Whale sharks
Whale sharks, scientifically known as Rhincodon typus, are the largest fish species currently in existence, often exceeding lengths of twelve meters (forty feet). These gentle giants are filter feeders, primarily consuming plankton, small schooling fish, and swimming crustaceans by swimming with their mouths open and using their extensive gill rakers to trap food. Typically found in warm waters, they exhibit migratory behavior and are known to gather in large groups, particularly at seasonal locations like the Ningaloo Coral Reef in Western Australia, where feeding conditions are optimal.
Reproductive habits of whale sharks are intriguing; they are ovoviviparous, meaning the female retains fertilized eggs in her uterus until they hatch, releasing live young into the water. A single female can carry up to three hundred embryos at different sizes. Despite their immense size, adult whale sharks have few natural predators, although orcas have been known to attack them. However, human activities pose significant threats, including small-scale fisheries targeting whale sharks. Conservation efforts have been initiated, such as the Philippines' ban on commercial fishing of these animals, as they are classified as endangered by the IUCN Red List due to declining populations. Whale sharks continue to capture the interest of researchers and enthusiasts alike, making them a vital subject for marine conservation.
Whale sharks
Whale Shark Facts
Classification:
- Kingdom: Animalia
- Subkingdom: Bilateria
- Phylum: Chordata
- Subphylum: Vertebrata
- Superclass: Gnathostomata
- Class: Chondrichthyes
- Subclass: Elasmobranchii (sharks, skates, and rays)
- Order: Orectolobiformes (carpet sharks)
- Family: Rhincodontidae
- Genus and species:Rhincodon typus
- Geographical location: Worldwide, generally in warm temperate and tropical seas except for the Mediterranean, where it is conspicuously absent
- Habitat: Coastal and oceanic
- Gestational period: Unknown
- Life span: Unknown
- Special anatomy: The largest shark species, reaching total lengths of at least twelve meters (forty feet); broadly blunt and flattened snout with a very large mouth located almost at the very tip of the animal and containing many minute teeth; a small whisker, or barbel, located on the nostrils above either side of the mouth; eyes located just behind the mouth, comparatively very small; prominent longitudinal ridges extending down the length of the body trunk; large heterocercal tail that is asymmetric (as in all sharks), with the upper lobe larger than the lower lobe and a prominent lateral keel located where the tail joins the body trunk; body coloration characterized by a checkerboard pattern of light stripes and spots against a dark grayish or brownish background on the upper surfaces and pale white below
The whale shark, Rhincodon typus, is the largest extant species of fish in the world and may attain lengths in excess of twelve meters (forty feet). Despite its large size, the whale shark is a generally placid and inoffensive creature that feeds upon plankton, swimming crustaceans, and small species of schooling fish in the water. The whale shark is a filter feeder, and it cruises through the water column with its mouth open. Any food in the water is caught by the large and extensive gill rakers, which are large, sievelike structures located upon the gill arches. The water then passes to the exterior of the animal through the gill arches, and the trapped food is swallowed. The whale shark occasionally exhibits more specialized feeding behavior, hanging almost vertically in the water and opening its mouth. The powerful suction caused by the opening of the mouth draws in water and any food in the vicinity.
Whale sharks are believed to be highly migratory, and are encountered either singly or in groups of up to several hundred individuals. Although comparatively little is known about the lives of these animals, they have been observed to congregate at various locations, such as Ningaloo Coral Reef in Western Australia, at predictable seasonal times. These aggregations are believed to be associated with particularly favorable feeding conditions, such as the spawning cycles of corals. When they are not congregating at these sites, whale sharks are known to make long voyages. Satellite tracking of radio tags attached to whale sharks for periods as long as thirteen months have demonstrated that these fish may range far and wide, crossing the oceans.
Reproductive and Life Cycles
Although a few young whale sharks have been maintained in captivity for short intervals, much remains to be learned about their biology, including their reproductive cycles. As in all sharks and rays, fertilization is internal. The males possess pelvic fins that are modified to form claspers, which are long, cylindrical structures. These are inserted into the female reproductive tract to release sperm, which fertilize the eggs of the female. Whale sharks were once believed to be an oviparous (egg-laying) species, but more recent evidence has demonstrated that they are ovoviviparous. In this style of reproduction, the fertilized eggs are maintained within the female’s uterus both prior to and after hatching. At about the time that the yolk of the embryonic egg has been completely absorbed, the embryos are released into the water to fend for themselves. A single pregnant female was found to have three hundred embryos contained within the uterus, ranging in size from forty-two to sixty-four centimeters (sixteen to twenty-six inches) in length. The length of the gestation period is unknown.
The life span of whale sharks is not known, and because of its immense size it is believed to have few natural enemies upon achieving adulthood. At least report, however, has described a fatal attack on a whale shark by killer whales (orcas) in the Gulf of California, so these latter must be considered as whale shark predators. Humans pose a threat to these gentle giants as well; small harpoon fisheries that fish for whale sharks have existed in some Pacific cultures, including the Philippines. However, in 1998, the government of the Philippines protected the whale shark, outlawing the commercial fishing of the animal. The IUCN Red List of Threatened Species lists the whale shark as endangered and notes its decreasing population.
Principal Terms
cartilage: a tough and flexible tissue that constitutes the skeleton of all sharks and rays
caudal fin: the tail fin of fishes, which supplies the forward thrust in locomotion
clasper: a modification of the pelvic fins in male sharks and rays which acts as the male sexual organ in internal fertilization of the female
elasmobranchs: the suborder of the sharks and rays
gill rakers: sievelike strainers found upon the gill arches of whale sharks, used to filter food from water taken in though the mouth
heterocercal: a type of caudal fin in which the spinal column extends into the upper lobe, producing an asymmetrical and distinctly sharklike tail
Bibliography
Last, P. R., and J. D. Stevens. Sharks and Rays of Australia. CSIRO, 1994.
Parker, Steve, and Jane Parker. The Encyclopedia of Sharks. Firefly Books, 1999.
"Rhincodon typus." IUCN Red List of Threatened Species, International Union for Conservation of Nature and Natural Resources, 2016, www.iucnredlist.org/details/19488/0. Accessed 5 Oct. 2016.
Springer, V. G., and J. P. Gold. Sharks in Question: The Smithsonian Answer Book. Smithsonian Institution Press, 1989.
Tricas, T. C., et al. Sharks and Rays. Time-Life Books, 1997.