The whale shark, Rhincodon typus, is a slow-moving filter feeding shark. It is the largest extantfish species. The largest confirmed individual had a length of 12.65 metres (41.50 ft) and a weight of more than 21.5 tonnes (47,000 lb), and there are unconfirmed reports of considerably larger whale sharks. This distinctively-marked fish is the only member of its genus Rhincodon and itsfamily, Rhincodontidae (called Rhinodontes before 1984), which belongs to the subclassElasmobranchii in the class Chondrichthyes. The species originated about 60 million years ago.
The whale shark is found in tropical and warm oceans and lives in the open sea with a lifespan of about 70 years.[3] Although whale sharks have very large mouths, as filter feeders they feed mainly, though not exclusively, on plankton, which are microscopic plants and animals. However, the BBC program Planet Earth filmed a whale shark feeding on a school of small fish. The same documentary showed footage of a whale shark timing its arrival to coincide with the mass spawning of fish shoals and feeding on the resultant clouds of eggs and sperm.
The species was distinguished in April 1828 after the harpooning of a 4.6 metres (15.1 ft) specimen in Table Bay, South Africa. Andrew Smith, a military doctor associated with British troops stationed in Cape Town described it the following year.[4] He published a more detailed description in 1849. The name “whale shark” comes from the fish’s physiology, being as large as many whales and also a filter feeder like many whale species.
Distribution and habitat
The whale shark inhabits all tropical and warm-temperate seas. They are known to migrate every spring to the continental shelf of the central west coast of Australia. The coral spawning of the area’s Ningaloo Reef provides the whale shark with an abundant supply of plankton. Primarilypelagic, seasonal feeding aggregations occur at several coastal sites such as the southern and eastern parts of South Africa; Gladden Spit in Belize; Ningaloo Reef in Western Australia; Great Rann of Kutch in India; Útila in Honduras; Donsol, Pasacao and Batangas in the Philippines; offIsla Mujeres and Isla Holbox in Yucatan, Mexico; Ujung Kulon National Park in Indonesia; Nosy Be in Madagascar Off Tofo Reef near Inhambane in Mozambique, and the Tanzanian islands ofMafia, Pemba, Zanzibar and, very rarely, Eilat, Israel. Although typically seen offshore, it has been found closer to land, entering lagoons or coral atolls, and near the mouths of estuaries and rivers. Its range is generally restricted to about ±30° latitude. It is capable of diving to depths of 700 metres (2,300 ft), and is migratory.[3]
Description
As a filter feeder it has a capacious mouth which can be up to 1.5 metres (4.9 ft) wide and can contain between 300 and 350 rows of tiny teeth.[5] It has five large pairs of gills. Two small eyes are located towards the front of the shark’s wide, flat head. The body is mostly grey with a white belly; three prominent ridges run along each side of the animal and the skin is marked with a “checkerboard” of pale yellow spots and stripes. These spots are unique to each individual and are useful for counting populations. Its skin can be up to 10 centimetres (3.9 in) thick. The shark has a pair each of dorsal fins and pectoral fins. Juveniles’ tails have a larger upper than lower fin while the adult tail becomes semi-lunate (crescent-shaped). Spiracles are just behind the eyes.
The whale shark is not an efficient swimmer since it uses its entire body, unusual for fish, to attain an average speed of around 5 kilometres per hour (3.1 mph). The whale shark is largest animal in the world outside of the large cetaceans. The average size of adult whale sharks is estimated at 9.7 metres (31.82 ft) and 9 tonnes (20,000 lb).[7] The largest verified specimen was caught on November 11, 1947, nearBaba Island, in Karachi, Pakistan. It was 12.65 metres (41.50 ft) long, weighed more than 21.5 tonnes (47,000 lb), and had a girth of 7 metres (23.0 ft).[7] Stories exist of vastly larger specimens — quoted lengths of 18 metres (59 ft) are not uncommon in the popular shark literature — but no scientific records support their existence. In 1868 the Irish natural scientist Edward Perceval Wright obtained several small whale shark specimens in the Seychelles, but claimed to have observed specimens in excess of 15 metres (49.2 ft), and tells of reports of specimens surpassing 21 metres (68.9 ft).In a 1925 publication, Hugh M. Smith described a huge animal caught in a bamboo fish trap in Thailandin 1919. The shark was too heavy to pull ashore, but Smith estimated that the shark was at least 17 metres (56 ft) long, and weighed approximately 37 tonnes (82,000 lb). These measurements have been exaggerated to 43 tonnes (95,000 lb) and a more precise 17.98 metres (58.99 ft) in recent years. A shark caught in 1994 off Tainan County, southern Taiwan reportedly weighed 35.8 tonnes (79,000 lb).[6] There have even been claims of whale sharks of up to 23 metres (75 ft). In 1934 a ship named the Maurguani came across a whale shark in the Southern Pacific Ocean, rammed it, and the shark consequently became stuck on the prow of the ship, supposedly with 4.6 metres (15.1 ft) on one side and 12.2 metres (40.0 ft) on the other.[8] No reliable documentation exists for these claims and they remain “fish-stories”.
Diet
The whale shark is a filter feeder — one of only three known filter feeding shark species (along with the basking shark and the megamouth shark). It feeds on macro-algae, plankton, krill, Christmas Island red crab larvae,[9] and small nektonic life such as small squid or vertebrates. It also feeds on small fish and the clouds of eggs and sperm during mass spawning of fish shoals.[1] The many rows of teeth play no role in feeding; in fact, they are reduced in size in the whale shark. Instead, the shark sucks in a mouthful of water, closes its mouth and expels the water through its gills. During the slight delay between closing the mouth and opening the gill flaps, plankton is trapped against the dermal denticles which line its gill plates and pharynx. This fine sieve-like apparatus, which is a unique modification of the gill rakers, prevents the passage of anything but fluid out through the gills, trapping anything above 2 to 3 millimetres (0.079 to 0.12 in) in diameter. Material caught in the filter between the gill bars is swallowed. Whale sharks have been observed “coughing” and it is presumed that this is a method of clearing a build up of food particles in the gill rakers. Whale sharks migrate to feed and possibly to breed.[3][10][11]
The whale shark is an active feeder, targeting concentrations of plankton or fish. It is able to ram filter feed or can gulp in a stationary position. This is in contrast to the passive feeding basking shark, which does not pump water. Instead, it swims to force water across its gills.
