Blue shark

Appearance

Blue sharks are light-bodied with long pectoral fins. Like many other sharks, blue sharks are countershaded: the top of the body is deep blue, lighter on the sides, and the underside is white. The male blue shark commonly grows to 1.82 to 2.82 m at maturity, whereas the larger females commonly grow to 2.2 to 3.3 m at maturity. Large specimens can grow to 3.8 m long. Occasionally, an outsized blue shark is reported, with one widely printed claim of a length of 6.1 m, but no shark even approaching this has been confirmed in this species. The blue shark is fairly elongated and slender in build and typically weighs from 27 to 55 kg in males and from 93 to 182 kg in large females. Occasionally, a female in excess of 3 m will weigh over 204 kg. The heaviest reported weight for the species was 391 kg. The blue shark is also ectothermic.

Habitat

The blue shark 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 but will tolerate temperatures of 21 °C or above. Records from the Atlantic show a regular clockwise migration within the prevailing currents.

Reproduction

They are viviparous, with a yolk-sac placenta, delivering four to 135 pups per litter. The gestation period is between 9 and 12 months. Females mature at five to six years of age and males at four to five. 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 three times as thick as male skin.

Food

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. Sharks have been observed and documented working together as a "pack" to herd prey into a concentrated group from which they can easily feed. Blue sharks rarely eat tuna, which have been observed taking advantage of the herding behavior to opportunistically feed on escaping prey. It is interesting to note that the observed herding behavior was undisturbed by different species of shark in the vicinity that normally would pursue the common prey. The blue shark can swim at fast speeds, allowing it to catch up to prey easily. Its triangular teeth allows it to easily catch hold of slippery prey.

Predators

Young and smaller individuals may get eaten by larger sharks, such as the great white shark and the tiger shark. This shark may host several species of parasites. For example, the blue shark is the definite host of the tetraphyllidean tapeworm, "Pelichnibothrium speciosum". It becomes infected by eating intermediate hosts, probably opah and/or longnose lancetfish.