The basking shark strains an ocean's worth of water through a metre-wide mouth
A basking shark cruises with its jaws gaping open, at walking pace, sieving plankton from the sea. A five-metre individual has been calculated to push some 450 tonnes of water through its gills in an hour. It is the world's second-largest fish, and completely harmless to people.
Adults usually measure around eight metres and weigh several tonnes; the biggest reliably recorded, caught in a herring net in Canada's Bay of Fundy in 1851, was just over twelve metres. Its gill slits nearly encircle the head, lined inside with dark, bristly rakers that trap food, and its liver can make up a quarter of its body weight, probably helping with buoyancy and energy storage. It also has the smallest brain for its weight of any shark, fitting a slow, passive life.
Unlike the whale shark and megamouth, the other plankton-eating sharks, it cannot suck or pump water. It relies entirely on swimming forward, which makes it a true ram feeder. It is choosy about where it eats: water around feeding sharks held about 75 percent more zooplankton than nearby, especially small crustaceans called copepods. Its hundreds of tiny hooked teeth seem useless for eating; one idea is that they help embryos feed on unfertilised eggs inside the mother.
Satellite tags revealed an unexpectedly restless animal. Rather than hibernating, as once assumed, basking sharks dive as deep as 900 metres in winter, tracking the plankton. Sharks tagged off Cape Cod in 2009 spent weeks deep below the surface as they headed south across the equator to Brazil, and one lingered for a month near the mouth of the Amazon. In summer they gather, sometimes swimming nose to tail in circles in what looks like courtship, and despite their bulk they can leap clean out of the sea.
Almost nothing is known about their breeding. Pregnancy may last one to three years, and only one pregnant female has ever been caught, carrying six young. Rotting carcasses have fooled beachcombers more than once, producing sea monster stories such as the Stronsay Beast. Heavy fishing for liver oil, fins and meat has wiped out some populations.
Source: Basking shark
