Whale shark
The whale shark is a slow-moving, filter-feeding carpet shark and the largest known extant fish species. The largest confirmed individual had a length of. The whale shark holds many records for size in the animal kingdom, most notably being by far the most massive living non-cetacean animal. It is the only living species of the genus Rhincodon and the only extant member of the family Rhincodontidae, which belongs to the subclass Elasmobranchii in the class Chondrichthyes. Before 1984 it was classified as Rhiniodon into Rhinodontidae. An extinct species, Rhincodon ferriolensis was described in Europe, in Late Burdigalian stage.
The whale shark is an active filter feeder, primarily consuming plankton, krill, fish eggs, and small schooling fish such as sardines and anchovies. The shark can process over 6,000 liters of water per hour through its specialized sieve-like gill pads. Highly migratory, the whale shark travels thousands of miles across tropical oceans to exploit seasonal food sources, with large, predictable feeding aggregations occurring at coastal sites such as Ningaloo Reef in Western Australia, the Yucatan Peninsula in Mexico, and off the coast of Gujarat and Kerala in India. Despite its immense size, the whale shark is docile and poses no significant threat to humans. It is currently listed as an Endangered species on the IUCN Red List owing to a population decline of more than 50% over the last 75 years, primarily as a result of targeted fishing, bycatch in other fisheries, and collisions with large ships.
Whale sharks inhabit the open waters of all tropical oceans. They are rarely found in water below. The lifespan of a whale shark is estimated to be between 80 and 130 years, based on studies of their vertebral growth bands and the growth rates of free-swimming sharks.
The species was distinguished in April 1828 after the harpooning of a specimen in Table Bay, South Africa. Andrew Smith, a military doctor associated with British troops stationed in Cape Town, described it the following year. 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.
Evolution
The whale shark is one of the most ancient shark species still extant today, with the earliest fossil teeth known from the Late Oligocene of South Carolina, US. Their teeth become more geographically widespread from the Early Miocene onwards, where they are known from other parts of the eastern United States, southern France, and Costa Rica.Description
Whale sharks possess a broad, flattened head with a large mouth and two small eyes located at the front corners. Unlike many other sharks, whale shark mouths are located at the front of the head rather than on the underside of the head. Their mouths can be exceptionally wide; a individual was reported to have a mouth measuring across. Whale shark mouths can contain over 300 rows of tiny teeth and 20 filter pads which it uses to filter feed. The spiracles are located just behind the eyes. Whale sharks have five large pairs of gills. Their skin is dark grey with a white belly marked with an arrangement of pale grey or white spots and stripes that is unique to each individual. The skin can be up to thick and is very hard and rough to the touch. The whale shark has three prominent ridges along its sides, which start above and behind the head and end at the caudal peduncle. The shark has two dorsal fins set relatively far back on the body, a pair of pectoral fins, a pair of pelvic fins and a single medial anal fin. The caudal fin has a larger upper lobe than the lower lobe.Whale sharks have been found to possess dermal denticles on the surface of their eyeballs that are structured differently from their body denticles. The dermal denticles, as well as the whale shark's ability to retract its eyes deep into their sockets, serve to protect the eyes from damage.
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 been shown to reform over a previously wounded area.
The complete and annotated genome of the whale shark was published in 2017.
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 two amino acid substitutions make the pigment more sensitive to blue light instead, the light that dominates the deep ocean. One of these mutations 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. 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. In the colder environment at 2,000 meters below the surface, it is activated again. The mutations thus allow the shark to see well at both ends of its great vertical range. The eyes have also lost all cone opsins except LWS.
Size
The whale shark is the largest non-cetacean animal in the world. The maximum size and growth patterns of the species are not well understood.Limited evidence, mostly from males, suggests that sexual maturity occurs around in length, with the possibility of females sexually maturing at a similar size or larger.
Various studies have aimed to estimate the growth and longevity of whale sharks, either by analyzing evidence from vertebral growth rings or measurements taken from re-sighted sharks over several years. This information is used to model growth curves, which can predict asymptotic length. The growth curves produced from these studies have estimated asymptotic lengths ranging from.
