Tuna - biology.

Tuna - biology. Tuna, common name for any of several large, pelagic, schooling fishes found in most of the waters of the world, long valued as food fish. A tuna has a sleek, rounded, streamlined body, tapering to a narrow junction with the tail. It is built for sustained speed. The pectoral fins fold into grooves on the body, and the eyes are flush with the body surface. A widely forked tail with rays extending over the last vertebra provides the driving force. On each side of the tail base are bony keels that are extensions of the caudal vertebrae. The design of the tail and the way the tendons connect it to the swimming muscles of the body are unusually successful. The body design is further improved by the well-developed vascular system under the skin, which keeps the body temperature higher than that of the water in which the fish is swimming. This increases the power output of the muscles and speeds the nerve impulses. Tuna are glistening blue above, gray spotted with silver below, and resemble the mackerel in general structure; certain species are called horse mackerel. They are distinguished from other fishes, however, by a series of finlets behind the second dorsal fin and the anal fin. When hooked, they offer great resistance and are therefore popular as game fish. During August and September, tuna approach the coastal areas to spawn, returning to deep water at the beginning of winter. They migrate great distances to spawning and feeding grounds; a fish tagged off California was caught off Japan ten months later. Because the tuna has no respiratory mechanism to ensure the flow of water over the gills, only the current caused by its swimming achieves this, so the tuna will die of anoxia if it stops swimming. The largest tuna, known as the bluefin tuna, weighs up to 817 kg (about 1,800 lb). It is found in the Atlantic and Pacific oceans and in the Mediterranean Sea and is especially abundant off the California coast. The yellowfin tuna occurs in both the Atlantic and Pacific oceans; it is caught in large numbers near Florida and Rhode Island. The Atlantic bonito weighs up to 2.7 kg (6 lb) and is found in the Atlantic Ocean from Nova Scotia to Brazil; the striped tuna, or skipjack, inhabits the Pacific Ocean and can weigh 22.7 kg (50 lb). The albacore, a fine food fish that can weigh up to 30 kg (66 lb), is thought to migrate between the United States and Japan. Tuna--mainly bluefin and yellowfin--are fished off the Atlantic and Pacific coasts of the United States, and signs of excessive harvesting are evident. Almost all the tuna landed at U.S. ports is used for canning. The liver of most types of tuna yields an oil that is often used in the processing of leather. Tuna accounts for almost 13 percent of seafood consumption in the United States. Due to pollution of the oceans, however, some tuna contains unsafe levels of mercury, which can cause neurological damage. In March 2004 the U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) issued guidelines recommending that pregnant women, nursing mothers, children, and women who may become pregnant limit their intake of tuna. The two agencies said this group should eat no more than 6 oz (170 g) of albacore tuna per week and no more than 12 oz (340 g) of light tuna per week. Albacore tuna is often sold as white tuna. Some consumer and environmental groups recommended lowering the consumption of tuna even further, especially albacore tuna. Scientific classification: Tuna belong to the family Scombridae in the order Perciformes. The bluefin tuna is classified as Thunnus thynnus, the yellowfin tuna as Thunnus albacares, the Atlantic bonito as Sarda sarda, the striped tuna as Euthynnus pelamis, and the albacore as Thunnus alalunga. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.