True crabs belong to the infraorder Brachyura. Other crustaceans that look like crabs but aren't are anomurans but these have only three obvious pairs of walking legs and a more mobile abdomen. Brachyurans can be divided into as many as 50 families.

Zoologists classify crabs as a Crustacean: no bones, having many jointed legs.

Crabs have five pairs of jointed legs.

Crabs were first believed to originate 550 million years ago; at the same time as the well-known multi-jointed trilobite which is related to the horseshoe crab, alive today. The largest crustacean in the world is the Giant Spider Crab of Japan; it can measured 12 feet (3.7 meters) long between its outstretched claws.

The smallest crab in the world is the Pea Crab (1/4 inch wide or 5mm) which lives inside a live oyster shell.

Crabs have three jaws.

The Sponge Crab uses live sponge to camouflage itself for protection from enemies.

Decorator Crabs cover their shell (carapace) with living organisms, usually algae, to mask their outline and become invisible when motionless. They may also use small stones or broken shells. It uses numerous hooked hairs to attach the objects to its shell and legs.

Hermit Crabs switch to bigger abandon shells as they grow larger.

Crabs can regenerate legs or claws when they are lost. Male Fiddler Crabs use a king of "sign language" to signal amorous intent to females by waving his large claw in a distinct pattern.

Crabs live in the ocean, on land and in trees. Ghost Crabs are white in color and come out at night. Crabs are fun, enlightening, exciting, interesting and educational.

Sally Lightfoot Crab Scientific name: Percnon gibbesi This flat crab has a brown or maroon-colored body and legs, with yellowish spots on the legs. It uses its small claws to pick algae film off of hard substrates. It is sometimes used to help control algae growth in the aquarium. (Source: Scott Michael, author of Reef Fishes, Microcosm Press) Degree of difficulty: beginner to advanced Maximum size: carapace (shell) 1.5 inches long Compatibility: More than one crab can be kept in the same aquarium. Juveniles are not a threat to fishes, but adults may eat small sleeping fish. It should not be kept with crustacean-feeding fish, such as groupers, snappers, hawkfish, larger wrasses, triggerfish, pufferfish, or porcupinefish. Food: omnivorous; feeds mainly on algae Habitat: This species is usually found in intertidal areas and often lives among the spines of sea urchins. Red Crab Scientific name: Uca sp. Given the proper environment, these crabs make an interesting display animal. They should have a deep substrate in which to dig their burrows. They are of little threat to most fish and invertebrates, but be careful with very small gobies and blennies, as these are in the most danger from the red crab. Males will have one very large, exaggerated claw, and females will have two small claws. These crabs will accept almost any food. The tank should have a tight top, as they can crawl out of aquariums. (Source: Mitch Carl, aquarist, Henry Doorly Zoo, Omaha, Nebraska) Degree of difficulty: relatively easy Maximum size: 1 to 3 inches long Compatibility: generally nonaggressive Food: herbivorous; will accept most frozen foods Habitat: The tank should provide equal areas of water and land, each with a deep substrate for the crab to dig in. Anemone Crab Neopetrolisthes ohshimai The anemone crab is a small, colorful crab with a porcelain-like shell. Like the clownfish, this crab has developed an immunity to anemone stings. This crab is usually found within the stinging tentacles of a number of anemone species where it uses its large well-developed claws to keep clownfishes from stealing its home. Anemone Hermit Crabs typically are found on coral reefs and sand flats. Average adult size: Adults can grow to be 5 inches across. Natural history: Anemone hermit crabs probably are the most common species of the hermit crabs. Their eyes are green, perched on red and white striped eye stalks. Hermit crabs have soft abdomens, so they keep their back half tucked away safely in a shell for protection. This species, Dardanus, typically has small sea anemones attached to its shell. This is one of the ocean's classic symbiotic relationships. A relationship in which both partners benefits. The hermit crab gains camouflage and protection. The anemone's stinging tentacles keep the hermit crab from becoming an easy lunch for a hungry fish. And, when the hermit crab does not move it blends in easily with the reef. The anemones benefit by being taken to many different locations to feed as the crab moves around the reef. Range: From South Africa north to the Society Islands. Crabs Of Australia Crustacean List By Common Name Common names of species described and pictured in this web site are listed below. To go directly to a particular animal in this list click on its name. If you prefer to look at the image gallery of a group click on a heading. Decorator Crabs This small cryptic-looking "decorator crab" appears to have mastered the art of disguise by attaching various anemones and coral polyps to its exoskeleton. Notice its greenish-colored eye at the end of its eye stalk (view of the crab's left side). A slow mover, even when approached, it seems to have complete confidence in its ability to remain inconspicuous. This particular specimen is about 1 inch in height. The alternate black and white bands on its legs distinguish this decorator crab from other, similar species. M WELCOME SPECIAL MEMBERSHIP Andre's Aquarium Club Ask all your aquarium related questions General page on crabs Crab_Species_Facts_and_Information

