Physical characteristics

In addition to having five pairs of legs, decapods are distinguished by the carapace that covers the dorsal portion of the head and thorax, forming a single functional unit termed the cephalothorax. The sides of the carapace extend downward to envelop the gills, forming lateral branchial chambers. Although the vast majority of decapods are aquatic and breathe with gills, some terrestrial forms have developed blood vessels in the inner surface of the branchial chambers so that they function as lungs.

Decapods have two pairs of antennae, as do all crustaceans. The first pair typically bears special chemosensory structures that govern the sense of smell, while the second pair is often elongate and tactile. The foremost legs often have claws that perform several functions related to feeding, mating, and defense.

The structure and function of the abdomen varies among the different decapod taxa. Lobsters and shrimplike forms have large muscular abdomens that terminate in a flattened tail fan. To evade predators, these animals snap their tail fans rapidly beneath their abdomens, which propel them backwards. Shrimps and other decapods that don't have heavy or thick exoskeletons use their abdominal appendages to swim forward. At the other extreme, the true crabs or brachyurans have lost most of their abdominal appendages; their abdomen plays no role in locomotion. The few remaining appendages in brachyurans are used only for egg attachment in females or as copulatory structures (gonopods) in males.

Decapods exhibit tremendous diversity in shape, size, and color. They range in size from minute parasitic pea crabs to the giant Japanese spider crab Macrocheira kaempferi, which has legs spanning up to 12 ft (3.7 m). Many species have distinctive color patterns, while others are able to change color by expanding or contracting specialized groups of pigment cells in the epidermis underlying their exoskeleton.

The many variations of the basic decapod body plan reflect the great success of this group and their adaptation to a wide range of habitat types and ecological roles. Although the familiar terms "shrimp," "lobster," and "crab" have no formal taxonomic meaning, they do represent three distinct and recognizable body plans. Of the shrimplike forms, the Dendrobranchiata and the Caridea are the most familiar; they include all commercially harvested shrimps and prawns.

The Dendrobranchiata have perhaps the least-modified body plan of all the Decapoda and also show little diversity in form. The first three pairs of legs on penaeid shrimps have small claws that are equal in size. This is a relatively small group of approximately 350 species, and is primarily tropical or subtropical in distribution. Some Caridea superficially resemble the penaeids, but the third pair of legs never has claws, and the abdomen usually has a pronounced hump in the middle. This diverse group shows an unusually wide range of variation in body form; there are about 1800 described species.

The Stenopodidea are a very small group (25 species) of tropical shrimps. Like the penaeids, they have claws on their first three pairs of legs, but their third pair is greatly enlarged. The ghost and mud shrimps (Thalassinidea) are an especially problematic group whose taxonomic affinities remain unclear. The Thalassinidea are thin-shelled burrowing forms that often have large claws on the first pair of legs and small claws on the second.

Heavily armored crustaceans with large muscular abdomens are commonly referred to as "lobsters"; again, this category combines groups with little relationship to one another besides their desirability as food. The Astacidea (crayfish and clawed lobsters) infraorder encompasses about 800 species that have large claws on the first pair of legs and small claws on the next two. The infraorder Palinura is a small (130 species) group found primarily in tropical and subtropical waters, which contains the spiny and slipper lobsters along with some deep-water forms that should probably be classified elsewhere. Slipper and spiny lobsters lack true claws on their front appendages and have a distinctive and unusual flattened larval form known as a phyllosoma.

Crabs have the most compact decapod body form. In the true crabs or Brachyura, the abdomen is greatly reduced in size, folded beneath the body, and involved only in reproduction. Only the first pair of legs has claws. This is a very successful group, accounting for roughly half the 10,000 known species of decapods. The Anomura is composed of a wide range of unusual crablike animals, including the hermit crabs, king crabs, sand crabs, and porcelain crabs. It is an almost entirely marine group, with about 1800 species worldwide. Like the Brachyura, the Anomura usually have claws on their first pair of legs; however, anomuran crabs can be distinguished by the fact that their last pair of legs is much smaller and often tucked into the gill chamber. In addition, their second pair of antennae is positioned beside the eyes rather than between them as in the brachyurans.

Anomurans have the widest range of variation in abdominal form of any of the decapods. Hermit crabs have elongate, soft, asymmetrical abdomens that they protect with an empty snail shell. The king crabs are believed to have evolved from hermit crabs; although the abdomen is tucked underneath the body, all retain the asymmetry of hermit crabs and some even have completely soft, unprotected abdomens. At the other extreme, porcelain crabs have thin flat abdomens superficially similar to those of true crabs, but have retained a tail fan and a limited ability to swim.

Sperm duct Intestine

Ventral nerve cord

Abdominal segments

Ventral nerve cord

Abdominal segments

Uropod

Pereiopod

Decapod anatomy. (Illustration by Christina St. Clair)

Uropod

Pereiopod

Cheliped

Decapod anatomy. (Illustration by Christina St. Clair)

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