Shrimps Crabs and Lobsters: Decapoda

Decapods, including crabs, shrimps, lobsters, and crayfishes, are among the most familiar of all crustaceans. They come in a wide variety of sizes, ranging from tiny pea crabs to the giant Japanese spider crab Macrocheira kaempferi with spidery legs spanning up to 12 feet (3.7 meters) across. Many species have distinctive color patterns, and some are able to change their colors. Despite their incredible variety, all decapods have the same basic body plan with three body regions: head, thorax, and abdomen. The head and thorax are closely joined together, or fused, to form the cephalothorax (SEH-feh-lo-THOR-acks). A shieldlike carapace (CARE-eh-pes) covers the cephalothorax. The carapace also covers the sides of the body and protects the breathing organs, or gills.

The head sometimes has a beaklike projection called a rostrum and two distinct pairs of long antennae. The first pair of antennae, or antennules (an-TEN-yuls), is branched, or biramous (BY-ray-mus). They are used mostly to detect odors. The second pair of antennae is uniramous (YU-neh-RAY-mus), or not branched. This pair is used mainly as organs of touch. The compound eyes are set on the tips of stalks. Each compound eye has multiple lenses. Depending on the species, the uniramous jaws, or mandibles, are used for slicing flesh, grinding plant materials, or crushing shells.

The first three segments of the thorax are closely joined, or fused, with the head. The appendages of these first three segments are called maxillipeds (mack-SIH-leh-pehds). Maxillipeds are thoracic (thuh-RAE-sik) limbs that work together with the mouthparts. The leglike limbs, or pereopods (PAIR-ee-oh-pawds), of the remaining five thoracic segments are either uniramous or weakly biramous and are used mainly for walking. The first few pairs of legs, especially the first pair, often have claws that are used for feeding, mating, and defense. Fast, slender claws are used to grab alert prey. Large strong claws with toothlike surfaces are used to crush the shells of clams, snails, and other hard-shelled prey.

The six-segmented abdomen has pairs of appendages underneath called pleopods (PLEE-oh-pawds). In lobsters, crayfishes, and shrimps, the abdomen is long, thick, and powerful and is used for swimming. At the end of the abdomen is a pair of slender biramous appendages, the uropods (YUR-oh-pawds). In between the uropods is a taillike segment called the telson. The telson is not tightly joined with, or fused, to the last abdominal segment. The telson and uropods work together to form a fanlike tail. Hard-bodied decapods, such as lobsters, snap their abdomens with fanlike tails forward underneath their bodies to propel themselves backward through the water. Shrimps and other softer-bodied species use their abdomens and tails to swim forward in the water. However, the bodies of crabs are relatively short and compact. They have lost most of their abdominal appendages. The crab's abdomen is short, flat, and folded forward under the body. It plays no role in swimming and has just a few pairs of pleopods that are used only for carrying eggs (females) or mating (males).