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Hermit Crab Scientific name: Dardanus sp. Although interesting to observe and useful as scavengers, larger hermit crabs are too destructive to be kept in a community aquarium. Some of the smaller varieties are not as great a threat to tankmates. Hermit crabs spend most of their time foraging and are often more active at night. (Source: Scott Michael, author of Reef Fishes, Microcosm Press) Degree of difficulty: relatively easy Maximum size: 1.5 to 6 inches long, depending on the species Compatibility: More than one hermit crab can be kept in the same aquarium, but larger individuals may prey on smaller ones. Larger hermit crabs are very destructive, feeding on other invertebrates and small sleeping fish. Smaller specimens should not be kept with crustacean-feeding fish, such as groupers, snappers, or hawkfish. All are potential prey for larger wrasses, triggerfish, pufferfish, and porcupine fish. Food: omnivorous; its diet should be varied and include plant material Habitat: These crabs can live on sand or hard bottoms. Provide them with extra shells to move into as they grow. Properly chosen hermit crabs that eat algae and are reef safe have no negative impact on a reef system. In fact, they are solely beneficial. The smaller species are able to get into tiny cracks and crevices where algae is growing that larger hermits cannot. They can also access spaces under rocks and corals where detritus or debris accumulates to remove it. Arrow Crab Scientific name: Stenorhynchus seticornis This species has extremely long legs and a triangular-shaped body with a long rostrum (spiny tentacle). The claws are also very long and often have purple tips. The arrow crab is golden brown, with fine dark lines. It is a short-lived species, both in the wild and in the aquarium (where it usually lives for no more than one year). (Source: Scott Michael, author of Reef Fishes, Microcosm Press) Degree of difficulty: relatively easy Maximum size: carapace (shell) 2.5 inches long Compatibility: Keep only one per tank or a mated pair. It should not be kept with crustacean-feeding fish, such as groupers, snappers, larger wrasses, triggerfish, pufferfish, or porcupinefish. Food: omnivorous; its diet should be varied and include plant material Habitat: This species is often found living among the tentacles of the condylactis anemone in the wild but does not require an anemone in captivity. Purple Shore Crab Hemigrapsus nudus The purple shore crab is commonly found on the open rocky seashores of the pacific coast of North America. It has a purple and red shell with a white underbelly, and grows to about 2 inches in length. This small crab can be seen scavenging the seashore where it feeds on algae and dead animal matter. www.beachwatchers.wsu.edu EMERALD CRAB Add 1 small per 20 gallons of water The Emerald Crab is a recent introduction into the saltwater tank and a long awaited one. This bright green crab loves the reef atmosphere and will spend its day hiding and crawling among the rocks while it feeds on algae. The reason this crab is such a fine addition is because of its appetite for Green Bubble algae which can grow in either new or established tanks. Their claws are designed specifically to pull at algae. These creatures are aggressive algae eaters and are perfect for the reef tank with their docile demeanor. This species is a relatively new introduction and shouldn't harm any other inverts or corals in the tank as they concentrate on feeding on the algae in the tank. SPONGE CRAB Sponge crabs are able to carry a piece of living sponge on their back by holding it in position with their last two pairs of legs. These legs are smaller, directed more or less upwards and bear small pincers. Those crabs deprived of their personal sponge caps will go to great lengths to retrieve them. If all else fails they will cut and shape new caps. The Halloween crab The Halloween Hermit Crab originating from the Hawaiian Islands, is bright orange with red banding. They are a very colorful addition to the marine aquarium, and they aid in eating any left over food that may settle on the substrate. These crabs live in abandoned snail or triton shells, or it may resort to attacking and eating a snail so it can use the shell for its home. The Halloween Hermit Crab is a valuable addition to an aquarium because it will eat algae including green hair algae and cyanobacteria. In addition, it provides valuable aeration by sifting through the sand. These Hermit Crabs can become destructive in a reef aquarium when they attain a large size. If there is an insufficient amount of algae present, the Halloween Hermit Crab will need to be fed dried seaweed, as well as meaty items such as mysis shrimp.

BIOLOGY OF CRABS

Although crabs come in a variety of shapes and sizes they all have the same general body plan. All crabs have one pair of chelipeds and four pairs of walking legs. Also referred to as claws, nippers or pincers, the chelipeds are the first pair of legs on a crab and their most distinguishing structure. Chelipeds are used for holding and carrying food, digging, cracking open shells and warning off would be attackers.