Whale shark
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 Whale shark in the Georgia Aquarium.
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| Rhincodon typus (Smith, 1828) |
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 Range of whale shark
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The whale shark, Rhincodon typus, is a gentle and slow filter feeding shark that is the largest living fish species. This distinctively-marked shark is the only member of its genus Rhincodon and its family, Rhincodontidae (called Rhinodontes before 1984), which is grouped into the subclass Elasmobranchii in the class Chondrichthyes. The shark is found in tropical and warm oceans and lives in the open sea. The species is believed to have originated about 60 million years ago.[1]
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[edit] Naming
The species was first identified in April 1828 following the harpooning of a 4.6 m (15 ft) specimen in Table Bay, South Africa. It was described the following year by Andrew Smith, a military doctor associated with British troops stationed in Cape Town. He proceeded to publish a more detailed description of the species in 1849. The name "whale shark" comes from the fish's large size and eating habits; that is, a shark as large as a whale that shares a similar filter feeder eating mode.
[edit] 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 such as Ningaloo Reef in Western Australia; Útila in Honduras; Donsol in the Philippines; and Pemba and Zanzibar off the coast of Tanzania. Its range is restricted to about ±30 ° latitude. It is found to a depth of 700 m.[2] The whale shark is solitary and rarely seen in groups unless feeding at locations with an abundance of food. Males range over longer distances than females (which appear to favour specific locations). Whale sharks eat more than 2 tonnes of food a day.
[edit] Anatomy and appearance
As a filter feeder it has a capacious mouth which can be up to 1.5 m (5 ft) wide and can contain up to 300 rows of tiny teeth.[3] 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 whale shark and because of this they can be used to identify each animal and hence make an accurate population count. Its skin can be up to 10 cm (about 4 in.) thick. 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 since the entire body is used for swimming, which is unusual for sharks and contributes to an average speed of only around 5 km/h. The largest specimen regarded as accurately recorded was caught on November 11, 1947, near the island of Baba, not far from Karachi, Pakistan. It was 12.65 m (41.5 ft) long, weighed more than 21.5 tons (47,300 lb), and had a girth of 7 m (23 ft).[4] Stories exist of vastly larger specimens - Quoted lengths of 18 m (59 ft) are not uncommon in the popular shark literature - but no scientific records exist to support their existence. In 1868 the Irish natural scientist E. Perceval Wright spent time in the Seychelles, during which he managed to obtain several small whale shark specimens, but claimed to have observed specimens in excess of 15 m (49 ft), and tells of reports of specimens surpassing 21 m (69 ft).
In a 1925 publication, Hugh M. Smith describes a huge whale shark caught in a bamboo fish trap in Thailand in 1919. The shark was too heavy to pull ashore, but Smith estimated that the shark was at least 17 m (55.7 ft) and weighed approximately 37 tonnes (81,500 lb), which have been exaggerated to an accurate measurement of 17.98 m and weight 43 tonnes in recent years. There have even been claims of whale sharks of up to 23 m (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 m (15 ft) on one side and 12.2 m (40 ft) on the other.[5] No reliable documentation exists of those claims and they remain little more than "fish-stories".
[edit] 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 phytoplankton, macro-algae, plankton, krill and small nektonic life, such as small squid or vertebrates. The many rows of teeth play no role in feeding - 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 (anything above 2 to 3 mm in diameter is trapped). Any 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.[2][6][7] The shark can circulate water at a rate up to 1.7 L/s (3.5 US pint/s). The whale shark is an active feeder and targets concentrations of plankton or fish by olfactory cues rather than simply 'vacuuming' constantly. 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 between the full and quarter moons of May, June and July, .
The whale shark does not need to swim forward when feeding; it is often observed in a vertical position, 'bobbing' up and down swallowing water and actively filtering it for food.[6][2]
[edit] Behaviour towards divers
This species, despite its enormous size, does not pose any significant danger to humans. Divers and snorkelers can swim with this giant fish without any risk apart from unintentionally being hit by the shark's large tail fin.
The shark is often seen by divers in Thailand, the Maldives, the Red Sea, Western Australia (Ningaloo Reef), Gladden Spit Marine Reserve in Belize, Sodwana Bay (Greater St. Lucia Wetland Park) in South-Africa and at the Galapagos islands. They are regularly seen from December to May in the Philippines ( at 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.
[edit] Reproduction
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 female in July 1996 which was pregnant with 300 pups indicates that they are viviparous with ovoviviparous development.[2][8] The eggs remain in the body and the females give birth to live young which are 40 to 60 cm long. 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.
[edit] Conservation status
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.
[edit] Whale sharks in captivity
A whale shark is featured as the main attraction of Osaka Aquarium Kaiyukan and as of 2005, three whale sharks are being studied in captivity at the Okinawa Churaumi Aquarium in Japan. Four whale sharks, two males, Ralph and Norton, and two females, Alice and Trixie, are held in the Georgia Aquarium, in Atlanta. The two females were added on June 3, 2006 in hopes that reproduction in whale sharks could be studied in captivity. All four whale sharks were imported from Taiwan.
[edit] See also
[edit] Notes
- ^ Jurassic Shark (2000) documentary by Jacinth O'Donnell; broadcast on Discovery Channel, August 5, 2006
- ^ a b c d Ed. Ranier Froese and Daniel Pauly. Rhincodon typus. FishBase. Retrieved on 17 September 2006.
- ^ Compagno, L.J.V.. Species Fact Sheet, Rhincodon typus. Food and Agriculture Organization of the United Nations. Retrieved on 19 September 2006.
- ^ Gerald L. Wood, "Animal Facts and Feats", 1990.
- ^ Xavier Maniguet, "Jaws of Death"; 1991.
- ^ a b Martin, R. Aidan.. Elasmo Research. ReefQuest. Retrieved on 17 September 2006.
- ^ Whale shark. Icthyology at the Florida Museum of Natural History. Retrieved on 17 September 2006.
- ^ Dr. Eugenie Clark. Frequently Asked Questions. Retrieved on 26 September 2006.
[edit] References
- Norman, Brad (2000). Rhincodon typus. 2006 IUCN Red List of Threatened Species. IUCN 2006. Retrieved on 11 May 2006. Database entry includes justification for why this species is vulnerable
- FAO web page on Whale shark
- "Rhincodon typus". FishBase. Ed. Ranier Froese and Daniel Pauly. November 2004 version. N.p.: FishBase, 2004.
- Rhincodon typus (TSN 159857). Integrated Taxonomic Information System. Accessed on 16 November 2005.
[edit] External links
- Whale shark, Rhincodon typus MarineBio"
- ECOCEAN Whale Shark Photo-identification Library
- Whale Shark Project
- Whale sharks
- TimeAsia.com: Best of Asia, Best Animal Encounter
- Whale Shark profile and whale sharks in Belize from The Nature Conservancy
