Basking shark - WikiHQ

The basking shark (Cetorhinus maximus) is the second-largest living shark and fish, after the whale shark. It is one of three plankton-eating shark species, along with the whale shark and megamouth shark. Typically, basking sharks reach in length, but large individuals have been known to grow more than long. It is usually greyish-brown, with mottled skin, with the inside of the mouth being white in colour. The caudal fin has a strong lateral keel and a crescent shape. Other common names include bone shark, elephant shark, sailfish, and sunfish.

The basking shark is a cosmopolitan migratory species found in all the world's temperate oceans. A slow-moving filter feeder, its common name derives from its habit of feeding at the surface, appearing to be basking in the warmer water there. It has anatomical adaptations for filter-feeding, such as a greatly enlarged mouth and highly developed gill rakers. Its snout is conical, and the gill slits extend around the top and bottom of its head. The gill rakers, dark and bristle-like, are used to catch plankton as water filters through the mouth and over the gills. The teeth are numerous, often numbering 100 per row. The teeth are very small, have a single conical cusp, are curved backwards, and are the same on both the upper and lower jaws. This species has the smallest weight-for-weight brain size of any shark, reflecting its relatively passive lifestyle.

Basking sharks have been shown from satellite tracking to overwinter in both continental shelf (less than ) and deeper waters. They may be found either in small shoals or alone. Despite their large size and threatening appearance, basking sharks are not aggressive and are harmless to humans.

The basking shark has long been a commercially important fish as a source of food, shark fin, animal feed, and shark liver oil. Overexploitation has reduced its populations to the point where some have disappeared and others need protection.

Taxonomy

The basking shark is the only extant member of the family Cetorhinidae, part of the mackerel shark order Lamniformes. Johan Ernst Gunnerus first described the species as Cetorhinus maximus from a specimen found in Norway. The genus name Cetorhinus was published without an etymology, but is presumed to be from the Greek ketos, meaning "marine monster" or "whale", and rhine, an ancient obscure Greek word meaning "shark". The species name maximus is from Latin and means "greatest". Following its initial description, more attempts at naming included: Squalus isodus, by Italian zoologist Saverio Macri (1754–1848) in 1819; Squalus elephas, by Charles Alexandre Lesueur in 1822; Squalus rashleighanus, by Jonathan Couch in 1838; Squalus cetaceus, by Laurens Theodorus Gronovius in 1854; Cetorhinus blainvillei, by the Portuguese biologist Felix Antonio de Brito Capello (1828–1879) in 1869; Selachus pennantii, by Charles John Cornish in 1885; Cetorhinus maximus infanuncula, by Dutch zoologists Antonius Boudewijn Deinse (1885–1965) and Marcus Jan Adriani (1929–1995) in 1953; and Cetorhinus maximus normani, by Siccardi in 1961. In Orkney, it is called hoe-mother (contracted homer), meaning "the mother of the piked dogfish".

Evolutionary history

The oldest known members of Cetorhinidae are members of the extinct genus Keasius, from the middle Eocene of Antarctica, the Eocene of Oregon, and possibly the Eocene of Russia. Members of the modern genus Cetorhinus appear during the Miocene, with members of the modern species appearing during the Late Miocene. The association of Pseudocetorhinus from the Late Triassic of Europe with Cetorhinidae is doubtful.

Range and habitat

The basking shark is a coastal-pelagic shark found worldwide in boreal to warm-temperate waters. It lives around the continental shelf and occasionally enters brackish waters. It characteristically migrates with the seasons.

Anatomy and appearance

The basking shark regularly reaches in length with some individuals reaching . The average length of an adult is around weighing about . A specimen trapped in a herring net in the Bay of Fundy, Canada in 1851 has been credited as the largest recorded. Its weight has been estimated at . It is the second-largest extant fish species, after the whale shark. The two species can be easily distinguished by the basking shark's cavernous jaw, up to in width, longer and more obvious gill slits that nearly encircle the head and are accompanied by well-developed gill rakers, smaller eyes, much larger overall size, and smaller average girth. Great whites possess large, dagger-like teeth, while basking shark teeth are much smaller at , and hooked; only the first three or four rows of the upper jaw and six or seven rows of the lower jaw function. In behaviour, the great white is an active predator of large animals, not a filter feeder.

Other distinctive characteristics include a strongly keeled caudal peduncle, highly textured skin covered in placoid scales and a mucus layer, a lunate caudal fin, and a pointed snout—distinctly hooked in younger specimens. In large individuals, the dorsal fin may flop to one side when above the surface. Colouration is highly variable (and likely dependent on observation conditions and the individual's condition): commonly, the colouring is dark brown to black or blue dorsally, fading to a dull white ventrally. The sharks are often noticeably scarred, possibly through encounters with lampreys or cookiecutter sharks. The basking shark's liver, which may account for 25% of its body weight, runs the entire length of the abdominal cavity and is thought to play a role in buoyancy regulation and long-term energy storage.

On several occasions, decomposing basking shark remains were the most likely explanations for "globsters," or initially unidentified remains found washed up on beaches or otherwise encountered. Examples include the Stronsay Beast from the early 1800s and the Zuiyo-maru case of the 1970s.

