Cetorhinus maximus infanunculaDeinse & Adriani, 1953
Cetorhinus normaniSiccardi, 1961
Hanovera auratavan Beneden, 1871
Halsydrus pontoppidiani*Neill, 1809
Polyprosopus macerCouch, 1862
Scoliophis atlanticus*Anonymous, 1817
Selachus pennantiiCornish, 1885
Squalis gunneri*Blainville, 1816
Squalis shavianus*Blainville, 1816
Squalus cetaceusGronow, 1854
Squalus elephasLesueur, 1822
Squalus gunnerianusBlainville, 1810
Squalus homianusBlainville, 1810
Squalus isodusMacri, 1819
Squalus maximusGunnerus, 1765
Squalus pelegrinusBlainville, 1810
Squalus rashleighanusCouch, 1838
Squalus rhinoceros*DeKay, 1842
Squalus rostratusMacri, 1819
Tetraoras angiova*Rafinesque, 1810
Tetroras angiova*Rafinesque, 1810
Tetroras maccoyiBarrett, 1933
----
* ambiguous synonym
Thebasking shark (Cetorhinus maximus) is the second-largest livingshark andfish, after thewhale shark.[4] It is one of threeplankton-eating shark species, along with the whale shark andmegamouth shark. Typically, basking sharks reach 7.9 m (26 ft) in length, but large individuals have been known to grow more than 10 m (33 ft) long. It is usually greyish-brown, with mottled skin, with the inside of the mouth being white in colour. Thecaudal 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 acosmopolitanmigratory species found in all the world's temperate oceans. A slow-movingfilter 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 developedgill 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 and very small and often number 100 per row. The teeth 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.[5]
Basking sharks have been shown from satellite tracking tooverwinter in both continental shelf (less than 200 m or 660 ft) and deeper waters.[6] They may be found in either small shoals or alone. Despite their large size and threatening appearance, basking sharks are not aggressive and are harmless to humans.
The basking shark is the onlyextant member of the familyCetorhinidae, part of the mackerel shark orderLamniformes.Johan Ernst Gunnerus first described the species asCetorhinus maximus, from a specimen found inNorway, naming it. The genus nameCetorhinus comes from the Greekketos, meaning "marine monster" or "whale", andrhina, an ancient obscure Greek word meaning "shark".[8] The species namemaximum is from Latin and means "greatest". Following its initial description, more attempts at naming included:Squalus isodus, in 1819 by Italian zoologist Saverio Macri (1754–1848);Squalus elephas, byCharles Alexandre Lesueur in 1822;Squalus rashleighanus, byJonathan Couch in 1838;Squalus cetaceus, byLaurens Theodorus Gronovius in 1854;Cetorhinus blainvillei by the Portuguese biologist Felix Antonio de Brito Capello (1828–1879) in 1869;Selachus pennantii, byCharles John Cornish in 1885;Cetorhinus maximus infanuncula, by Dutch zoologists Antonius Boudewijn Deinse (1885–1965) and Marcus Jan Adriani (1929–1995) in 1953; andCetorhinus maximus normani, by Siccardi in 1961.[9] InOrkney, it is called hoe-mother (contracted homer), meaning "the mother of thepiked dogfish".[10]
The oldest known members of Cetorhinidae are members of the extinct genusKeasius, from the middleEocene of Antarctica, the Eocene of Oregon and possibly the Eocene of Russia. Members of the modern genusCetorhinus appear during theMiocene, with members of the modern species appearing during the Late Miocene. The association ofPseudocetorhinus from the Late Triassic of Europe with Cetorhinidae is doubtful.[11]
The basking shark is a coastal-pelagic shark found worldwide inboreal to warm-temperate waters. It lives around thecontinental shelf and occasionally entersbrackish waters.[12] It is found from the surface down to at least 910 m (2,990 ft). It prefers temperatures of 8 to 14.5 °C (46.4 to 58.1 °F) but has been confirmed to cross the much warmer waters at the equator.[13] It is often seen close to land, including in bays with narrow openings. The shark followsplankton concentrations in the water column, so it is often visible at the surface.[14] It characteristically migrates with the seasons.[15]
