Whale shark
Whale shark Temporal range:
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Whale shark in the Andaman Sea around the Similan Islands | |
The size of various whale shark individuals with a human for scale | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Chondrichthyes |
Subclass: | Elasmobranchii |
Subdivision: | Selachimorpha |
Order: | Orectolobiformes |
Family: | Rhincodontidae |
Genus: | Rhincodon A. Smith, 1829[7][6] |
Species: | R. typus
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Binomial name | |
Rhincodon typus | |
Range of whale shark[needs update] | |
Synonyms | |
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The whale shark (Rhincodon typus) is a slow-moving,
Whale sharks inhabit the open waters of all tropical oceans. They are rarely found in water below 21 °C (70 °F).[2] Whale sharks' lifespans are estimated to be between 80 and 130 years, based on studies of their vertebral growth bands and the growth rates of free-swimming sharks.[9][10][11] Whale sharks have very large mouths and are filter feeders, which is a feeding mode that occurs in only two other sharks, the megamouth shark and the basking shark. They feed almost exclusively on plankton and small fishes and pose no threat to humans.
The species was distinguished in April 1828 after the harpooning of a 4.6 m (15 ft) specimen in Table Bay, South Africa. Andrew Smith, a military doctor associated with British troops stationed in Cape Town, described it the following year.[12] The name "whale shark" refers to the animal's appearance and large size; it is a fish, not a mammal, and (like all sharks) is not closely related to whales.[13] In addition, its filter feeding habits are similar to baleen whales.
Description
Whale sharks possess a broad, flattened head with a large mouth and two small eyes located at the front corners.
Whale sharks have been found to possess
Evidence suggests that whale sharks can recover from major injuries and may be able to regenerate small sections of their fins. Their spot markings have also been shown to reform over a previously wounded area.[21]
The complete and annotated genome of the whale shark was published in 2017.[22]
Rhodopsin, the light-sensing pigment in the rod cells of the retina, is normally sensitive to green and used to see in dim light, but in the whale shark (and the bottom-dwelling cloudy catshark) two amino acid substitutions make the pigment more sensitive to blue light instead, the light that dominates the deep ocean. One of these mutations also makes rhodopsin vulnerable to higher temperatures. In humans, a similar mutation leads to congenital stationary night blindness, as the human body temperature makes the pigment decay.[23][24] This pigment becomes unstable in shallow water, where the temperature is higher and the full spectrum of light is present. To protect from this instability, the whale shark deactivates the pigment when in shallow water (as otherwise the pigment would hinder full color vision). In the colder environment at 2,000 meters below the surface where the shark dives, it is activated again.[25] The mutations thus allow the shark to see well at both ends of its great vertical range.[26][27] The eyes have also lost all cone opsins except LWS.[28]
Size
The whale shark is the largest non-
Large whale sharks are difficult to measure accurately, both on the land and in the water. When on land, the total length measurement can be affected by how the tail is positioned, either angled as it would be in life or stretched as far as possible. Historically, techniques such as comparisons to objects of known size and knotted ropes have been used for in-water measurements, but these techniques may be inaccurate.[33] In 2011, laser photogrammetry was proposed to improve the accuracy of in-water measurements.[33][35]
Reports of large individuals
Since the 1800s, there have been accounts of very large whale sharks. Some of these are as follows:
In 1868, the Irish natural scientist
Hugh M. Smith described a huge animal caught in a bamboo fish trap in Thailand in 1919. The shark was too heavy to pull ashore, and no measurements were taken. Smith learned through independent sources that it was at least 10 wa (a Thai unit of length measuring between a person's outstretched arms). Smith noted that one wa could be interpreted as either 2 m (6.6 ft) or the approximate average of 1.7 to 1.8 m (5.6–5.9 ft), based on the local fishermen.[37] Later sources have stated this whale shark as approximately 18 m (59 ft), but the accuracy of the estimate has been questioned.[14][8]
In 1934, a ship named the Maunganui came across a whale shark in the southern Pacific Ocean and rammed it. The shark became stuck on the prow of the ship, supposedly with 15 ft (4.6 m) on one side and 40 ft (12.2 m) on the other, suggesting a total length of about 55 ft (17 m).[38][39]
