Rhincodon typus
The whale shark (Rhincodon typus) is a species of orectolobiform elasmobranch, the only member of the family Rhincodontidae and of the genus Rhincodon; It is the largest extant fish in the world, at approximately 12m in length. It is fed by filtering. It inhabits warm tropical and subtropical waters. It has been suggested that it has inhabited the Earth for 60 million years. In Mexico it is considered threatened.
Name
The first identified specimen measured 4.6 meters in length and was speared and caught off the coast of Table Bay, South Africa, in 1828. The specimen sold for £600,000,000, and its holotype is on display at the Museum of Natural History of Paris. The first scientific citation was given the following year by Andrew Smith, a military doctor with ties to the British Army, who was stationed in Cape Town. In 1849 he published a more detailed description of the species. He was assigned the name "whale shark"; due to the physiology of the fish, since it is a shark but has a size comparable to that of a whale. In Vietnamese religion he is revered as a deity, and is called 'Ca Ong', which literally means 'Lord Fish'. It is also called domino fish, checkerboard, or lady fish, after the classic board game.
Genomic characteristics
The whale shark genome is made up of approximately 3.2 Gb. The genome of this animal has a GC content of 42% and 50% of transposable genetic elements (27% are LINEs). The whale shark genome is predicted to be made up of 28,483 protein-coding genes. This shark is one of the animals with the slowest evolutionary rate among vertebrates. It is known that most of the genes of this animal are ancestral (>684 MA), a few appeared 684-93 MA ago, and several (35%) have appeared recently (93 MA-present). This indicates the conservation of a large number of genes in this species and a potential for recent gene innovation, in the last 94 MA.
Relation between physiological and genomic characteristics
The great longevity of the whale shark is explained by the high body mass, in addition to the extremely low mass- and temperature-adjusted basal metabolic rate. It also has a relatively large genome and intronic size due to the expansion of multiple types of repetitive elements. In addition, the coding sequence of this shark is much smaller than that of many chordate species, which could be related to a lower adjusted basal metabolic rate and therefore greater longevity. Neuronal connectivity genes are increased in size in this species. It seems that these genes could be related to greater longevity. The size of numerous genes related to metabolism and telomere maintenance has been shown to be correlated with basal metabolic rate, body weight, and longevity. This would explain the fact that there is an evolutionary relationship between gene size and numerous physiological traits in whale sharks and other chordates.
Habitat
Inhabits warm oceans and seas, near the tropics, although some have been observed in cooler waters, such as those off the New York coast. They are thought to be pelagic fish, but in certain seasons they migrate large distances to coastal areas, such as Ningaloo Reef in Western Australia, Utila in Honduras, Donsol and Batangas in the Philippines, the island of Holbox in the state of Quintana Roo, the Yucatan and Baja California peninsulas, Mexico, coasts of Venezuela (ocumare de la Coast) the islands of the Zanzibar archipelago (Pemba and Unguja), off the coast of Tanzania, in Ceiba and in the Pearl archipelago in Panama. Although it is common to find it offshore, it is also possible to spot it near the coast, entering in lagoons or coral atolls, and near the mouths of rivers. It usually stays within ±31° latitude, and at a depth of 700 meters. It usually acts solitary, although occasionally they form groups to feed in areas with large concentrations of food. Males can be found in more disparate places, while females prefer to stay in more specific places.
Anatomy
Its belly is totally white, while its back is grayish in color, darker than most sharks, with a multitude of spots and horizontal and vertical lines of white or yellowish color, in such a way that it resembles a a chess board. These spots represent a unique pattern in each specimen, which is why they are used to identify them and to census their population. Their skin can be up to 10 centimeters thick. Its body is streamlined, elongated and robust, and presents several longitudinal ridges on the head and back. Its head is wide and flattened, and on its sides are two small eyes, behind which are the spiracles. Its huge mouth can be up to 1.5 meters wide, large enough to hold a side-swimming seal, and its jaws contain many rows of small teeth. It has five large pairs of gills, the indentations are huge. It has a pair of dorsal fins and pectoral fins, the latter being very powerful. The tail of these beings can measure more than 2.5 meters from side to side. In young whale sharks the upper fin of the tail is larger than the lower fin, whereas the tail of an adult is crescent-shaped, and is what provides propulsion. However, the whale shark is not an efficient swimmer, since it uses its entire body to swim, which is not usually the case with fish, and for this reason it moves at an average speed of 5 km/h, a relatively slow speed for such a huge fish.
