Extremely large, inoffensive shark (Cetorhinus maximus), second only to the whale shark as the largest living fish; length up to 10 m/33 ft, weight c.6000 kg/13 200 lb; lives in oceanic surface waters feeding entirely upon plankton filtered by stiff bristles on its long gill arches. (Family: Cetorhinidae.)
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Cetorhinus maximus (Gunnerus, 1765) |
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The basking shark, Cetorhinus maximus, is the second largest fish, after the whale shark.
Like other large sharks, basking sharks are at risk of extinction due to a combination of low resilience and overfishing through increasing demands for the sharks' fins, flesh and organs.
Taxonomy
This shark is called the basking shark because it is most often observed when feeding at the surface and looks like it is basking. It is also called bone shark, elephant shark, hoe mother, sun-fish and sailfish.
Distribution and habitat
The basking shark is a coastal-pelagic shark found worldwide in boreal to warm-temperate waters around the continental shelves.
Anatomy and appearance
The basking shark is one of the largest known sharks, second only to the whale shark. There are reports from Norway of three basking sharks over 12 m (the largest being 13.7 m), but those are considered dubious since few if any sharks anywhere near such size have been caught in the area since. These sharks possess the typical lamniform body plan and have been mistaken for great white sharks. The sharks are often noticeably scarred, possibly through encounters with lampreys or cookiecutter sharks.
Diet
The basking shark is a passive filter feeder, eating zooplankton, small fish and invertebrates from the water at a rate of up to 2,000 tons of water per hour. Unlike the megamouth shark and whale shark, basking sharks do not appear to actively seek their quarry, but do possess large olfactory bulbs that may guide them in the right direction. Contrary to the other large filter feeders it relies only of the water that is pushed throught the gills by swimming while the megamouth shark and whale shark can suck or pump water through its gills.
Behaviour
Although basking sharks are often sighted close to land and in enclosed bays during warmer months, they are highly migratory and seem to disappear entirely during autumn and winter (when the plankton is scarce at the surface).
These sharks have few predators, but orcas, tiger sharks are known to feed on them, and the aforementioned lampreys are often seen attached to them, although it is unlikely that they are able to cut through the shark's thick skin.
Even though the basking shark is large and slow it can breach the surface and has been reported jumping fully out of the water.
Reproduction
Basking sharks are ovoviviparous: the developing embryos first rely on a yolk sac, and as there is no placental connection, they later feed on unfertilized ova produced by the mother (a behaviour known as oophagy).
The onset of maturity in basking sharks is not known but is thought to be between the age of 6 and 13 and at a length of between 4.6 and 6 m.
The seemingly useless teeth of basking sharks may play a role in courtship behaviour, possibly as a means for the male to keep hold of the female during mating.
Importance to humans
Historically, the basking shark has been a staple of fisheries because of its slow swimming speed, unaggressive nature and previously abundant numbers.
Basking sharks and cryptozoology
On several occasions corpses initially thought to be sea serpents or plesiosaurs, have later been identified as mostly likely to be the decomposing carcasses of basking sharks, as for example in the Stronsay beast and the Zuiyo Maru cases.
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