Shrimp differ from their close relatives, the lobsters and crabs, in that they are primarily swimmers rather than crawlers. As with other crustaceans, the body is covered with a smooth exoskeleton that must be periodically shed and re-formed as the animal grows. In some areas of the United States the term prawn is loosely applied to any large shrimp.
Tropical shrimp have bizarre shapes and colors.
One of the most unusual shrimp is the pistol shrimp, a burrow dweller whose third right appendage is adapted into a huge claw with a moveable finger that can be snapped shut with so much force that the resulting sound waves kill or stun nearby prey. Shrimp are widely distributed in temperate and tropical salt- and freshwaters. They may grow as long as 9 in. They swim forward by paddling their abdominal swimmerets and can move backward with swift strokes of their fanlike tails. The common commercial shrimp, of the genus Peneus, is found in coastal waters from Virginia south.