The largest specimen on record was captured on November 11, 1947, very close to Baba Island, near Karachi, Pakistan. It was 12.65 meters long and weighed over 21.5 tons. However, there are many stories of larger whale sharks, mentioning lengths of over 18 metres, which are not uncommon by the way. in popular literature, but there are no records or scientific evidence to support its existence. In 1868, the Irish botanist Edward Perceval Wright, while vacationing in the Seychelles, observed some specimens of whale sharks, and claimed to have seen specimens of more than 15 meters in length, and even some that exceeded 21 meters.
In a 1925 publication, Hugh McCormick Smith describes an enormous whale shark caught in a Thai bamboo fish trap in 1919. The shark was too heavy to land on dry land, but Smith estimated its Its length was at least 17 meters, and its weight was around 37 tons, although these figures were later exaggerated, even stating that it measured 17.98 meters and weighed 43 tons. There have even been reports of 23-meter whale sharks. In 1934, the ship Maurguani encountered a whale shark while sailing in the South Pacific Ocean, and struck it, causing it to get stuck in the bow of the ship, it is said to have been 4, 6m on one side of the ship and 12.3m on the other. However, there is no reliable documentation of any of these facts, so they remain little more than "marine legend".
Food
It is one of three species of sharks that feed by filtering the water, along with the basking shark (Cetorhinus maximus) and the largemouth shark (Megachasma pelagios). It feeds mainly on phytoplankton, nekton, macroalgae, and krill, but sometimes also on crustaceans, such as crab larvae, squid, and schools of small fish, such as anchovies, sardines, mackerel, and tuna. The numerous teeth that it has do not play any decisive role in feeding, in fact, they are small. Instead of teeth, the whale shark sucks in a large amount of water, and when it closes its mouth it filters it through its gill combs. In the short time interval between closing its mouth and opening its gill combs, plankton become trapped in the dermal denticles. This filtering mechanism prevents the passage of any fluid between the gills, and everything that measures more than 2 or 3 millimeters in diameter is trapped and immediately engulfed. These sharks have been observed to emit a kind of cough, which is a cleaning mechanism to expel the accumulation of food particles in the gills.
It locates fish or plankton concentrations by scent cues, but instead of constantly drinking water, it is able to pump it through its gills, and can absorb water at a rate of 1.7 l/s. The whale shark does not need to move while feeding, and is often seen upright and moving up and down while actively pumping and filtering the water, unlike the basking shark (Cetorhinus maximus), which has a more passive way of feeding and does not pump the water, but when swimming it drives the water towards its gills.
They congregate around the reefs of the Caribbean coast of Belize, supplementing their daily diet with the roe of the cubera snapper, which deposits them in the phases of the full moon and the waxing and waning quarter of the Moon in the months of May, June and July.
Relationship with humans
This species, despite its enormous size, does not pose any danger to humans. It is a widely cited example given the fame that sharks have as people-eaters. They are actually quite affectionate, and are often playful with divers. There are even unconfirmed reports of whale sharks surfacing face up for belly-scratching by a diver to remove parasites.
This shark is very often observed by divers and by tourists aboard boats off the Yucatan peninsula on the island of Holbox, in the Bay Islands of Honduras, in the Maldives, the Galapagos Islands of Ecuador; in the Philippines, Thailand, the Red Sea, Ningaloo Reef and Christmas Island in Western Australia, Tofo Beach in Mozambique, and Sodwana Bay in South Africa. Some of these locations, such as Western Australia, have become focal points for the ecotourism industry.