The basking shark regularly reaches 7–8.5 m (23–28 ft) in length with some individuals reaching 9–11 m (30–36 ft).[16][17][18][19][20] The average length of an adult is around 7.9 m (26 ft) weighing about 4.65 t (4.58 long tons; 5.13 short tons).[16] Historical sightings suggest basking sharks around 12 m (39 ft) in length, including three basking sharks estimated at ~40fod (12.5 metres (41 ft)) and a one ~45 fod (14 metres (46 ft)) were reported between 1884 and 1905, but these visual estimates lack good evidence.[16][21][20][22] A 12.27 m (40.3 ft) specimen trapped in a herring net in theBay of Fundy, Canada, in 1851 has been credited as the largest recorded.[16][23] Its weight has been estimated at 16 t (16 long tons; 18 short tons).[24][16] A study looking at the growth and longevity of the basking shark suggested that individuals larger than ~10 m (33 ft) are unlikely.[25] It is the second-largest extant fish species, after the whale shark.[4]
Beached basking shark
They possess the typical sharklamniform body plan and have been mistaken forgreat white sharks.[26] The two species can be easily distinguished by the basking shark's cavernous jaw, up to 1 m (3 ft 3 in) in width, longer and more obviousgill slits that nearly encircle the head and are accompanied by well-developedgill rakers, smaller eyes, much larger overall size and smaller average girth. Great whites possess large, dagger-like teeth; basking shark teeth are much smaller 5–6 mm (0.20–0.24 in) 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 keeledcaudal peduncle, highly textured skin covered inplacoid scales and a mucus layer, a pointed snout—distinctly hooked in younger specimens—and a lunatecaudal fin.[27] In large individuals, thedorsal 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 withlampreys orcookiecutter 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 inbuoyancy regulation and long-term energy storage.
On several occasions, "globster" corpses initially identified by non-scientists as asea serpents orplesiosaurs have later been identified as likely to be the decomposing carcasses of basking sharks, as in theStronsay Beast and theZuiyo-maru cases.[28]
Basking sharks do not hibernate and are active year-round.[6] In winter, basking sharks often move to deeper depths, even down to 900 m (3,000 ft) and have been tracked making vertical movements consistent with feeding on overwintering zooplankton.[29]
They are slow-moving sharks (feeding at about 2 knots (3.7 kilometres per hour; 2.3 miles per hour))[30] and do not evade approaching boats (unlike great white sharks). They are not attracted tochum.
The basking shark is large and slow, but it canbreach jump entirely out of the water.[31] This behaviour could be an attempt to dislodge parasites orcommensals.[15] Such interpretations are speculative, however, and difficult to verify; breaching in large marine animals such as whales and sharks might equally well beintraspecificthreat displays of size and strength.
Argos system satellite tagging of 20 basking sharks in 2003 confirmed basking sharks move thousands of kilometres during the summer and winter, seeking the richest zooplankton patches, often alongocean fronts.[6][32] They shed and renew their gill rakers in an ongoing process, rather than over one short period.[33]
A 2009 study tagged 25 sharks off the coast ofCape Cod,Massachusetts, and indicated at least some migrate south in the winter. Remaining at depths between 200 and 1,000 metres (660 and 3,280 ft) for many weeks, the tagged sharks crossed the equator to reach Brazil. One individual spent a month near the mouth of theAmazon River. They may undertake this journey to aid reproduction.[13][34]
On 23 June 2015, a 6.1-metre-long (20 ft), 3,500-kilogram (7,716 lb) basking shark was caught accidentally by afishing trawler in theBass strait nearPortland,Victoria, in southeastAustralia, the first basking shark caught in the region since the 1930s, and only the third reported in the region in 160 years.[35][36] The whole shark was donated to theVictoria Museum for research, instead of the fins being sold for use inshark fin soup.[37][38]
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.[15] Small schools in theBay of Fundy and theHebrides have been seen swimming nose to tail in circles; their social behaviour in summer months has been studied and is thought to represent courtship.[41]
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.
