Shrimp

I

Introduction

Shrimp, 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.

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II

Range and Habitat of Shrimp

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 Characteristics of Shrimp

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.

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