Whale shark
| Whale shark | ||||||||||||||||
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 Whale shark from Taiwan in the Georgia Aquarium
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| Rhincodon typus (Smith, 1828) | ||||||||||||||||
 Range of whale shark
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Whale shark is the common name for a very large, slow, filter-feeding shark, Rhincodon typus, characterized by a large, terminal mouth with small teeth, spotted body, nasal barbels, and sieve-like apparatus for straining plankton. The whale shark 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), and is grouped into the subclass Elasmobranchii in the class Chondrichthyes.
The whale shark is ubiquitous in tropical and warm oceans and lives in the open sea. The species is believed to have originated about 60 million years ago (O'Donnell 2000).
Although there are some active fisheries for whale sharks, for most humans the greatest significance of whale sharks is perhaps the wonder they bring, whether seen in the wild, on eco-tours or diving trips, or in aquariums. Their great size, coupled with the mystery of these animals, and their gentle nature, adds to the human experience of nature. Ecologically, they are part of marine food chains, with principally their young falling prey to blue sharks, marlins, and other large predators.
Distribution and habitat
The whale shark inhabits the world's tropical and warm-temperate oceans. Its range is restricted to about ±30° latitude.
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 and Batangas in the Philippines; and the Tanzanian islands of Pemba and Zanzibar. Though it is often seen offshore, the whale shark has also been found closer to shore, entering lagoons or coral atolls, and near the mouths of estuaries and rivers. It is found to a depth of 700 meters (2,300 ft) (Froese and Pauly 2006).
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 favor specific locations).
Anatomy and appearance
The whale shark body is mostly gray with a white belly. Three prominent ridges run along each side of the animal, with the lower ridges extending into strong caudal keels near the tail (Grzimek et al. 2004). 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 centimeters (3.9 in) thick. The shark has a pair 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).
As a filter feeder, the whale shark has a capacious mouth which can be up to 1.5 meters (4.9 ft) wide and can contain between 300–350 rows of tiny teeth (FAO 2006). The mouth is on the front end of the head, rather than on the underside as in other large sharks. Two small eyes are located towards the front of the shark's wide, flat head. The whale shark's spiracles are just behind the eyes (Spiracles are small openings on the surface of sharks and rays that usually lead to respiratory systems and often used to pump water through the gills while the animal is at rest.)
The whale shark has five large pairs of gills. Dermal denticles (small outgrowths that cover the skin of many cartilaginous fish, including sharks, and are similar in structure to teeth) line its gill plates and pharynx. This fine rake-like apparatus is a unique modification of the gill rakers and is used for straining plankton from the water.
The whale shark is not an efficient swimmer since the entire body is used for swimming, which is unusual for fish and contributes to an average speed of only around 5-kilometer-per-hour (3.1 mph).
The whale shark is the largest fish in the world, with an average size of 5.5 to 10 meters (18-32.8 feet) (Grzimek et al. 2004). 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 meters (41.50 ft) long, weighed more than 21.5 metric tons (47,300 pounds), and had a girth of 7 meters (23.0 ft) (Wood 1982). Stories exist of vastly larger specimens—quoted lengths of 18 meters (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 meters (49.2 ft), and tells of reports of specimens surpassing 21 meters (68.9 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 meters (56 ft) long, and weighed approximately 37 metric tons (81,500 pounds), which have been exaggerated to an accurate measurement of 17.98 meters (58.99 ft) and weight 43 tons in recent years. There have even been claims of whale sharks of up to 23 meters (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 meters (15.1 ft) on one side and 12.2 meters (40.0 ft) on the other (Maniguet 1994). No reliable documentation exists of those claims and they remain little more than "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 plankton, including phytoplankton and zooplankton such as krill, as well as small nektonic life, such as small squid or vertebrates.
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 that 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 (Froese and Pauly 2006; Martin 2006; Martins and Knickle 2006).
