Deorator Crab (Composcia retusa)

Representatives of the family Majidae are commonly called spider or Decorator Crabs.They have a distinctive triangular-shaped body which tapers towards the front, although the carapace is rounded in a few species. The world's largest crab, the Japanese spider crab, belongs to this family. Its carapace is 40 cm (16 in) wide, and its monstrous clawed walking legs are up to 2 m (6.5 in) in length. Decorator Crabs are master camouflage artists, using their claws to adorn themselves with bits and pieces of algae, sponges, and hydroids. The carapace is usually covered with spines or knobs, and is equipped with hooked hairs that are used to attach the various items. The camouflage is so effective that it is difficult to detect these creatures unless they are moving. Frequently only the black tear-shaped eyes are visible. The 6 cm wide (2.5 in) species shown here is one of the most artistic members of the family, choosing several species and colors for its decoration. It occurs to depths of 10-15 m (33-50 ft).

Harlequin Shrimp (Hymenocera picta)

Harlequin Shrimps live a relatively reclusive life style, hiding among reef crevices. Their highly distinctive color pattern, flag-like antennae, and broad extensions on the ends of the first walking legs, which bear the pincers, make them easy to identify. This spectacular shrimp usually occurs in pairs, and males are almost always smaller than females. The genus Hymenocera was formerly believed to consist of two species, Hymenocera picta form the Pacific and Hymenocera elegans form Indian Ocean. However, the two are now considered to represent a single species, which varies in color depending on locality. Animals from the Pacific tend to have brighter red markings. The distributional range extends from Panama to East Africa and Red Sea. Surprisingly, this 5 cm-long (2 in) shrimp feeds exclusively on several species of starfish, including the crown-of-thorns. It eats the starting from the tip of the leg and works towards the central disk, keeping its victim alive and fresh as long as possible. 

Blue-spotted Hermit Crab (Dardanus guttatus)

The Hermit Crab's practice of using a dead using a dead mollusc shell as a portable home is well known. They are often seen lugging their cumbersome shells in broad daylight, unlike most crabs, which avoid daytime fish predators by being active only at night. Furthermore, they are not vulnerable when shedding their hard outer covering (moulting), a typical crab trait during periods of growth. The Hermit Crab simply finds a larger shell and moves into its new quarters. If you attempt to dislodge a hermit crab form its shell, you will find it an impossible task, without physically damaging the animal. After moving into a new shall, the crab's flexible abdomen adopts a twisted shape that perfectly follows the bend of the shell. It can constrict or expand the abdominal muscles so that the body presses tightly against the shell and fills every empty space. Most common Hermit Crabs belong to the family Diogenidae. They occur from the shallow intertidal zone down to depths of at least 100 m (330 ft).

Coral Crab (Trapezia rufopunctata)

This unmistakable crab is easy to find if you are familiar with its preferred habitat. Coral Crabs live exclusively among the branches of pocillopora corals, and wedge themselves so tightly among the branches that the only way to remove them is to remove the entire coral head from the water. Their characteristic pattern, red spots on a white or pink background, is similar to that of two small fishes (a blenny and scorpionfish), which also inhabit the same coral. The family Trapeziidae is one of the easiest crab groups to recognize, as the members have a squarish carapace, and relatively long, flattened clawed legs (chelipeds). They are true crabs (brachyurans) as evidenced by the shortened antennae, and a very short abdomen which is folded under the body. Although many species exhibit considerable variation in color pattern, many are entirely or at least partly red. All crustaceans have jointed limbs, which are readily regenerated if broken or self amputated when escaping a predator.

Soldier Crab (Mictyris longicarpus)

Soldier Crabs are named for their blue and grey military colors, and habit of moving in mass information over sandy beaches. Unlike most crabs that walk sideways, Soldier Crabs often walk forward. All are bright blue to blue-grey on the back with contrasting pale walking legs the color varies depending on the species. The body shape is nearly spherical and the walking legs are positioned underneath the body, tending to lift the animal well above the ground. The characteristic clawed appendages are thrust upward into a prominent position if the crab is threatened. Soldier Crabs also have a large pair of flattened appendages near the mouth, know as the third maxillipeds, which assist in tearing up and manipulating food. An extensive sand flat, periodically inundated by the rising tides is its preferred habitat. At low tide they emerge form the beach in large numbers, to feed on detritus. When the tide rises, or if they are disturbed, the crabs burrow into the sand in a peculiar, cork-screw fashion.

Mantis Shrimp (Odontodactylus scyallarus)

Mantis Shrimps are descended form an ancient lineage, having split away mainstream crustacean evolution some 400 million years ago. Individuals often grow as large as 30 cm (12 in). This colorful creature is often seen shuffling across the bottom, or with its head stuck out from its burrow entrance. Photographers can trick the creature into posing, by placing small stones in the burrow. the animal promptly emerges and clears away the unwanted debris. Mantis Shrimps are voracious predators, feeding mainly at night on fishes and other crustaceans, molluscs and worms. The victim's shell or body is crushed with the expanded claws on their second pair of legs. Captive specimens can break the glass of an aquarium with these powerful pincers. Needless to say, these animals should not be handled! Odontodactylus is easily distinguished by its bright green body color, blue head, and reddish antennae and thoracic limbs. Its preferred habitat is sandy or rubble bottoms down to at least 70 m (230 ft).

Swimmer Crab (Charybdis)

Swimmer Crabs have flattened, paddle-like rear legs used for swimming or burrowing in the sand. Most species are nocturnal predators of molluscs, crustaceans and occasional fishes, and live on the bottom. Some, which attain a large size, are renowned for their edibility; in fact the family includes most of the crabs sold in markets and restaurants. Swimmers are classified as true crabs (brachyarans) as the first pair of legs are clawed and the last pair are not much smaller than the others. In addition, the antennae are very short. A conspicuous feature that separates them from the other major group of crabs (the anomurans) is the shape of the abdomen very short, flat, and folded under the body. In contrast, many anomurans have a lobster-like abdomen, although some species hold it under the body as do the true crabs. All crabs lay huge number of eggs, which are carried on the paired appendages (pleopods) between the thorax and the abdomen.