Shrimp

Shrimps are decapod crustaceans from the infraorder Caridea found around the world in fresh and salt water and in both shallow and deep waters. There are about 2,000 species of shrimps; they range in length from a fraction of an inch to up to eight inches. Most are about 1.5–3 inches long, with the larger species commonly known as prawns. They are closely related to crabs, crayfish, and lobsters—the main difference is that they have a semitransparent body, a flexible abdomen, and a fan-like tail allowing them to swim easily. They often eat small plants, animals, or even carrion. Female shrimps may lay anywhere between 1,500 and 14,000 eggs depending on species and conditions.

The various types of shrimps include the common European or sand shrimp (Crangon vulgaris and Crago septemspinosus) found in the North Atlantic; the Peneus setiferus found off the east coast of North America from Mexico as far north as North Carolina; the brown-grooved shrimp (Peneus aztecus); and the pink-grooved shrimp (Peneus duorarum). The edible shrimps in the West Indies are Xiphocaris, which live in fresh water, with those from the genus Macrobrachium often being found in rivers in tropical countries.

Shrimps form the food source for numerous creatures, with the smallest type of shrimp, often known as krill—of which there are 82 species—providing the major part of the diet for baleen whales (such as blue whales, southern right whales, or humpback whales), and also for seals, penguins, squid, and fish. Blue whales consume many tons of krill each week. Krill, which are between one and two centimeters long as adults, live in vast swarms of up to 35 pounds per cubic yard in some parts of the Antarctic Ocean. It is not unknown for there to be [Page 1601]tens of millions of krill in a single swarm, and this is what often attracts whales and other animals. Krill, which can often swim a few centimeters per second, can molt, leaving behind some of their skin, or exuvia, as a decoy for predators.

There is commercial fishing of krill in the Southern Ocean and the waters around Japan—krill is consumed by people in both Japan and Russia. However, its major use is as aquaculture feed, fish bait, and food for livestock and pets. To satisfy demand for these uses, there has been a recent increase in fishing fleets specifically designed to fish and process krill. This started in the Soviet Union during the 1960s, followed by Japan in 1972, and later by Poland, Chile, and South Korea. Ukraine also established its own krill fleet in 1991, and Japan, Poland, and the Ukraine are now the largest krill-fishing nations in the world.

The increase in whaling in the 19th century and for much of the 20th century allowed the krill population to increase. However, even with a much reduced whale population today, scientists have noticed a decline in krill numbers that they suspect is connected to the warming of the oceans from the greenhouse effect. If the cause of this is correct, this would affect the ecoenvironment of the oceans, as krill are an important part of the marine food chain. Krill may also have been affected by events such as the coccolithophore bloom in the Bering Sea in 1998, which resulted in a decline of diatoms, on which coccolithophores and krill both feed. Without the ability to feed on small coccolithophores, the population of krill off the coasts of western Alaska fell drastically with the result that there was a fall in other forms of fauna, particularly salmon.

...