Life history

thumb|alt=Shot of head in profile with partially opened mouth|Head of a basking shark

Basking sharks do not hibernate and are active year-round.

Surfacing behaviors

They are slow-moving sharks (feeding at about ) and do not evade approaching boats (unlike great white sharks). They are not attracted to chum.

The basking shark is large and slow, but it can breach jump entirely out of the water. This behaviour could be an attempt to dislodge parasites or commensals. They shed and renew their gill rakers in an ongoing process, rather than over one short period.

A 2009 study tagged 25 sharks off the coast of Cape Cod, Massachusetts, and indicated at least some migrate south in the winter. Remaining at depths between for many weeks, the tagged sharks crossed the equator to reach Brazil. One individual spent a month near the mouth of the Amazon River. They may undertake this journey to aid reproduction.

On 23 June 2015, a , basking shark was caught accidentally by a fishing trawler in the Bass strait near Portland, Victoria, in southeast Australia, the first basking shark caught in the region since the 1930s, and only the third reported in the region in 160 years. The whole shark was donated to the Victoria Museum for research, instead of the fins being sold for use in shark fin soup.

While basking sharks are not infrequently seen in the Mediterranean Sea and records exist in the Dardanelles Strait, it is unclear whether they historically reached deeper basins of the Sea of Marmara, Black Sea, and Sea of Azov.

thumb|left|A basking shark filter feeding

Basking sharks are usually solitary, but during summer months in particular, they aggregate in dense patches of zooplankton, where they engage in social behaviour. They can form sex-segregated shoals, usually in small numbers (three or four), but reportedly up to 100 individuals.

Predators

Basking sharks have few predators. White sharks have been reported to scavenge on the remains of these sharks. Killer whales have been observed feeding on basking sharks off California in the US and New Zealand. Lampreys are often seen attached to them, although they are unlikely to be able to cut through the shark's thick skin.

Diet

thumb|Basking shark filter feeding at [[Dursey Sound|alt=Basking shark filter feeding]]

The basking shark is a ram feeder, filtering zooplankton, very small fish, and invertebrates from the water with its gill rakers by swimming forwards with its mouth open. A basking shark has been calculated to filter up to of water per hour swimming at an observed speed of . Basking sharks feed preferentially in zooplankton patches dominated by small planktonic crustaceans called calanoid copepods (on average 1,700 individuals per cubic metre of water). They will also feed on copepods of the genera Pseudocalanus and Oithona. Basking sharks sometimes congregate in groups of up to 1,400 spotted along the northeastern U.S. Samples taken near feeding sharks contained 2.5 times as many Calanus helgolandicus individuals per cubic metre, which were also found to be 50% longer. Unlike the megamouth shark and whale shark, the basking shark relies only on the water it pushes through its gills by swimming; the megamouth shark and whale shark can suck or pump water through their gills. In females, only the right ovary appears to function, and it is currently unknown why only one of the organs seems to function.

Gestation is thought to span over a year (perhaps two to three years), with a small, though unknown, number of young born fully developed at . Only one pregnant female is known to have been caught; she was carrying six unborn young.

Mating is thought to occur in early summer, and birthing in late summer, following the female's movement into shallow waters.

The age of maturity is thought to be between the ages of six and 13 and at a length of . Breeding frequency is thought to be two to four years.

The exact lifespan of the basking shark is unknown, but experts estimate it to be about 50 years. The generation cycle is estimated to be 22-34 years. confirmations of the species became very scarce. Management plans have been declared to promote effective conservation. In June 2018, the Department of Conservation classified the basking shark as "Threatened - Nationally Vulnerable" under the New Zealand Threat Classification System.

The eastern north Pacific Ocean population is a U.S. National Marine Fisheries Service species of concern, one of those species about which the U.S. Government's National Oceanic and Atmospheric Administration has some concerns regarding status and threats, but for which insufficient information is available to indicate a need to list the species under the U.S. Endangered Species Act (ESA).

The IUCN Red List indicates this as an endangered species. As of March 2010, it was also listed under Annex I of the CMS Migratory Sharks Memorandum of Understanding.

Once considered a nuisance along the Canadian Pacific coast, basking sharks were the target of a government eradication programme from 1945 to 1970. Since 1994, basking shark sightings in the area became rare, and in 2008 efforts were made to determine whether any sharks still lived in the area and monitor their potential recovery. One shark was spotted in June 2024 near Wallace Island Marine Provincial Park.

References

;General references

David A Ebert, Sharks, Rays and Chimaeras of California,
Marine Conservation Society Basking shark page
FAO Figis Species Fact Sheet for basking shark

External links

Irish Basking Shark Project
BBC Wildlife Finder – video news and news from the BBC archive
ARKive entry on the Basking Shark
Fisheries & Oceans Canada – Basking sharks on the west coast of Canada
Basking Sharks in the Isle of Man
Basking sharks featured on RNZ Critter of the Week, 24 Jan 2020
Basking Shark - Video on Check123 - Video Encyclopedia