Basking shark
The basking shark (Cetorhinus maximus) is the second largest living fish, after the whale shark, and one of three plankton-eating sharks along with the whale shark and megamouth shark. Adults typically reach 6–8 m (20–26 ft) in length. They are usually greyish-brown, with mottled skin. The caudal fin has a strong lateral keel and a crescent shape.
The basking shark is a cosmopolitan migratory species, found in all the world's temperate oceans. A slow-moving filter feeder, its common name derives from its habit of feeding at the surface, appearing to be basking in the warmer water there. It has anatomical adaptations for filter feeding, such as a greatly enlarged mouth and highly developed gill rakers. Its snout is conical and the gill slits extend around the top and bottom of its head. The gill rakers, dark and bristle-like, are used to catch plankton as water filters through the mouth and over the gills. The teeth are very small and numerous, and often number one hundred per row. The teeth have a single conical cusp, are curved backwards, and are the same on both the upper and lower jaws. This species has the smallest weight-for-weight brain size of any shark, reflecting its relatively passive lifestyle.
Basking sharks are believed to overwinter in deep waters. They may be found in either small schools or alone. Small schools in the Bay of Fundy and the Hebrides have been seen swimming nose to tail in circles in what may be a form of mating behaviour. Despite their large size and threatening appearance, basking sharks are not aggressive and are harmless to humans.
It has long been a commercially important fish, as a source of food, shark fin, animal feed, and shark liver oil. Overexploitation has reduced its populations to the point where some have disappeared and others need protection.
Taxonomy:
The basking shark is the only member of the family Cetorhinidae, part of the mackerel shark order Lamniformes. Gunnerus was the first to describe and name the species Cetorhinus maximus from a specimen found in Norway. The genus name Cetorhinus comes from the Greek ketos which means marine monster or whale and rhinos meaning nose; the species name maximus is from Latin and means "greatest". In the following centuries there were more attempts at naming: Squalus isodus, in 1819 by Macri; Squalus elephas, by Lesueur in 1822; Squalus rashleighanus, by Couch in 1838; Squalus cetaceus, by Gronow in 1854; Cetorhinus blainvillei by Capello in 1869; Selachus pennantii, by Cornish in 1885; Cetorhinus maximus infanuncula, by Deinse and Adriani in 1953; and finally Cetorhinus maximus normani, by Siccardi in 1961. Other names include bone shark, elephant shark, hoe-mother (sometimes contracted to homer), sail-fish, and sun-fish.
Range and habitat:
The basking shark is a coastal-pelagic shark found worldwide in boreal to warm-temperate waters around the continental shelves, and entering into brackish waters on occasions. It prefers 8 to 14.5 °C (46.4 to 58.1 °F) temperatures, but has been confirmed to cross the much-warmer waters at the equator. It is ften seen close to land, including bays with narrow openings. The shark follows plankton concentrations in the water column, so is often visible at the surface. It characteristically migrates with the seasons. The basking shark is found from the surface down to at least 910 m (2,990 ft).
Surfacing behaviors:
They are slow-moving sharks (feeding at about 2 kn (3.7 km/h; 2.3 mph)) and do not evade approaching boats (unlike great white sharks). They are not attracted to chum.
Though the basking shark is large and slow, it can breach, jumping entirely out of the water. This behaviour could be an attempt to dislodge parasites or commensals. Such interpretations are speculative, however, and difficult to verify; breaching in large marine animals such as whales and sharks might equally well be intraspecific threat displays of size and strength.
Migration:
Argos system satellite tagging confirmed basking sharks move thousands of kilometres during the winter, seeking plankton blooms. They shed and renew their gill rakers in an ongoing process, rather than over one short period.
A 2009 study tagged 25 sharks off the coast of Cape Cod, Massachusetts, and indicated at least some migrate south in the winter. Remaining at depths between 200 and 1,000 metres (660 and 3,280 ft) for many weeks, the tagged sharks crossed the equator to reach Brazil. One individual spent a month near the mouth of the Amazon River. They may undertake this journey to aid reproduction.
On 23 June 2015, a 6.1-meter-long (20 ft), 3,500 kilograms (7,716 lb) basking shark was caught accidentally by a fishing trawler in the Bass strait near Portland, Victoria, in Southeast Australia, the first basking shark caught in the region since the 1930s, and only the third reported in the region in 160 years. The whole shark was donated to the Victoria Museum for research, instead of the fins being sold for use in shark fin soup.
While basking sharks are not infrequently seen in the Mediterranean Sea and records exist in the Dardanelles Strait, it is unclear whether they historically reached into deeper basins of Sea of Marmara, Black Sea and Azov Sea.
Reproduction:
Basking sharks are ovoviviparous: the developing embryos first rely on a yolk sac, with no placental connection. Their seemingly useless teeth may play a role before birth in helping them feed on the mother's unfertilized ova (a behaviour known as oophagy). In females, only the right ovary appears to function.
Gestation is thought to span over a year (perhaps two to three years), with a small, though unknown, number of young born fully developed at 1.5–2 m (4 ft 11 in–6 ft 7 in). Only one pregnant female is known to have been caught; she was carrying six unborn young. Mating is thought to occur in early summer and birthing in late summer, following the female's movement into shallow waters.
The age of maturity is thought to be between the ages of six and 13 and at a length of 4.6–6 m (15–20 ft). Breeding frequency is thought to be two to four years.
The exact lifespan of the basking shark is unknown, but experts estimate to be about 50 years.
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