespots, are found along the front and rear margins. This makes a total of eleven. |
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