About Cetorhinus maximus (Gunnerus, 1765)
The basking shark, scientifically named Cetorhinus maximus, is the second largest living shark and fish, after the whale shark. It is one of three plankton-eating shark species, alongside the whale shark and megamouth shark. Typically, basking sharks reach 7.9 m (26 ft) in length, though large individuals have been recorded growing over 10 m (33 ft) long. Its usual colouration is greyish-brown, with mottled skin and a white-coloured mouth interior. Its caudal fin is crescent-shaped and has a strong lateral keel. Other common names for this species are bone shark, elephant shark, sailfish, and sunfish. The basking shark is a cosmopolitan migratory species that lives in all of the world's temperate oceans. It is a slow-moving filter feeder, and its common name comes from its habit of feeding at the surface, where it appears to be basking in the warmer water there. It has specific anatomical adaptations for filter-feeding: a greatly enlarged mouth and highly developed gill rakers. It has a conical snout, and its gill slits extend around the top and bottom of its head. The gill rakers are dark and bristle-like, and they catch plankton as water filters through the mouth and over the gills. Basking sharks have numerous teeth, often with 100 teeth per row. The teeth are very small, have a single conical cusp, are curved backwards, and are identical on both upper and lower jaws. This species has the smallest brain size per unit body weight of any shark, which reflects its relatively passive lifestyle. Satellite tracking shows that basking sharks overwinter in both continental shelf waters less than 200 m (660 ft) deep and deeper waters. They can be found either alone or in small shoals. Despite their large size and threatening appearance, basking sharks are not aggressive and are harmless to humans. The basking shark has long been a commercially important fish, used as a source of food, shark fin, animal feed, and shark liver oil. Overexploitation has reduced its populations to the point that some local populations have disappeared, and remaining populations require protection. The basking shark is a coastal-pelagic shark found worldwide in boreal to warm-temperate waters. It lives around the continental shelf and occasionally enters brackish waters. It can be found from the surface down to at least 910 m (2,990 ft) deep. It prefers water temperatures between 8 and 14.5 °C (46.4 to 58.1 °F), but has been confirmed to cross the much warmer waters of the equator. It is often seen close to land, including in bays with narrow openings. The shark follows concentrations of plankton in the water column, so it is often visible at the surface. It characteristically migrates with changing seasons. Basking sharks are ovoviviparous: developing embryos first rely on a yolk sac, with no placental connection to the mother. Their seemingly non-functional teeth may play a pre-birth role in helping embryos feed on the mother's unfertilized ova, a behaviour called oophagy. In females, only the right ovary appears to be functional, and researchers currently do not know why only one of the two ovaries works. Gestation is thought to last more than a year, and may last two to three years. A small, still unknown number of fully developed young are born, measuring 1.5–2 m (4 ft 11 in – 6 ft 7 in) at birth. Only one pregnant female basking shark has ever been caught; this individual carried six unborn young. Mating is thought to occur in early summer, and birthing in late summer, after the female moves into shallow waters. Sexual maturity is thought to occur between the ages of six and 13 years, when individuals reach a length of 4.6–6 m (15–20 ft). Breeding is thought to happen every two to four years. The exact lifespan of the basking shark is unknown, but experts estimate it to be around 50 years. The species' generation cycle is estimated to be 22 to 34 years.
