Shrimp - biology.

Shrimp - biology. I INTRODUCTION Shrimp, any of about 2,000 species of small, aquatic animals related to crabs, lobsters, and crayfish. Shrimp range in size from animals not much bigger than a fingernail to ones over 20 cm (8 in) long. Larger species are often known as prawns, although many people interchange the two names for all species. Shrimp live in a wide variety of freshwater and saltwater habitats, including lakes, coral reefs, and the depths of the sea. Some shrimp are good swimmers and spend nearly their entire life swimming in open water. But many species are bottom-dwellers and swim only occasionally. These shrimp crawl over submerged rocks, sand, or mud on the lake or ocean floor. Nearly all shrimp escape from danger by swimming backwards, with rapid flicks of their fanlike tails. Shrimp are an important part of the food web in the waters where they live. Open-water shrimp feed on zooplankton, microscopic animals that drift in the water. Bottom-dwellers feed on the dead remains of small animals and plants. Freshwater shrimp are an important food for fish, while marine shrimp are eaten by many kinds of animals, including birds and seals. Shrimp are also a popular food for humans, and shrimp fishing is a commercially important business. Shrimp are also raised and harvested in shrimp farms, a rapidly expanding industry in some parts of the world. II RANGE AND HABITAT Shrimp are found in all the world's oceans, as well as in freshwater lakes and streams. In the oceans, some open-water species live at depths around 5 km (3 mi), but most are found closer to the surface. To avoid predators, most open-water shrimp carry out daily migrations. They feed near the water's surface at night but spend the daytime hiding in the murky depths of the ocean, about 200 m (660 ft) down. Most of the shrimp caught as human food are bottom-dwellers that live on shallow seabeds. These species are often scavengers, feeding on remains of dead animals. Scavenging shrimp are also common in eel-grass beds that grow close to the shore. In the western Atlantic, the Sargassum shrimp, also known as the brown glass shrimp, is one of the few species that lives in a floating habitat--the clumps of seaweed that drift in the Sargasso Sea. Rather than venturing out into the open, some shrimp spend their lives hidden away. Many shelter under rocks or in coral crevices. Others live in caves, where they feed on dead remains. Burrowing shrimp excavate homes in soft sediment. In Southeast Asia, one kind of burrowing shrimp can cause problems in rice fields, which are submerged underwater. The digging action of burrowing shrimp creates muddy waters that block photosynthesis and decrease the oxygen levels in the water, thereby smothering young rice shoots. III PHYSICAL DESCRIPTION Shrimp are crustaceans, animals with a hard shell called an exoskeleton that forms a head, thorax, and abdomen. In shrimp that live near the water's surface, the exoskeleton is often transparent, making them less visible to predators. Bottom-dwelling shrimp are typically brown or green, while most deep-sea species are red. Shrimp that live in coral reefs can be quite vivid in color, and include some of the most beautifully colored animals in the seas. Structurally, shrimp are similar to crabs and lobsters but have bodies flattened from side to side. The exoskeleton covering the shrimp's head and thorax forms a rigid shield known as the carapace. The exoskeleton of the abdomen is flexible. The abdomen is packed with muscle, and it can flex rapidly to give the tail a powerful flick. The abdominal muscles form the white meat of shrimp, the tastiest part of the shrimp for humans. Shrimp have two pairs of whiplike antennae that contain sensory organs for taste and touch, which help shrimp find food. Bottom-dwelling species are quickly attracted by the scent of dead remains. Many shrimp species also have bulging compound eyes that are made up of hundreds of lenses joined together. The eyes act as an early warning system against predators--shrimp react instantly if they see anything moving overhead. Deep-sea shrimp have light-producing organs, called photophores, that scientists believe may help identify mates in the dark depths of the sea. Shrimp have a large number of specialized legs. Eight pairs of legs are located on the thorax. Three front pairs, called maxillipeds, are used for feeding, while the remaining five, called pereiopods, are used for clinging and for walking. The pereiopods can be long and delicate, and they are sometimes thinner than a human hair. The abdomen has five pairs of legs, called pleopods or swimmerets. These legs are short and rounded, and they are used for swimming. Pleopods can also be used for burrowing, and some females use their pleopods to carry eggs. IV SHRIMP BEHAVIOR AND REPRODUCTION Shrimp do not have large brains, but their behavior can be surprisingly complex. Many shrimp live alone, but some species live in large groups and show synchronized behavior. For example, pink shrimp are bottom-dwellers found along the East Coast of the United States. When darkness falls, millions of pink shrimp move into action together to feed on the seabed. Further out to sea, open-water shrimp often swim in tightly knit swarms. Like a school of fish, these shrimp move in the same direction, behaving as a single unit. Some of the most elaborate behavior is shown by cleaner shrimp, which live in coral reefs. These brightly