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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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