The basking shark is a ramfeeder, filteringzooplankton, very small fish, andinvertebrates from the water with its gill rakers by swimming forwards with its mouth open. A 5-metre-long (16 ft) basking shark has been calculated to filter up to 500 short tons (450 t) of water per hour swimming at an observed speed of 0.85 metres per second (3.1 km/h; 1.9 mph).[33] Basking sharks are not indiscriminate feeders on zooplankton. Samples taken in the presence of feeding individuals recorded zooplankton densities 75% higher than adjacent non-feeding areas.[42] Basking sharks feed preferentially in zooplankton patches dominated by small planktonic crustaceans calledcalanoid copepods (on average 1,700 individuals per cubic metre of water). They will also feed on copepods of the generaPseudocalanus andOithona.[43] Basking sharks sometimes congregate in groups of up to 1,400 spotted along the northeastern U.S.[44] Samples taken near feeding sharks contained 2.5 times as manyCalanus helgolandicus individuals per cubic metre, which were also found to be 50% longer. Unlike themegamouth shark andwhale 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.[9]
Basking sharks areovoviviparous: the developing embryos first rely on ayolk sac, with noplacental connection. Their seemingly useless teeth may play a role before birth in helping them feed on the mother's unfertilizedova (a behaviour known asoophagy).[45] In females, only the rightovary appears to function, and it is currently unknown why only one of the organs seems to function.[46]
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 1.5–2 m (4 ft 11 in – 6 ft 7 in). Only one pregnant female is known to have been caught; she was carrying six unborn young.[47]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 4.6–6 m (15–20 ft). 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.[48][49]
Aside from direct catches, by-catches in trawl nets have been one of several threats to basking sharks. In New Zealand, basking sharks had been abundant historically; however, after the mass by-catches recorded in the 1990s and 2000s,[50] confirmations of the species became very scarce.[12] Management plans have been declared to promote effective conservation.[51][52] In June 2018 theDepartment of Conservation classified the basking shark as "Threatened - Nationally Vulnerable" under theNew Zealand Threat Classification System.[53]
The "wonderful fish" described inHarper's Weekly on 24 October 1868, was likely the remains of a basking shark.
Historically, the basking shark has been a staple of fisheries because of its slow swimming speed, placid nature, and previously abundant numbers. Commercially, it was put to many uses: the flesh for food and fishmeal, the hide forleather, and its largeliver (which has a highsqualene content) foroil.[15] It is currently fished mainly for its fins (forshark fin soup). Parts (such ascartilage) are also used intraditional Chinese medicine and as anaphrodisiac in Japan, further adding to demand.
As a result of rapidly declining numbers, the basking shark has been protected in some territorial waters and trade in its products is restricted in many countries underCITES. Among others, it is fully protected in the United Kingdom and the Atlantic and Mexican Gulf regions of the United States.[47] Since 2008, it has been illegal to fish for, or retain if accidentally caught, basking sharks in waters of the European Union.[47] It is partially protected in Norway and New Zealand, as targetedcommercial fishing is illegal, but accidental bycatch can be used (in Norway, any basking shark caught as bycatch and still alive must be released).[2][47][55] As of March 2010, it was also listed under Annex I of theCMSMigratory Sharks Memorandum of Understanding.[56]
Once considered a nuisance along theCanadian 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,[57] and in 2008 efforts were made to determine whether any sharks still lived in the area and monitor their potential recovery.[58] One shark was spotted in June 2024 nearWallace Island Marine Provincial Park.[57]
It is tolerant of boats and divers approaching it and may even circle divers, making it an important draw for dive tourism in areas where it is common.[59]
^abC. Knickle; L. Billingsley & K. DiVittorio."Biological Profiles basking shark". Florida Museum of Natural History.Archived from the original on 21 August 2006. Retrieved24 August 2006.
^abcdCompagno, Leonard J. V. (1984)."CETORHINIDAE – Basking sharks"(PDF).Sharks of the World: An annotated and illustrated catalogue of shark species known to date. Food and Agriculture Organization of the United Nations.[permanent dead link]
^Collett, Robert (1905).Meddelelser om Norges Fiske i Aarene 1884–1901 (3die Hoved-Supplement til "Norges Fiske", III Slutning). Forhandlinger i Videnskabs–selskabet i Christiania.
^Pauly, D. (2002).Growth and Mortality of the Basking Shark Cetorhinus maximus and their Implications for Management of Whale Sharks Rhincodon typus. Vol. Elasmobranch biodiversity, conservation, and management : proceedings of the international seminar and workshop, Sabah, Malaysia, July 1997. Fowler, Sarah L., Reed, Tim M., Dipper, Frances, 1951-, IUCN--The World Conservation Union. Species Survival Commission. Gland, Switzerland: IUCN.ISBN2-8317-0650-5.OCLC50526779.
^"Basking Shark". San Francisco State University. Retrieved17 October 2014.
^"Basking shark". redorbit.com. 19 March 2007. Retrieved17 October 2014.
^Sims, DW (2000). "Filter-feeding and cruising swimming speeds of basking sharks compared with optimal models: they filter-feed slower than predicted for their size".Journal of Experimental Marine Biology and Ecology.249 (1):65–76.Bibcode:2000JEMBE.249...65S.doi:10.1016/s0022-0981(00)00183-0.PMID10817828.