colored animals feed on dead scales and parasites that they pick from the skin of living fish. To attract a fish, a cleaner shrimp positions itself in a prominent place, known as a cleaning station, where it carries out a stylized dance. Interested fish recognize these movements and respond with a distinctive still pose, indicating they are ready for grooming. Cleaner shrimp groom fish many times their own size, and they may even enter a fish's gills and mouth. In shrimp reproduction, the male deposits sperm into a gelatinous mass produced by the female and located between her fourth pair of walking legs. The female then lays eggs, in batches of up to 15,000 at a time. The sperm fertilizes the eggs. Females usually care for the fertilized eggs in a brood chamber found on the underside of her tail. In some species, the eggs are scattered in the water and left to develop on their own. As with crabs and lobsters, the eggs hatch to produce drifting larvae. The larvae change shape as they grow, developing an adult body after molting their skin several times (see Metamorphosis). Once the change in shape is complete (from 30 to 160 days, depending on the species), the young shrimp are sexually mature and able to breed. The life spans of shrimp vary among species--some die once they have bred, but others live for six or seven years. V TYPES OF SHRIMP True shrimp belong to the order Decapoda--a group containing crustaceans that have ten walking legs. In the marine waters of the United States, three of the most common commercial species are the pink shrimp, the brown shrimp, and the white shrimp. Found in the waters along the East Coast and in the Gulf of Mexico, these shrimp have a maximum length of about 20 cm (8 in). On the West Coast, the Franciscan Bay shrimp is caught commercially. Freshwater shrimp are most common in warm parts of the world. They include river shrimp, edible shrimp that are trapped or farmed in the tropics, and also all true shrimp that have become adapted for life in caves. Cave shrimp are typically pale, with vestigial (nonfunctioning) eyes, and they find their way mainly by touch. Decapod shrimp also include animals that are rarely eaten by humans. Among the most specialized are pistol shrimp, also known as snapping shrimp, which grow to 6 cm (2.4 in) long. These shrimp have a pair of enlarged pincers, one of which can be almost as long as the rest of the animal's body. The pincer can snap shut, creating a shock wave that is powerful enough to stun small fish. Some pistol shrimp form mutually beneficial partnerships with fish called gobies. A pistol shrimp shares its food with the goby and digs a burrow for it on the seabed. In turn, the goby warns the pistol shrimp of a predator's approach, at which point both will hide in the burrow. Many other crustaceans are commonly called shrimp, but they are not members of the order Decapoda. They differ from true shrimp in their anatomy and in their ways of life. These animals include tadpole shrimp, which breed in temporary pools in deserts, and brine shrimp, which live in salt lakes. Mantis shrimp are often caught in shrimp nets. They have a pair of strong grasping legs that can painfully wound fishermen who handle them carelessly. Mantis shrimp are larger than most true shrimp, measuring up to 56 cm (22 in) long. VI SHRIMP AND HUMANS Humans have eaten shrimp for centuries, and shrimp are a traditional ingredient in many regional cuisines. Shrimp meat is high in protein but low in fat, which makes it a highly nutritious food. Each year in the United States, more than 125,000 tons of shrimp are caught, while the worldwide total is nearly 3 million tons. At sea, shrimp are usually caught by trawlers, motorized boats that drag pouched-shaped nets over the seabed. In the United States, shrimp harvesting occurs mostly along the southeastern coast and in the Gulf of Mexico, where the shallow seabeds provide good shrimping grounds. Farmed shrimp are raised from eggs, typically in tropical coastal ponds. This method of raising shrimp developed in the Asia, but it is now widespread throughout the tropical world. Farmed shrimp currently make up between a third and a half of all shrimp that enter the market. Shrimp breed rapidly, which helps them to withstand annual harvesting. Even so, shrimp fishing has its limits, and there are signs of overharvesting in some parts of the world. In addition, because shrimp fisheries trawl the seabed, other animals are often caught in the nets-turtles are particularly at risk. Shrimp farming also can cause environmental problems. In many parts of the tropics, mangrove swamps are cleared to make room for shrimp farms, removing habitats that are important breeding grounds for fish. Many shrimp farms are also short-lived--in some regions, disease and pollution limit their working life to as little as five years. VII SCIENTIFIC CLASSIFICATION True shrimp belong to the order Decapoda. The Sargassum shrimp is a member of the family Palaemonidae. It is classified as Leander tenuicornis. In the family Penaidae, the pink shrimp is classified as Penaeus duorarum, the brown shrimp is Penaeus aztecus, and the white shrimp is Penaeus setiferus. The Franciscan Bay shrimp is a member of the family Crangonidae and is classified as Crangon franciscorum. The river shrimp is a member of the Palaemonidae family and is classified as Macrobrachium carcinus. Contributed By: David Burnie Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.