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Whale shark

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Whale shark
Whale shark in the Maldives
Scientific classification
Kingdom:
Animalia
Phylum:
Chordata
Class:
Chondrichthyes
Order:
Orectolobiformes
Family:
Rhincodontidae
Genus:
Rhincodon

Smith, 1829
Species:
R. typus
Binomial name
Rhincodon typus

The whale shark, Rhincodon typus, is a gentle and slow filter feeding shark which is the largest living shark (and fish) species, the only member of its genus Rhincodon. It is a distinctively-marked member of the subclass Elasmobranchii of the class Chondrichthyes and lives in tropical and warm oceans. The species is believed to have originated about 60 million years ago.[1]

Naming

The species was first identified in 1828 off the coast of South Africa. The family Rhincodontidae was not finalized until 1984. The name "whale shark" comes from the fish's large size; that is, a shark as large as a whale.

Distribution and habitat

The whale shark inhabits the world's tropical and warm-temperate oceans. While thought to be primarily pelagic, seasonal feeding aggregations of the sharks occur at several coastal sites like Ningaloo Reef in Western Australia as well as Pemba, Útila, Honduras, Donsol, Philippines, and Zanzibar on the coast of East Africa. Its range is restricted to about ±30 ° latitude from these sites. The whale shark is solitary and rarely seen in groups unless feeding at locations with abundance of food like Ningaloo reef. Males range over longer distances than females, who seem to be attached to specific areas.

Anatomy and appearance

The greatest size accurately recorded was 12 meters (39 ft) long, with unofficial accounts of 18 meters (59 ft), weighing up to 12,000 kg (approximately 26,000 lb). It should not be confused with the basking shark (Cetorhinus maximus), the second largest fish. As a member of the order Orectolobiformes, it is a filter feeder and as such the shark has a capacious mouth which can be up to 1.5 m (5 ft) wide and contain up to 300 rows of tiny teeth. As part of its feeding process, it also 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 whale shark. Because of the spots' uniqueness they can be used to identify each animal and make an accurate population count. But so far the whale sharks have not been fully and accurately counted. Its skin can be up to 10 cm thick (about 4 in.). The shark has two pairs each of dorsal fins and pectoral fins. A juvenile whale shark's tail has a larger upper fin than lower fin while the adult tail becomes semi-lunate, or crescent-shaped. The shark's spiracles are just behind the eyes. The whale shark is not an efficient swimmer; the entire body is in motion while the animal swims. The result of this motion, one that is very unusual for sharks, is an average speed of around 5 km/h.

Diet

A size comparison of a whale shark and a human

The whale shark feeds on phytoplankton, macro-algae, plankton, krill or nektonic life (small squid or vertebrates). The many rows of teeth play no role in feeding, instead, water is actively drawn into the mouth and is passed over gill rakers and then out through the gill arches. Any material caught in the rakers is swallowed. The shark can circulate water at a rate up to 1.7 L/s (3.5 US pint/s). The sharks, however, are active feeders and target concentrations of plankton or fish by olfactory cues rather than simply 'vacuuming' constantly. According to sailors, whale sharks congregate at reefs off the Belizean Caribbean coast, supplementing their ordinary diet by feeding on the roe of giant cubera snappers, which spawn in these waters in May, June and July, and between the full and quarter moons of these months. Whale sharks are also regualarly sighted off the Honduran Bay Islands, particularly Utilla.

Behaviour

When it is explained that most sharks are not dangerous to humans, this species is used as the leading example. Divers and snorkelers can swim around the giant fish without any problems, apart from the risk of hit by the animals large and muscular tail fin.

The shark is often seen in Thailand, the Maldives, the Red Sea, Western Australia (Ningaloo Reef), Gladden Spit Marine Reserve in Belize, and at the Galapagos islands. They are regularly seen from December to May in the Philippines (Donsol). Lucky divers have also come across whale sharks in the Seychelles and in Puerto Rico. Between December and September, they are well known to swim along the bay of La Paz in the Mexican Baja California. Sometimes, they are accompanied by smaller fish.

Reproduction

Like most sharks, the reproductive habits of the whale shark are obscure. Based on the study of a single egg recovered off the coast of Mexico in 1956, it was believed to be oviparous, but the capture of a pregnant female in July 1996 containing 300 young whale sharks[1] indicates that they are viviparous with ovoviviparous development. The eggs remain in the body and the females give birth to live 40 to 60 cm young. It is believed that they reach sexual maturity at around 30 years and the life span has been estimated to be between 60 and 150 years.

Importance to humans

The whale shark is targeted by artisanal and commercial fisheries in several areas where they seasonally aggregate. The population is unknown and the species is considered vulnerable by the IUCN. Whale sharks are known to frequent the waters off Donsol in the Sorsogon province of the Philippines. A whale shark is also featured as the main attraction of Osaka Aquarium Kaiyukan.

As of 2005, three whale sharks are being studied in captivity at the Okinawa Churaumi Aquarium in Japan. Four whale sharks, including two males, Ralph and Norton, and two females, Alice and Trixie, are held in the Georgia Aquarium, which opened in 2005, in Atlanta. The two females were added on June 3, 2006 in hopes that their reproduction could be studied in captivity. All four whale sharks were imported from Taiwan by UPS.

See also

References

  • Template:IUCN2006 Database entry includes justification for why this species is vulnerable
  • FAO web page on Whale shark
  • Froese, Rainer; Pauly, Daniel (eds.). "Rhincodon typus". FishBase. November 2004 version.
  • "Rhincodon typus". Integrated Taxonomic Information System. 16 November. {{cite web}}: Check date values in: |date= and |year= / |date= mismatch (help)
  1. ^ Jurassic Shark (2000) documentary by Jacinth O'Donnell; broadcast on Discovery Channel, August 5, 2006

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