The whale shark is an active feeder and targets concentrations of plankton or fish by olfactory cues. Rather than simply "vacuuming" constantly, it is able to pump water over its gills. The shark can circulate water at a rate up to 1.7 L/s (3.5 U.S. pint/s). 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. This is in contrast to the basking shark, which is a passive feeder and does not pump water; it relies on its swimming to force water over its gills (Froese and Pauly 2006; Martin 2006).
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.
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 ovoviviparous (Froese and Pauly 2006). (Ovoviviparous animals develop within eggs that remain within the mother's body up until they hatch or are about to hatch. It is similar to vivipary in that the embryo develops within the mother's body, but unlike the embryos of viviparous species, ovoviviparous embryos are nourished by the egg yolk rather than by the mother's body.)
The eggs remain in the body and the females give birth to live young which are 40 centimeters (15.7 in) to 60 centimeters (23.6 in) long. It is believed that they reach sexual maturity in around 30 years and the life span has been estimated to be between 70 and 180 years.
Naming
The species was first identified in April 1828, following the harpooning of a 4.6-meter (15.1 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 physiology; that is, a shark as large as a whale that shares a similar filter feeder eating mode.
Whale sharks and humans
Whale sharks, despite their enormous size, do not pose any significant danger to humans. It is a frequently cited example when educating the public about the popular misconceptions of all sharks as "man-eaters."
Whale sharks actually are quite gentle and can be playful with divers. There are unconfirmed reports of sharks lying still, upside down on the surface to allow divers to scrape parasites and other organisms from their bellies. Divers and snorkellers 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 The Bay Islands in Honduras, Thailand, the Maldives, the Red Sea, Western Australia (Ningaloo Reef), Gladden Spit Marine Reserve in Belize, Tofo Beach in Mozambique, Sodwana Bay (Greater St. Lucia Wetland Park) in South Africa and at the Galapagos Islands.
The highest concentration of whale sharks to be found anywhere in the world is in the Philippines. From January to May, they congregate in the shallow coastal waters of Sorsogon province (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 Mexico's Baja California. Sometimes, they are accompanied by smaller fish, in particular, the remora.
Whale sharks are popular attractions in aquariums, where they also are able to be studied.
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 International Union for Conservation of Nature and Natural Resources (IUCN). It is protected in some countries, although there appear to be few actual threats to this species (Grzimek et al. 2004).
ReferencesISBN links support NWE through referral fees
- Food and Agriculture Organization of the United Nations. (FAO). 2006. FAO/SIDP species identification sheets: Rhincodon typus. (From L. J. V. Compagno, 1984, FAO Species Catalogue. Vol. 4. Sharks of the World. An Annotated and Illustrated Catalogue of Shark Species Known to Date. Part 1. Hexanchiformes to Lamniformes.)
- Froese, R., and D. Pauly. 2006. Rhincodon typus. FishBase. (Drawn from J. G. Colman, 1997. A review of the biology and ecology of the whale shark. J. Fish Biol. 51(6): 1219-1234.). Retrieved December 9, 2007.
- Grzimek, B., D. G. Kleiman, V. Geist, and M. C. McDade. Grzimek's Animal Life Encyclopedia. Detroit: Thomson-Gale, 2004. ISBN 0307394913
- Maniguet, X. 1994. The Jaws of Death: Shark as Predator, Man as Prey. Dobbs Ferry, NY: Sheridan House. ISBN 0924486643
- Martin, R. A. 2006. Building a better mouth trap. Elasmo Research, ReefQuest. Retrieved December 8, 2007.
- Martins, C., and C. Knickle. 2006. Whale shark. Florida Museum of Natural History. Retrieved December 9, 2007.
- O'Donnell, J. 2000. Jurassic Shark. Discovery Channel. August 5, 2006.
- Wood, G. L. 1982. The Guinness Book of Animal Facts and Feats. Enfield, Middlesex: Guinness Superlatives. ISBN 0851122353
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