Basking shark

Basking shark Temporal range: Early Oligocene–Present[1] PreЄ Є O S D C P T J K Pg N
Conservation status
Vulnerable  (IUCN 3.1)[2]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Superorder:	Selachimorpha
Order:	Lamniformes
Family:	Cetorhinidae T. N. Gill, 1862
Genus:	Cetorhinus Blainville, 1816
Species:	C. maximus
Binomial name
Cetorhinus maximus (Gunnerus, 1765)
Range of the basking shark
Synonyms
Cetorhinus blainvillei Capello, 1869 Cetorhinus maximus infanuncula Deinse & Adriani, 1953 Cetorhinus normani Siccardi, 1961 Hanovera aurata van Beneden, 1871 Halsydrus pontoppidiani* Neill, 1809 Polyprosopus macer Couch, 1862 Scoliophis atlanticus* Anonymous, 1817 Selachus pennantii Cornish, 1885 Squalis gunneri* Blainville, 1816 Squalis shavianus* Blainville, 1816 Squalus cetaceus Gronow, 1854 Squalus elephas Lesueur, 1822 Squalus gunnerianus Blainville, 1810 Squalus homianus Blainville, 1810 Squalus isodus Macri, 1819 Squalus maximus Gunnerus, 1765 Squalus pelegrinus Blainville, 1810 Squalus rashleighanus Couch, 1838 Squalus rhinoceros* DeKay, 1842 Squalus rostratus Macri, 1819 Tetraoras angiova* Rafinesque, 1810 Tetroras angiova* Rafinesque, 1810 Tetroras maccoyi Barrett, 1933 * ambiguous synonym

The basking shark (Cetorhinus maximus) is the second largest living fish, after the whale shark, and one of three plankton-eating sharks besides the whale shark and megamouth shark. It is a cosmopolitan migratory species, found in all the world's temperate oceans. It is 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 basking shark is usually greyish-brown, with mottled skin. The caudal (tail) fin has a strong lateral keel and a crescent shape. The teeth of the basking shark are very small and numerous, and often number one hundred per row. The teeth have a single conical cusp, are curved backwards, and are the same on both the upper and lower jaws. Adults typically reach 6–8 m (20–26 ft) in length.

Basking sharks are believed to overwinter in deep waters. They may be found in either small schools or alone. Small schools in the Bay of Fundy and the Hebrides have been seen swimming nose to tail in circles in what may be a form of mating behaviour. Despite their large size and threatening appearance, basking sharks are not aggressive and are harmless to humans.

It 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 member of the family Cetorhinidae, part of the mackerel shark order Lamniformes. Gunnerus was the first to describe and name the species Cetorhinus maximus from a specimen found in Norway. The genus name Cetorhinus comes from the Greek ketos which means marine monster or whale and rhinos meaning nose; the species name maximus is from Latin and means "greatest". In the following centuries there were more attempts at naming: Squalus isodus, in 1819 by Macri; Squalus elephas, by Lesueur in 1822; Squalus rashleighanus, by Couch in 1838; Squalus cetaceus, by Gronow in 1854; Cetorhinus blainvillei by Capello in 1869; Selachus pennantii, by Cornish in 1885; Cetorhinus maximus infanuncula, by Deinse and Adriani in 1953; and finally Cetorhinus maximus normani, by Siccardi in 1961.^[3] Other names include bone shark, elephant shark, hoe-mother (sometimes contracted to homer), sail-fish, and sun-fish.^[4]

Range and habitat

The basking shark is a coastal-pelagic shark found worldwide in boreal to warm-temperate waters around the continental shelves. It prefers 8 to 14.5 °C (46.4 to 58.1 °F) temperatures, but has been confirmed to cross the much-warmer waters at the equator.^[5] It is often seen close to land, including bays with narrow openings. The shark follows plankton concentrations in the water column, so is often visible at the surface. It characteristically migrates with the seasons.^[6] The basking shark is found from the surface down to at least 910 m (2,990 ft).^[7]

On 23 June 2015, a 6.1-meter-long (20 ft), 3,500 kilograms (7,716 lb) 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.^[8] The whole shark was donated to the Victoria Museum for research, instead of the fins being sold for use in shark fin soup.^[9][10] (Video)

Anatomy and appearance

The largest accurately measured specimen was trapped in a herring net in the Bay of Fundy, Canada, in 1851.^[11] Its total length was 12.27 m (40.3 ft), and it weighed an estimated 19 t (19 long tons; 21 short tons).^[12] Dubious reports from Norway mention three basking sharks over 12 m (39 ft), the largest at 13.7 m (45 ft), during 1884 to 1905,^[13] these are dubious because few anywhere near that size have been caught in the area since. On average, the adult basking shark reaches a length of 6–8 m (20–26 ft) and weighs about 5.2 t (5.1 long tons; 5.7 short tons).^[12] Some specimens still surpass 9–10 m (30–33 ft), but after years of large-scale fishing, specimens of this size have become rare.

