Salmon shark

The salmon shark (Lamna ditropis) is a species of mackerel shark found in the northern Pacific ocean. As an apex predator, the salmon shark feeds on salmon, squid, sablefish, and herring. They are known for their ability to maintain stomach temperature (homeothermy), which is unusual among fish. This shark has not been demonstrated to maintain a constant body temperature. The salmon shark is also known for an unexplained variability in the sex ratio between eastern and western populations in the northern Pacific. Adult salmon sharks are medium grey to black over most of the body, with a white underside with darker blotches. Juveniles are similar in appearance, but generally lack blotches. The snout is short and cone-shaped, and the overall appearance is similar to a small great white shark. The eyes are positioned well forward, enabling binocular vision to accurately locate prey. Salmon sharks generally grow to between 200 and 260 cm (6.6–8.6 ft) in length and weigh up to 220 kg (485 lb). Males appear to reach a maximum size slightly smaller than females. Unconfirmed reports exist of salmon sharks reaching as much as 4.3 m (14.2 ft); however, the largest confirmed reports indicate a maximum total length of about 3.0 m (10 ft). The maximum reported weight of this heavily built shark is over 450 kg (992 lb). Salmon sharks are ovoviviparous, with a litter size of two to six pups. As with other Lamniformes shark species, salmon sharks are oophagous, with embryos feeding on the ova produced by the mother. Females reach sexual maturity at eight to ten years, while males generally mature by age five. Reproduction timing is not well understood, but is believed to be on a two-year cycle with mating occurring in the late summer or early fall. Gestation is around nine months. Some reports indicate the sex ratio at birth may be 2.2 males per female, but the prevalence of this is not known. As with only a few other species of fish, salmon sharks have the ability to regulate their body temperature. This is accomplished by vascular counter-current heat exchangers, known as retia mirabilia, Latin for 'wonderful nets'. Arteries and veins are in extremely close proximity to each other, resulting in heat exchange. Cold blood coming from the gills to the body is warmed by blood coming from the body. This results in blood coming from the body losing its heat so that by the time it interacts with cold water from the gills, it is about the same temperature, so no heat is lost from the body to the water. Blood coming towards the body regains its heat, allowing the shark to maintain its body temperature. This minimizes heat lost to the environment, allowing salmon sharks to thrive in cold waters. Their homeothermy may also rely on SERCA2 and ryanodine receptor 2 protein expression, which may have a cardioprotective effect. They are common in continental offshore waters, but range from inshore to just off beaches. They occur singly, in feeding aggregations of several individuals, or in schools. Tagging has revealed a range which includes sub-Arctic to subtropical waters.