A 2020 study looked at the growth of whale shark individuals over a 10-year period around the Ningaloo Reef and concluded the species exhibits sexual dimorphism with regard to size, with females growing larger than males. The study found that males on average reach in length. The same study had less female data but estimated an average length of around. This value dropped to if data from aquarium whale sharks was included. The authors noted that these estimates represent average asymptotic size and are not the maximum sizes possible. Additionally, they acknowledged the potential for regional size variation.
Most previous growth studies have had data predominately from males and none have data from sharks over ~. Not all previous studies created separate growth curves for males and females, instead combining data from both sexes. Those studies that made sex-specific growth curves have estimated large asymptotic length estimates for males, with lengths of or more. However, mostly immature males were available in these studies, with few adults to constrain the upper portion of the growth curves.
The largest total length for the species is uncertain due to a lack of detailed documentation of the largest reported individuals. Whale sharks as large as in length have been reported in scientific literature. Most whale sharks observed are smaller.
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. Various forms of photogrammetry have been used to improve the accuracy of in-water measurements, including underwater and aerial techniques.
Reports of large whale sharks
Since the 1800s, there have been accounts of very large whale sharks. Some of these are as follows:In 1868, the Irish natural scientist Edward Perceval Wright obtained several small whale shark specimens in the Seychelles. Wright was informed of one whale shark that was measured as exceeding. Wright claimed to have observed specimens over and was told of specimens upwards of.
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. Smith noted that one wa could be interpreted as either or the approximate average of, based on the local fishermen. Later sources have stated this whale shark as approximately, but the accuracy of the estimate has been questioned.
In 1934, a ship named the 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 on one side and on the other, suggesting a total length of about.
Scott A. Eckert & Brent S. Stewart reported on satellite tracking of whale sharks from 1994 to 1996. Out of the 15 individuals tracked, two females were reported as measuring and. A long whale shark was reported as being stranded along the Ratnagiri coast in 1995. A female individual with a standard length of and an estimated total length at was reported from the Arabian Sea in 2001. In a 2015 study reviewing the size of marine megafauna, McClain and colleagues considered this female as being the most reliable and accurately measured.
On 7 February 2012, a large whale shark was found floating off the coast of Karachi, Pakistan. The length of the specimen was said to be between, with a weight of around.
Distribution and habitat
The whale shark inhabits all tropical and warm-temperate seas. The fish is primarily pelagic, and can be found in both coastal and oceanic habitats. Tracking devices have shown that the whale shark displays dynamic patterns of habitat utilization, likely in response to availability of prey. Whale sharks observed off the northeast Yucatan Peninsula tend to engage in inshore surface swimming between sunrise and mid-afternoon, followed by regular vertical oscillations in oceanic waters during the afternoon and overnight. About 95% of the oscillating period was spent in epipelagic depths, but whale sharks also took regular deep dives, often descending in brief "stutter steps", perhaps for foraging. The deepest recorded dive was. Whale sharks were observed to remain continuously at depths of greater than for three days or more.The whale shark is migratory and has two distinct subpopulations: an Atlantic subpopulation, from Maine and the Azores to Cape Agulhas, South Africa, and an Indo-Pacific subpopulation which holds 75% of the entire whale shark population. It usually roams between 30°N and 35°S where water temperatures are higher than but have been spotted as far north as the Bay of Fundy, Canada and the Sea of Okhotsk north of Japan and as far south as Victoria, Australia.
Seasonal feeding aggregations occur at several coastal sites such as the Persian Gulf and Gulf of Oman, Ningaloo Reef in Western Australia, Darwin Island in the Galápagos, Quintana Roo in Mexico, Mafia Island of Pwani Region in Tanzania, Inhambane province in Mozambique, the Philippines, around Mahe in the Seychelles, the Gujarat and Kerala coasts of India, Taiwan, southern China and Qatar.
In 2009, more than 400 whale sharks gathered off the Yucatan Coast. It was one of the largest gatherings of whale sharks recorded. Aggregations in that area are among the most reliable seasonal gatherings known for whale sharks, with large numbers occurring in most years between May and September. Associated ecotourism has grown rapidly to unsustainable levels.