Whale shark  (Rhincodon typus)

The whale shark is the largest fish in the world; with its vast size it resembles the whales from which its common name is derived. The head is flattened and the wide mouth, positioned at the tip of the snout, stretches almost as wide as the body. The dorsal fin is particularly large and the tail has a half-moon shape. The patterning of the body is very distinctive with its dark greyish-blue colour on the back and sides, and array of pale yellow blotches; the undersurface is pale (5). Stout ridges travel the length of the body, ending at the tail shaft (6). Five massive gill slits occur on the side of the head and within these there is a sieve like structure of cartilage (5). Curiously, the mouth contains around 300 tiny teeth although the function of these is unknown (6).

Found throughout the world's oceans in temperate and tropical waters, most commonly in a global band around the equator between 30° to 40° latitude (2).

The whale shark inhabits shallow coastal areas as well as the open ocean. This species prefers warm water, with surface temperature between 21° to 30° centigrade (5), but can tolerate water temperatures experienced on deep dives (over 1,000 metres) as low as 3° centigrade (7).

These sharks are usually solitary, but loose groups of up to 100 individuals have been sighted, often when they are feeding (5). Whale sharks appear to be highly migratory (2), and have been tracked for thousands of kilometres (7). Individuals who regularly visit the Ningaloo Reef in Australia, between March and May every year, appear to be mainly immature males (8). It is not clear whether movements across deep ocean basins follow prey routes or are undertaken for other reasons. Very little is known about the reproduction of the world's largest fish, but in 1995, one pregnant female was captured who contained nearly 300 foetuses (5). The species is ovoviviparous; the young hatch from eggs retained within the mother so that she then gives birth to live young.

Whale sharks are fairly docile creatures and feed on planktonic organisms and small fish by suction filter-feeding (2). This species is thought to be a more dynamic filter-feeder than, for example, the basking shark, actively sucking food in through their vast mouths and passing the water over the gill arches, where prey are retained and then swallowed (5). They have also been observed actively swimming through shoals of fish with their mouth agape or hanging vertically in the water and drawing food into their mouths (8).

Whale sharks have been fished throughout their range, and their flesh is highly valued in some Asian markets. The recent increase in the demand for shark-fin soup threatens this species; in 1999, a large whale shark fin sold for around £11,000 (9). Although little is known about the ecology of this species, it is likely to be long-lived with a slow reproduction rate, making populations particularly vulnerable to exploitation. Where these shy creatures regularly come close to shore, they have become important tourist attractions, but the impact of shark-watching tours is at present poorly understood (5).

The catching of whale sharks is now prohibited in the Philippines and international conservation and management plans are encouraged by its listing on Appendix II of the Convention on Migratory Species (CMS) (4). In an historic move in 2002, the whale shark was included on Appendix II of the Convention on International Trade in Endangered Species (CITES) (3). These awesome creatures are now an important part of the tourism industry in Thailand, South Africa, Seychelles, Mozambique, Honduras and the Maldives (10). They make annual visits to the northwest coast of Australia, where they are found within the Ningaloo Marine Park and provide a massive tourist attraction. The Australian Department of Conservation and Land Management (CALM) has produced strict guidelines and protection measures in order to minimise the impact of shark-watching tours, and research projects in the area hope to understand these mysterious giants further (8).

Fur further information on the whale shark see:

• The Whale Shark Research Group:
  www.whaleshark.org
• Whale Shark Project:
  www.whalesharkproject.org

Authenticated (26/03/08) by Dr Juerg Brunnschweiler.