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Basking shark
Cetorhinus maximus
This giant shark is easily recognised by its body
size and extremely large gill slits, which almost completely encircle
the head. It is the second largest fish in the world, surpassed only
by the whale shark, Rhincodon typus. Adults frequently reach
lengths over 7 meters (ca 23 feet) and in some cases even up to 9
meters (ca 30 feet). Reports of basking sharks reaching 15 meters
(ca 50 feet) or more have been made, but should not be taken for certain
since they are often estimates, based on records of sightings.
Feeding basking shark (Photo D. Burton)
The basking shark is a planktivorous (plankton eating) filter feeder,
just like the whale shark and the megamouth shark. It swims slowly
but powerful at a constant speed near the surface of the water with
the mouth open, enabling water to pass through the mouth and out through
the gills. Small crustaceans and other plankton organisms get stuck
on the inside of the gill arches, which are covered with a sticky,
mucous secretion. Unlike the whale shark and the megamouth shark,
Megachasma pelagios, that can actively pump water through their
gills by gulping, the basking shark is solely depending on the passive
flow of water for ingesting its food. To be able to support its huge
body with energy the basking shark has to filter a lot of water. Studies
have been made that suggest that as much as 1800 tons (ca 4 million
lb) of water passes through the mouth every hour.
Gill arch from basking shark (Photo D. Burton)
The basking shark is a temperately distributed species, which only
occasionally travels into warmer waters. Except for the eastern Gulf
of Mexico, and a few other places, the species is not found in the
tropics. It is a highly migratory species and coordinates its migration
according to the abundance of plankton. In the spring, when the amount
of plankton is increasing in the cold temperate waters, the basking
shark is found off the eastern coast of North America from North Carolina
to New York, then migrates in the summer to the northern part of its
range, the eastern tip of Canada, and vanishes in the autumn and winter.
The same pattern can be observed off the British Isles, with the exception
that a few individuals, mostly males, may be found in that locale
throughout the winter.
The disappearance of the basking shark in the winter has long been
a mystery to scientist. Since the basking sharks periodically sheds
the gillrakers during the colder months and because the amount of
plankton varies with the season the question arise as to how the shark
feeds during the winter. One theory suggests that the shark hibernates
on the bottom until the plankton blooms again and the gillrakers are
regenerated. Another explanation is that the shark changes the diet
and feeds in deeper water or on the bottom until the gillrakers grow
back, and still another one supports an overall reduced activity while
the shark relies on its stored oil (in the liver) for energy.
The reproduction of the basking shark is an area that, like the winter-disappearance,
is wrapped in mystery. Very little is known, but the species is probably
ovoviviparous and the embryos in the oviducts most likely feed on
unfertilized eggs produced by the mother. This theory is supported
by the fact that nonpregnant females have been found with several
million tiny eggs in their ovaries. Unfortunately no pregnant females
have been captured. The basking shark is believed to have a long gestation
period, three and a half years, and to give birth to relatively few
young. The length of the gestation period are estimated and not a
hundred percent certain, but if it proves to be true it would be the
longest known gestation period of any vertebrate, 18 to 20 months
longer than the spiny dogfish, Squalus acanthias, the currant
record holder.
The basking shark matures late, males possibly at 12 to 16 years or
more, and is thought to grow slowly. These factors, in combination
with the small size of existing populations, make the species sensitive
to overexploitation. The slow, cruising manner, in which the shark
swims, makes it an easy prey for harpooners. The captured basking
shark is valued not only for the fins and the meat, but also for the
large, oil-rich liver.
 Further reading
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