Shrimp are decapod crustaceans with elongated bodies and a primarily swimming mode of locomotion – most commonly Caridea and Dendrobranchiata. More narrow definitions may be restricted to Caridea, to smaller species of either group or to only the marine species. Under a broader definition, shrimp may be synonymous with prawn, covering stalk-eyed swimming crustaceans with long narrow muscular tails (abdomens), long whiskers (antennae), and slender legs. Any small crustacean which resembles a shrimp tends to be called one. They swim forward by paddling with swimmerets on the underside of their abdomens, although their escape response is typically repeated flicks with the tail driving them backwards very quickly. Crabs and lobsters have strong walking legs, whereas shrimp have thin, fragile legs which they use primarily for perching.
Shrimp are widespread and abundant. There are thousands of species adapted to a wide range of habitats. They can be found feeding near the seafloor on most coasts and estuaries, as well as in rivers and lakes. To escape predators, some species flip off the seafloor and dive into the sediment. They usually live from one to seven years. Shrimp are often solitary, though they can form large schools during the spawning season.
They play important roles in the food chain and are an important food source for larger animals ranging from fish to whales. The muscular tails of many shrimp are edible to humans, and they are widely caught and farmed for human consumption. Commercial shrimp species support an industry worth 50 billion dollars a year, and in 2010 the total commercial production of shrimp was nearly 7 million tonnes. Shrimp farming became more prevalent during the 1980s, particularly in China, and by 2007 the harvest from shrimp farms exceeded the capture of wild shrimp. There are significant issues with excessive bycatch when shrimp are captured in the wild, and with pollution damage done to estuaries when they are used to support shrimp farming. Many shrimp species are small as the term shrimp suggests, about 2 cm (0.79 in) long, but some shrimp exceed 25 cm (9.8 in). Larger shrimp are more likely to be targeted commercially and are often referred to as prawns, particularly in Britain.
Shrimp are swimming crustaceans with long narrow muscular abdomens and long antennae. Unlike crabs and lobsters, shrimp have well developed pleopods (swimmerets) and slender walking legs; they are more adapted for swimming than walking. Historically, it was the distinction between walking and swimming that formed the primary taxonomic division into the former suborders Natantia and Reptantia. Members of the Natantia (shrimp in the broader sense) were adapted for swimming while the Reptantia (crabs, lobsters, etc.) were adapted for crawling or walking. Some other groups also have common names that include the word "shrimp"; any small swimming crustacean resembling a shrimp tends to be called one.
The following description refers mainly to the external anatomy of the common European shrimp, Crangon crangon, as a typical example of a decapod shrimp. The body of the shrimp is divided into two main parts: the head and thorax which are fused together to form the cephalothorax, and a long narrow abdomen. The shell which protects the cephalothorax is harder and thicker than the shell elsewhere on the shrimp and is called the carapace. The carapace typically surrounds the gills, through which water is pumped by the action of the mouthparts. The rostrum, eyes, whiskers and legs also issue from the carapace. The rostrum, from the Latin rōstrum meaning beak, looks like a beak or pointed nose at the front of the shrimp's head. It is a rigid forward extension of the carapace and can be used for attack or defense. It may also stabilize the shrimp when it swims backward. Two bulbous eyes on stalks sit either side of the rostrum. These are compound eyes which have panoramic vision and are very good at detecting movement. Two pairs of whiskers (antennae) also issue from the head. One of these pairs is very long and can be twice the length of the shrimp, while the other pair is quite short. The antennae have sensors on them which allow the shrimp to feel where they touch, and also allow them to "smell" or "taste" things by sampling the chemicals in the water. The long antennae help the shrimp orient itself with regard to its immediate surroundings, while the short antennae help assess the suitability of prey.
Eight pairs of appendages issue from the cephalothorax. The first three pairs, the maxillipeds, Latin for "jaw feet", are used as mouthparts. In Crangon crangon, the first pair, the maxillula, pumps water into the gill cavity. After the maxilliped come five more pairs of appendages, the pereiopods. These form the ten decapod legs. In Crangon crangon, the first two pairs of pereiopods have claws or chela. The chela can grasp food items and bring them to the mouth. They can also be used for fighting and grooming. The remaining four legs are long and slender, and are used for walking or perching.
The muscular abdomen has six segments and has a thinner shell than the carapace. Each segment has a separate overlapping shell, which can be transparent. The first five segments each have a pair of appendages on the underside, which are shaped like paddles and are used for swimming forward. The appendages are called pleopods or swimmerets, and can be used for purposes other than swimming. Some shrimp species use them for brooding eggs, others have gills on them for breathing, and the males in some species use the first pair or two for insemination. The sixth segment terminates in the telson flanked by two pairs of appendages called the uropods. The uropods allow the shrimp to swim backward, and function like rudders, steering the shrimp when it swims forward. Together, the telson and uropods form a splayed tail fan. If a shrimp is alarmed, it can flex its tail fan in a rapid movement. This results in a backward dart called the caridoid escape reaction (lobstering).
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