Scott A. Eckert & Brent S. Stewart reported on satellite tracking of whale sharks from between 1994 and 1996. Out of the 15 individuals tracked, two females were reported as measuring 15 m (49 ft) and 18 m (59 ft) respectively.[40] A 20.75 m (68.1 ft) long whale shark was reported as being stranded along the Ratnagiri coast in 1995.[41][42] A female individual with a standard length of 15 m (49.2 ft) and an estimated total length at 18.8 m (61.7 ft) was reported from the Arabian Sea in 2001.[43] In a 2015 study looking into the size of marine megafauna, McClain and colleagues considered this female as being the most reliable and accurately measured.[8]
On 7 February 2012, a large whale shark was found floating 150 kilometres (93 mi) off the coast of Karachi, Pakistan. The length of the specimen was said to be between 11 and 12 m (36 and 39 ft), with a weight of around 15,000 kg (33,000 lb).[44]
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Jaws
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Teeth
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Eye
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Close up showing eyeball denticles
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Top of head
Distribution and habitat
The whale shark inhabits all tropical and warm-temperate seas. The fish is primarily
The whale shark is migratory
Seasonal feeding aggregations occur at several coastal sites such as the
In 2011, more than 400 whale sharks gathered off the
Growth and reproduction
Growth, longevity, and reproduction of the whale shark are poorly understood.[11][32][31] There was uncertainty as to whether vertebrae growth bands are formed annually or biannually, which is important in determining the age, growth, and longevity of whale sharks.[30][9][11] A 2020 study compared the ratio of Carbon-14 isotopes found in growth bands of whale shark vertebrae to nuclear testing events in the 1950-60s, finding that growth bands are laid down annually. The study found an age of 50 years for a 10 m (33 ft) female and 35 years for a 9.9m male.[31] Various studies looking at vertebrae growth bands and measuring whale sharks in the wild have estimated their lifespans from ~80 years and up to ~130 years.[9][10][11]
Evidence suggests that males grow faster than females in the earlier stages of life but ultimately reach a smaller maximum size.[29] Whale sharks exhibit late sexual maturity.[31] One study looking at free-swimming whale sharks estimated the age at maturity in males at ~25 years.[11]
Pupping of whale sharks has not been observed, but mating has been witnessed twice in
The capture of a ~10.6 m (35 ft) female in July 1996 that was pregnant with ~300 pups indicated that whale sharks are ovoviviparous.[10][56][57] The eggs remain in the body and the females give birth to live young which are 40 to 60 cm (16 to 24 in) long. Evidence indicates the pups are not all born at once, but rather the female retains sperm from one mating and produces a steady stream of pups over a prolonged period.[58]
On 7 March 2009, marine scientists in the Philippines discovered what is believed to be the smallest living specimen of the whale shark. The young shark, measuring only 38 cm (15 in), was found with its tail tied to a stake at a beach in Pilar, Sorsogon, Philippines, and was released into the wild. Based on this discovery, some scientists no longer believe this area is just a feeding ground; this site may be a birthing ground, as well. Both young whale sharks and pregnant females have been seen in the waters of St Helena in the South Atlantic Ocean, where numerous whale sharks can be spotted during the summer.[59][60]
In a report from Rappler last August 2019, whale sharks were sighted during WWF Philippines' photo identification activities in the first half of the year. There were a total 168 sightings – 64 of them “re-sightings” or reappearances of previously recorded whale sharks. WWF noted that “very young whale shark juveniles" were identified among the 168 individuals spotted in the first half of 2019. Their presence suggests that the Ticao Pass may be a pupping ground for whale sharks, further increasing the ecological significance of the area.[61]
Diet
The whale shark is a
The whale shark is an active feeder, targeting concentrations of plankton or fish. It is able to ram filter feed or can gulp in a stationary position. This is in contrast to the passive feeding basking shark, which does not pump water. Instead, it swims to force water across its gills.[10][65]
A juvenile whale shark is estimated to eat 21 kg (46 pounds) of plankton per day.[67]
The BBC program Planet Earth filmed a whale shark feeding on a school of small fish. The same documentary showed footage of a whale shark timing its arrival to coincide with the mass spawning of fish shoals and feeding on the resultant clouds of eggs and sperm.[63]
