The blue shark (Prionace glauca) is a species of requiem shark, family Carcharhinidae, that inhabits deep waters in the world’s temperate and tropical oceans. Preferring cooler waters, blue sharks migrate long distances, for example from New England to South America. Although generally lethargic, they can move very quickly. Blue sharks are viviparous and are noted for large litters of 25 to over 100 pups. They feed primarily on small fish and squid, although they can take larger prey. Blue sharks often school segregated by sex and size, and this behavior has led to their nickname “wolves of the sea”.
Blue sharks are light-bodied with long pectoral fins. The top of the body is deep blue, lighter on the sides, and the underside is white. It grows to 3.8 meters (12.5 feet) long and can weigh up to 204 kilograms (450 lb). The highest reported weight was 391 kilograms (862 lb). They are easy to distinguish from other sharks.
They are viviparous, with a yolk-sac placenta, delivering 4 to 135 pups per litter. The gestation period is between 9 and 12 months. Females mature at 5 to 6 years of age and males at 4 to 5. Courtship is believed to involve biting by the male, as mature specimens can be accurately sexed according to the presence or absence of bite scarring. Female blue sharks have adapted to the rigorous mating ritual by developing skin 3 times thicker than male skin.
Range and habitat
The blue shark is the most widely distributed animal in the world. It is an oceanic and epipelagic shark found worldwide in deep temperate and tropical waters from the surface to about 350 meters. In temperate seas it may approach shore where it can be observed by divers, while in tropical waters it inhabits greater depths. It lives as far north as Norway and as far south as Chile. Blue sharks are found off the coasts of every continent, except Antarctica. Its greatest Pacific concentrations occur between 20° and 50° North but with strong seasonal fluctuations. In the tropics it spreads evenly between 20° N and 20° S. It prefers waters with a temperature range of 7–16 °C (45–61 °F) but will tolerate temperatures of 21 °C (70 °F) or above. Records from the Atlantic show a regular clockwise migration within the prevailing currents.
Squid are important prey for blue sharks, but their diet includes other invertebrates such as cuttlefish and pelagic octopuses, as well as lobster, shrimp, crab, a large number of bony fishes, small sharks, mammalian carrion and occasional sea birds. Whale and porpoise blubber and meat have been retrieved from the stomachs of captured specimens and they are known to take cod from trawl nets. Blue sharks rarely eat tuna.
Adult blue sharks do not suffer predation on a regular basis, except by humans. Young and smaller individuals may get eaten by any sufficiently larger sharks such as the Great White Shark and the Tiger Shark . However, they are host to several species of parasites. For example, the blue shark is the definite host of the tetraphyllidean tapeworm, Pelichnibothrium speciosum (Prionacestus bipartitus). They become infected by eating intermediate hosts, probably Opah, (Lampris guttatus), and/or longnose lancetfish, (Alepisaurus ferox).
Blue sharks are the most heavily fished sharks mainly as by-catch. It is estimated that 10 to 20 million individuals are killed each year as a result of fishing. The flesh is edible, but not widely sought after; it is consumed fresh, dried, smoked and salted and diverted for fishmeal. The skin is used for leather, the fins for shark-fin soup and the liver for oil. Blue sharks are occasionally sought as game fish.
Blue sharks are not considered an aggressive species and rarely attack humans. Most interactions take place in deep water and on small boats as they rarely venture close to shore. As of 2009 there have been 13 attacks on humans and 4 fatalities. 
Blue sharks, like most pelagic sharks, tend to fare poorly in captivity. Attempts at keeping them using circular tanks with long glide paths, and pools with 3 meters (9.8 ft) central depth gently ascending to zero depth have met with mixed results at best; most specimens last less than 30 days. As with other pelagic sharks, they seem to have trouble avoiding walls or other obstacles. In one case at Sea World San Diego, the blue did fairly well until bull sharks were added to the tank; the bulls ate the blues. The captivity record for blue sharks as of 2008 was held by The New Jersey Aquarium for a specimen that lasted roughly 7 months before expiring of an apparent bacterial infection.
Source : www.wikipedia.org