Shrimps are common, small invertebrates that occur in all marine ecosystems; in addition, some species have adapted to living in freshwater. All members of this group (class Crustacea, order Decapoda) are adapted for swimming. Most species, however, are bottom-dwelling animals that swim only occasionally.
The body of most species of shrimps is compressed side-ways, or it may be more cylindrical in cross-section. The body consists of a well-developed thorax and abdomen enclosed in a tough carapace made of chitin, which often extends to the base of the legs, protecting the delicate gills. The first three pairs of thoracic limbs (or maxillipeds) are modified for use in feeding, specifically for grasping food. The other five pairs of thoracic legs, the first of which is usually larger than the others, have pinching claws that serve in handling prey as well for defensive purposes. These legs are also used for walking. The head is well developed and bears stalked eyes, a pair of mandibles, a pair of antennae, and smaller antennules. The antennae may be considerably longer than the body. Both the antennules and antennae play an important sensory role, detecting prey as well as changes in salinity and water temperature. At the end of the abdomen there is often a swimming fin formed by structures called the uropods and telson.
Unlike crabs and lobsters, their decapod relatives, shrimps can be highly gregarious and may swim and feed in large schools. Many species of shrimp are nocturnal, remaining concealed amid seaweed or hidden in the crevices of coral reefs during the day. Some species bury themselves in the sand, the only tell-tale sign of their presence being their long tentacles. At night they emerge to feed on smaller crustaceans, small fish, worms, and the eggs and larvae of a wide range of species.
One group of shrimps has developed an unusual means of capturing prey. The pistol or snapping shrimps (Alphaeidae) live in burrows that they excavate in sand on the seabed. One of their front claws is greatly enlarged, typically measuring more than half of the body length. The tip of this claw is modified as a broad baseplate, to which is attached a hinged joint; this is reminiscent of old-time muskets that had a powder pan which was ignited when a hammer hit it. The purpose of this device in the snapping shrimps is not primarily to grasp passing prey, but to stun them. When the shrimp feels threatened or detects potential prey nearby, the "hammer" is pulled back so that it is at a right angle to the base of the claw. When the hammer is released it produces a loud snapping noise, the shock wave of which can be sufficient to stun or even kill a small prey individual. The prey is then dragged into the shrimp's burrow and consumed. Pistol shrimps are also highly territorial, and use their snapping mechanism to deter other shrimps, and other invertebrates, from invading their territory and tunnels.
A number of shrimp species have developed elaborate social relationships with other marine animals. Certain species of shrimps live among the spines of sea urchins and the tentacles of sea anemones, feeding on plankton and small crustaceans. They also feed on the detritus produced as the urchin or anemone eats. The precise benefit to the host is not clear, but the shrimps may help deter small grazing fishes, or they may keep the tentacles or spines of the host free of debris and algae. A much refined association involves the cleaner shrimps, such as species of Periclimenes and Stenopus, which perform an essential service to many large fish by removing parasites from their body and cleaning injured tissues. To do so, the cleaner shrimps may have to enter the mouth of the host, a potentially lethal undertaking in view of the fact that most of the fish are large enough to make a meal out of the shrimp. However, the sanitary service is of such great importance to the fish that it never consumes its hygienist. Many fishes signal their desire to be cleaned by changing their body color, or by opening their mouth and extending their gill covers. In return for this service, the shrimps obtain much, if not all of their daily food requirements by eating the parasites or diseased flesh they find while cleaning. The cleaner shrimps are brightly colored and advertise their services to fish by perching in an exposed place and waving their long tentacles.
During the breeding season, many species of shrimp forsake their usual habitat in shallow water and migrate to deeper places where they mate and lay their eggs. Females lay huge numbers of eggs, often greater than half a million, which are released directly to the water and not retained on the body for hatching (crabs and lobsters do the latter). The microscopic eggs hatch into tiny larvae, known as nauplii, which drift with the current for several weeks before changing to the adult form. As the larvae grow, they undergo a number of molts until they acquire adult characters and eventually migrate toward shallower near-shore habitat where they live until the next breeding season.
Shrimps are an important part of the marine food web. They are eaten by a wide range of fishes, and even by marine mammals such as seals and whales. Larger species of shrimps are also sought out by commercial fisheries, which harvest huge amounts of these crustaceans for human consumption. Some species of shrimps are also cultivated in aquaculture in tropical countries.
See also Zooplankton.