The largest concentration of whale sharks in the world is found in the Philippines. Between the months of January and May, they congregate on the shallow shores of Donsol, in the province of Sorsogon. Some very lucky divers have found whale sharks in Puerto Rico, the Dominican Republic and the Seychelles. Between September and May, it is very common to come across a specimen in the bay of La Paz, in the Mexican state of Baja California Sur, as well as from May to September on the northeast coast of the Mexican state of Quintana Roo. Sometimes they are accompanied by small fish, such as the remora. Recently, they have been sighted in the vicinity of Tenggol Island, located on the west coast of Peninsular Malaysia, where there are also several coral reefs frequented by these sharks, such as Redang Island or Kapas Island.
Playback
Their reproductive habits are unclear. Through observations of a female in 1910 that had 16 eggs in one of its oviducts, it was erroneously deduced that they were viviparous. In 1956, an egg was studied on the coast of Mexico, and everything indicated that they were oviparous beings, but in July 1996 a female was discovered off the coast of Taiwan that had about 300 eggs (the highest number recorded of any shark species), which would show that they are ovoviviparous. The young hatch inside from their mother, who gives birth to them alive. Newborn sharks usually measure between 40 and 60 centimeters in length, but little is known about them, since young specimens are rarely seen, and morphometric studies have not been carried out, nor is much known about their growth rate. They are believed to reach sexual maturity at around 30 years (9 m), with an average life of about 100.
Conservation status
It is the target of artisanal fishing and the fishing industry in several coastal areas where it is occasionally seen. The population of this species is unknown, but it is considered by the IUCN as an endangered species. All fishing, sale, import and export of whale sharks for commercial purposes will be prohibited and penalized. In the Philippines, this law has been applied since 1998, and in Taiwan since May 2007, a country where approximately 100 specimens were killed each year.
In captivity
The main attraction at the Kaiyukan Aquarium, in Osaka, Japan, is a whale shark, and as of 2005, three specimens are being studied in captivity at Japan's Churaumi Aquarium. At the Georgia Aquarium, located in Atlanta, four more of these sharks are kept, two males, named Taroko and Yushan, and two females, named Alice and Trixie. In this same aquarium, Ralph and Norton, two captive male sharks, died on January 11, 2007 and June 13, 2007 respectively. The two males currently in the aquarium were added on June 3 of 2006 with the aim of studying the reproduction of this species in captivity. The six whale sharks were brought from Taiwan, where sharks are a rich and prized delicacy due to the texture and flavor of their meat.[citation needed]
Migration
Migration patterns in Mexico of whale sharks
Two studies have been carried out in parallel, one in the northwest of the country and the other in the southeast. It has been possible to verify that the species does not have established patterns in terms of the migration process. Approximately 20% of the young whale sharks that arrive in the Gulf of California are distributed between the bays of Los Angeles and La Paz. Most of them travel between 2,100 and 4,900 kilometers. The young specimens remain within the Gulf of California possibly to protect themselves from predators and to obtain food. Adults travel south and pregnant females leave the Gulf of California.
In a nine-year National Geographic Society-funded study at the Mote Marine Laboratory, the migration patterns of this species have been mapped via satellite. It has been observed that between May and September in a town in the northeast of the Yucatan peninsula, up to 800 specimens gather in a site rich in plankton. They return to the same site after six years and even return for several consecutive years, to later distribute themselves in neighboring areas.
The twelve main sighting locations are located between western Australia and Indonesia to Belize but cannot be compared in quantity with the one located in the Caribbean. At the moment there is no knowledge of its activity during the colder half of the year.
Etymology
Rhincodon: {gr, rhyngchos}, snout + {gr, odous}, tooth.
typus: {gr, typos, -ou}, stroke or printed mark.
Contenido relacionado
Plantago lanceolata
Fumaria capreolata
Lophopogon