The Sunday News
Taxonomy
THE whale shark, Rhincodon typus is a warm temperate species, slow-moving filter feeding shark and the largest known living fish species. It is, however, not a whale although it is known as a whale shark. The name Rhincodon typus is derived from the Greek words “rhyngchos” translated to rasp, “odous” translated to tooth and “typus” translated to type.
Geographical Distribution
Whale sharks have a very widespread distribution, found in almost all tropical and warm temperate seas, except in the Mediterranean. They are known to dwell in both deep and shallow coastal waters and the lagoons of coral islands and reefs.
They are known to undertake large-scale and localised transoceanic migrations and these different geographic locations appear to be preferred at various times of the year. These movements through water columns appear to be associated with feeding and breeding.
Habitat
Unlike most sharks from the same order (the Orectolobiformes), which are benthic species (which live on or near the bottom of the sea), the whale shark is a pelagic species (prefer open sea). Research shows that this shark prefers warm waters, with surface temperature around 21 to 30 degrees Celsius symbolised by high primary productivity (evidenced by the density of plankton). Whale sharks have smaller livers than most sharks and could conceivably control their resilience by swallowing some air like the sand tiger sharks (Ondontaspis taurus).
Biology
Distinctive Features
The whale shark is characterised by a streamlined body and a depressed, broad and flattened head. It has a pair of small eyes located back on the sides of the head. It has two dorsal fins that are set rearward on its body (the first being larger than the second), and ends with a large dual-lobbed caudal fin (or tail). The dual-lobbed caudal fin is semi-lunate in adults, whilst in juveniles the upper lobe is considerably longer than the lower lobe. Its skin can be up to 10 cm thick.
The whale shark’s back and sides are greyish to brownish, with an upper surface pattern of creamy white spots between pale, vertical and horizontal stripes (checkerboard like) and its belly is white. The distinctive markings in the whale sharks and other pelagic species could be linked to social activities such as postural displays and recognition processes. Some scientists are of the idea that these markings (pigment patterns) could be an adaptation for radiation shielding, important in species that spend a significant proportion of time in surface waters possibly exposed to high levels of ultraviolet radiation.
Whale sharks have a mouth that can be about 1,5 metres wide, containing 300 to 350 rows of tiny teeth in each jaw and 10 filter pads which it uses to filter feed. An individual tooth has a single, hooked tip. However, these teeth appear to have little use in feeding. Whale sharks have five large pairs of gills modified internally into filtering screens.
Size, age and growth
Accurate statistics about the sizes have not been publicised, however, various authors have provided estimates that range from 15-20 metres. They are believed to reach sexual maturity at minimum length of nine metres for both sexes. Like most sharks, female whale sharks are larger than males. Their age estimates are as high as 60 years and can go far beyond this estimate.
Feeding habits
The Whale Shark unlike most plankton feeding vertebrates does not rely on forward motion to operate its filtration mechanism, but is able to hang vertically in the water and draw feed by closing its gill slits and opening its mouth. After closing its mouth, the shark uses gill rakers that filter the nourishment from the water; practically nothing but water only goes through this sieve. Whale sharks have been observed “coughing”, presumably to clear a build-up of particles from the filter pads or to flush the gill rakers of accumulated food particles. Whale sharks have very large mouths, and as filter feeders they are believed to be able to sieve zooplankton as small as one millimetre in diameter through the fine mesh of their gill rakers, stereotypically feeding on a variety of planktonic (microscopic organisms) and nektonic (larger free-swimming) prey, schooling fishes and small crustaceans among other organisms.
Reproduction
The whale sharks are ovoviviparous (egg cases hatch in the mother’s uteri, hence giving birth to live young). They carry approximately 300 embryos which range from 48-65 centimetres in length. The pups are not all born at once, but rather the female retains sperm from one mating and produces a steady stream of pups over a lengthy period. The gestation period, birth localities, and frequency of reproduction for this species are not yet known. They reach sexual maturity at lengths of nine metres thus around an age of 30 years.
Predators and threats
Because of their enormous size they are not subject to extensive predation after reaching maturity. Only two reports have been registered of juvenile whale sharks being found in the stomachs of other animals: a blue shark and a blue marlin. However, their major threats are harpoons (a barbed spear-like missile attached to a long rope and thrown by hand or fired from a gun, used for catching whales and other large sea creatures) and entanglement fisheries in some regions of the world.
Danger to humans
They are generally considered as harmless to humans; though there are a few cases that were reported of whale sharks bumping sport-fishing boats, possibly after being provoked. They are more at risk from being struck accidentally by vessels while basking or feeding on the surface hence leading them to display harmful behaviour towards human beings.