Male basking shark

They possess the typical shark lamniform body plan and have been mistaken for great white sharks.^[14] 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 obvious gill slits that nearly encircle the head and are accompanied by well-developed gill rakers, smaller eyes, 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 and 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 pointed snout—distinctly hooked in younger specimens—and a lunate caudal fin.^[15] 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.

Life history

Head of a basking shark

Basking sharks do not hibernate, and are active year-round.^[16] In winter, basking sharks move to depths of 900 m (3,000 ft) to feed on deep-water plankton.

Migration

Satellite tagging confirmed basking sharks move thousands of kilometres during the winter, seeking plankton blooms. They shed and renew their gill rakers in an ongoing process, rather than over one short period.^[7]

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 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 the Amazon River. They may undertake this journey to aid reproduction.^[7][17]

They are slow-moving sharks (feeding at about 2 kn (3.7 km/h; 2.3 mph)) and do not evade approaching boats (unlike great white sharks). They are not attracted to chum.

Though the basking shark is large and slow, it can breach, jumping entirely out of the water.^[18] This behaviour could be an attempt to dislodge parasites or commensals.^[6] Such interpretations are speculative, however, and difficult to verify; breaching in large marine animals such as whales and sharks might equally well be intraspecific threat displays of size and strength.

Interactions

A basking shark filter feeding

Basking sharks are social animals and form sex-segregated schools, usually in small numbers (three or four), but reportedly up to 100 individuals.^[6] Their social behaviour is thought to follow visual cues. Although the basking shark's eyes are small, they are fully developed. They may visually inspect boats, possibly mistaking them for other basking sharks.^[19] Females are thought to seek shallow water to give birth.

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 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

Basking shark filter feeding at Dursey Sound

The basking shark is a passive feeder, filtering zooplankton, small fish, and invertebrates from up to 2,000 short tons (1,800 t) of water per hour.^[3] They feed at or close to the surface with their mouths wide open and gill rakers erect. Unlike the megamouth shark and whale shark, the basking shark does not appear to actively seek quarry, but it does possess large olfactory bulbs that may guide it. It 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.^[3]

Reproduction

Basking sharks are ovoviviparous: the developing embryos first rely on a yolk sac, with no placental connection. Their seemingly useless teeth may play a role before birth in helping them feed on the mother's unfertilized ova (a behaviour known as oophagy).^[20] In females, only the right ovary appears 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 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.^[21] 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 to be about 50 years.^[22][23]

Conservation

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).^[24]

The IUCN Red List indicates this as a Vulnerable species.

Importance to humans

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 for leather, and its large liver (which has a high squalene content) for oil.^[6] It is currently fished mainly for its fins (for shark fin soup). Parts (such as cartilage) are also used in traditional Chinese medicine and as an aphrodisiac 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 under CITES. It is fully protected in the UK, Malta, New Zealand,^[25] Florida and USA Gulf, and since 2008, it is subject to a target fishing and landed bycatch ban within EU waters.^[21] As of March 2010, it was also listed under Annex I of the CMS Migratory Sharks Memorandum of Understanding.^[26]

Once considered a nuisance along the Canadian Pacific coast, basking sharks were the target of a government eradication programme from 1945 to 1970. As of 2008, efforts were under way to determine whether any sharks still lived in the area and monitor their potential recovery.^[27]

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.

In 2015, a 6.3 meter basking shark was caught by a trawler in seas near Portland, in Southeastern Australia.^[28]

Basking sharks and cryptozoology

The "wonderful fish" described in Harper's Weekly on October 24, 1868, was likely the remains of a basking shark.

On several occasions, "globster" corpses initially thought to be sea serpents or plesiosaurs have later been identified as likely to be the decomposing carcasses of basking sharks, as in the Stronsay beast and the Zuiyo-maru cases.^[29]

See also

• List of prehistoric cartilaginous fish
• Shark liver oil
• Threatened sharks

References

General references

External links

Wikimedia Commons has media related to Cetorhinus maximus.

• Basking shark, Cetorhinus maximus MarineBio"
• Irish Basking Shark Project
• BBC Wildlife Finder - video news and news from the BBC archive
• ARKive entry on the Basking Shark
• Basking Shark Profile and Photos
• Basking Shark Project
• Fisheries & Oceans Canada - Basking sharks on the west coast of Canada
• Basking Sharks in the Isle of Man