Basking Shark

The basking shark Cetorhinus maximus is the world's second biggest fish. The largest accurately measured specimen was trapped in a herring net in the Bay of Fundy, Canada, in 1851. It was reported to be 12.27 m (40 ft 3 in) long and weighed an estimated 16 tonnes.

The biggest baskers are the females and they come to the surface when the plankton blooms. Males seem to swim deeper, coming to the surface only to mate. Both sexes are easily recognised by the enormous dorsal fin, the stout, sometimes bulbous, snout, and the enormous gill slits on each side of the body that almost meet on the dorsal surface. The animal's skin is covered by tiny, backward-facing dermal denticles and these are bathed in a thick, black, foul-smelling mucus. The body colour varies from dark grey to dusky brown.

The basking shark is a filter feeder. A large basker can plough through the surface waters with its metre-wide (3.3 ft wide) mouth agape at about 3-5 km/hr (1.9-3 mph), processing up to 7000 litres (1540 gallons) of seawater an hour. It sieves the water using enormous gill rakers (long processes of the gill arches), evolved to support a high flow rate. These huge comb-like structures filter out the tiny free-floating larvae of molluscs and crustaceans, such as crabs and lobsters, as well as other constituents of the zooplankton, such as fish eggs, copepods and barnacles. When it has sieved enough food, the great shark closes its cavernous mouth and swallows the accumulated mass in one enormous gulp.

The basking shark lives mainly in cold and temperate coastal and continental shelf waters, and is seen off the coasts of North and South America, Australia, New Zealand and Japan, as well as the British Isles and Europe. Occasionally it reaches the tropics, one specimen having turned up in Hawaii. It is also found in the Mediterranean.