Whale sharks are known to prey on a range of planktonic and small nektonic organisms that are spatiotemporally patchy. These include krill, crab larvae, jellyfish, sardines, anchovies, mackerels, small tunas, and squid. In ram filter feeding, the fish swims forward at constant speed with its mouth fully open, straining prey particles from the water by forward propulsion. This is also called ‘passive feeding’, which usually occurs when prey is present at low density.[68]
Due their mode of feeding, whale sharks are susceptible to the ingestion of microplastics. As such, the presence of microplastics in whale shark scat was recently confirmed.[69]
Relationship with humans
Behavior toward divers
Despite its size, the whale shark does not pose any danger to humans. Whale sharks are docile fish and sometimes allow swimmers to catch a ride,[70][71][72] although this practice is discouraged by shark scientists and conservationists because of the disturbance to the sharks.[73] Younger whale sharks are gentle and can play with divers. Underwater photographers such as Fiona Ayerst have photographed them swimming close to humans without any danger.[74]
The shark is seen by divers in many places, including the
Conservation status
There is currently no robust estimate of the global whale shark population. The species is considered
It is listed, along with six other species of sharks, under the
In 2010, the Gulf of Mexico oil spill resulted in 4,900,000 barrels (780,000 m3) of oil flowing into an area south of the Mississippi River Delta, where one-third of all whale shark sightings in the northern part of the gulf have occurred in recent years. Sightings confirmed that the whale sharks were unable to avoid the oil slick, which was situated on the surface of the sea where the whale sharks feed for several hours at a time. No dead whale sharks were found.[82]
This species was also added to
Hundreds of whale sharks are illegally killed every year in China for their fins, skins, and oil.[84]
In captivity
The whale shark is popular in the few public aquariums that keep it, but its large size means that a very large tank is required and it has specialized feeding requirements.[85] Their large size and iconic status have also fueled an opposition to keeping the species in captivity, especially after the early death of some whale sharks in captivity and certain Chinese aquariums keeping the species in relatively small tanks.[86][87]
The first attempt at keeping whale sharks in captivity was in 1934 when an individual was kept for about four months in a netted-off natural bay in
Since the mid-1990s, several other aquariums have kept the species in Japan (
Outside Asia, the first and so far only place to keep whale sharks is Georgia Aquarium in Atlanta, United States.[88] This is unusual because of the comparatively long transport time and complex logistics required to bring the sharks to the aquarium, ranging between 28 and 36 hours.[90] Georgia keeps two whale sharks: two males, Taroko and Yushan, who both arrived in 2007.[95] Two earlier males at Georgia Aquarium, Ralph and Norton, both died in 2007.[87] Trixie died in 2020. Alice died in 2021. Georgia's whale sharks were all imported from Taiwan and were taken from the commercial fishing quota for the species, usually used locally for food.[90][96] Taiwan closed this fishery entirely in 2008.[96]
Human culture
In Madagascar, whale sharks are called marokintana in Malagasy, meaning "many stars", after the appearance of the markings on the shark's back.[97]
In the Philippines, it is called butanding and balilan.[98] The whale shark is featured on the reverse of the Philippine 100-peso bill. By law snorkelers must maintain a distance of 4 ft (1.2 m) from the sharks and there is a fine and possible prison sentence for anyone who touches the animals.[99]
Whale sharks are also known as jinbei-zame in Japan (because the markings resemble patterns typically seen on jinbei); gurano bintang in Indonesia; and ca ong (literally "sir fish") in Vietnam.[100]
The whale shark is also featured on the latest 2015–2017 edition of the Maldivian 1000
See also
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Further reading
- Colman, J.G. (December 1997). "A review of the biology and ecology of the whale shark". J. Fish Biol. 51 (6): 1219–34. PMID 29991171.
- FAO web page on Whale shark
- "Whale Sharks, Whale Shark Pictures, Whale Shark Facts". Animals, Animal Pictures, Wild Animal Facts. 10 September 2010. Archived from the original on 16 January 2010.
External links
- Whale Shark Photograph-identification Library
- Whale Shark And Oceanic Research Center
- Maldives Whale Shark Research Program
- Whale Sharks: Gentle Giants of the Seas
- Foundation for the Protection of Marine Megafauna
- Whale shark, Rhincodon typus at marinebio.org
- Whale Shark Fact Sheet, Fisheries Western Australia
- Albino whale shark photographed in Galapagos
- Photographs National Geographic
- A whale shark recorded defecating
- Photos of Whale shark on Sealife Collection