Basking shark

The basking shark (Cetorhinus maximus), also known as the bone shark, is the second largest fish, after the whale shark. A cosmopolitan species, basking sharks are found in all the world's temperate oceans. They are the only surviving member of their family, Cetorhinidae. It has however been suggested that the related megamouth shark (Megachasma pelagios) also belongs here.

Like other large sharks, basking sharks are at risk of extinction due to a combination of low resilience and overfishing through mankind's increasing demands for the sharks' flesh and organs.

Naming

These sharks are named for their apparent basking behavior while feeding. They are most often seen while basking at the surface with their mouths open.

Anatomy and appearance

The basking shark is one of the largest sharks known to man, second only to the whale shark. The largest specimen considered to be accurately measured was the vast beast that became trapped in a herring net in Bay of Fundy, Canada, 1851. Its total length was 12.27 metre (40 ft 3 in), weighing an estimated sixteen tons. From Norway comes a report of three basking sharks over twelve metres (largest 13,7 m), but those are considered dubious since few if any sharks anywhere near such size have been caught in the area since. Normally the basking shark reaches a length of between 6 metres (20 feet) and a little over 8 metres (28 feet). Some specimens surpass 9 or even 10 metres, but after years of hard fishing, those huge specimens have become exceedingly rare. These sharks possess the typical lamniform body plan and have been mistaken for great white sharks: the two species can be easily distinguished, however, by the basking's cavernous jaw (up to a metre in width, held wide open whilst feeding), longer and more obvious gill slits (which nearly circle the entire head and are accompanied by well-developed gill rakers), smaller eyes, and usually smaller girth. Also, great whites possess large, dagger-like teeth, whilst those of the basking shark are greatly reduced (5-6 mm) and hooked; only the first 3-4 rows of the upper jaw and 6-7 rows of the lower are functional. There are also several behavioural differences between the two (see Behaviour).

Other distinctive characteristics of the basking shark include a strongly keeled caudal peduncle, highly textured skin covered in placoid scales and a layer of mucus, a pointed snout (which is distinctly hooked in younger specimens), and a lunate caudal fin. In large individuals the dorsal may flop over when above the surface. Coloration is highly variable and likely dependent on observation conditions and the state of the animal itself: dark brown to black or blue dorsally fading to a dull white ventrally is a common description. The sharks are often noticeably scarred, possibly through encounters with lampreys or cookiecutter sharks. The basking shark's liver, which may account for 25% of the shark's body weight, runs the entire length of the abdominal cavity and is thought to play a role in buoyancy regulation and long-term energy storage.

In females, only the right ovary appears to be functional: if so, this is a unique characteristic among sharks.

Diet

The basking shark is a filter feeder eating zooplankton from the water at a rate of 1,500 m³ or more 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 point the sharks in the right direction.

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; during this time they remain at the bottom in deep water where they may hibernate and lose their gill rakers: this hypothesis is however disputed.

They feed at or close to the surface with their mouths wide open and gill rakers erect. They are slow-moving sharks (feeding at about two knots) and do not attempt to evade approaching boats (unlike great whites). As filter feeders, they are also harmless to humans if left alone and will not be attracted to chum.

Basking sharks are social animals and form schools segregated by sex, usually in small numbers (3-4) but reportedly up to 100 individuals. Their social behaviour is thought to follow visual cues, as although the basking shark's eyes are small, they are fully developed; the sharks have been known to visually inspect boats, possibly mistaking them for conspecifics.^* Females are thought to seek out shallow water to give birth.

These sharks have few predators, but orcas, tiger sharks and the aforementioned lampreys are known to feed on them.

Even though the basking shark is considered to be slow and very large it can actually breach the water, i.e. jump fully out like some whales do.

Reproduction

Basking sharks are ovoviviparous: the developing embryos first rely on a yolk sac, and as there is no placental connection, they later rely on unfertilized ova produced by the mother (a behaviour known as oophagy). Gestation is thought to span over a year (but perhaps much longer), with a small and unknown number of young born fully developed at 1.5-2 metres (5-6.5 feet) in length. Mating is thought to occur in early summer and birthing in late summer, following the female's movement into shallow coastal waters.

The onset of maturity in basking sharks is not known with certainty but is thought to be between the 6-13th year of life and at a length of between 4.6-6 metres. Breeding frequency is also unknown, but is thought to be 2-4 years.

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 previous abundant numbers. Commercially it was put to many uses: the flesh for food and fishmeal, the hide for leather, and the liver for its oil. It now seems to be fished mainly for its fins (for shark fin soup). Parts (such as cartilage) are also used in traditional Chinese medicine and as an aphrodisiac in Japan, further adding to demand.

As a result of rapidly declining numbers, the basking shark has been protected and trade in its products restricted in many countries. It is fully protected in the UK, Malta, Florida and US Gulf and Atlantic waters, and targeted fishing for basking sharks is illegal in New Zealand.

Basking sharks and cryptozoology

Decomposing corpses of basking sharks have on several occasions been misidentified as sea serpents or plesiosaurs, for example the Stronsay beast and the Zuiyo Maru case.

References

David A Ebert, Sharks, Rays and Chimaeras of California, ISBN 0520234847

External links

ARKive entry on the Basking Shark

Sharks | Ovoviviparous fish

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