Basking shark

Basking shark

The **basking shark** (*Cetorhinus maximus*) is the second largest living fish, after the whale shark, and one of three plankton-eating sharks along with the whale shark and megamouth shark. Adults typically reach 6–8 m (20–26 ft) in length. They are usually greyish-brown, with mottled skin. The caudal fin has a strong lateral keel and a crescent shape.

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 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. This species has the smallest weight-for-weight brain size of any shark, reflecting its relatively passive lifestyle.

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 Italian Zoologist Saverio Macri (1754–1848); *Squalus elephas*, by Lesueur in 1822; *Squalus rashleighanus*, by Couch in 1838; *Squalus cetaceus*, by Gronow in 1854; *Cetorhinus blainvillei* by the Portuguese Biologist Felix Antonio De Brio Capello (1828–1879) in 1869; *Selachus pennantii*, by Cornish in 1885; *Cetorhinus maximus infanuncula*, by the Dutch Zoologists Antonius Boudewijn Deinse (1885–1965) and Marcus Jan Adriani (1929–1995) in 1953; and finally *Cetorhinus maximus normani*, by Siccardi in 1961. Other names include **bone shark**, **elephant shark**, **hoe-mother** (sometimes contracted to *homer*), **sail-fish**, and **sun-fish**with the latter two names potentially confusing it with the Mola mola or ocean sunfish, and the true sailfishes of the genus *Istiophorus*.

## Range and Habitat

The basking shark is a coastal-pelagic shark found worldwide in boreal to warm-temperate waters around the continental shelves, and entering into brackish waters on occasions. 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. 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. The basking shark is found from the surface down to at least 910 m (2,990 ft).

## Anatomy and Appearance

The largest accurately measured specimen was trapped in a herring net in the Bay of Fundy, Canada, in 1851. Its total length was 12.27 m (40.3 ft), and it weighed an estimated 19 t (19 long tons; 21 short tons). Reports from Norway mention three basking sharks over 12 m (39 ft), the largest at 13.7 m (45 ft), during 1884 to 1905, these however are considered 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). Some specimens still surpass 9–10 m (30–33 ft), but after years of large-scale fishing, specimens of this size have become rare.

The basking shark possesses the typical shark lamniform body plan and has often been mistaken for great white sharks. 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, 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 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. 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

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

### 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). In females, only the right ovary appears to function, and it is currently unknown why only one of these 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 1.5–2 m (4 ft 11 in–6 ft 7 in). Only one pregnant female basking shark is known to have been caught; she was carrying six unborn pups. 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.

## Behavior

Basking sharks 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, at speeds up to 5 meters per second (11 mph). This behaviour could be an attempt to dislodge parasites or commensals. 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.

### Migration

Argos system 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.

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.

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. 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 into deeper basins of Sea of Marmara, Black Sea and Azov Sea.

### Interactions

Basking sharks are social animals and form sex-segregated schools, usually in small numbers (three or four), but reportedly up to 100 individuals. 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. 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

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. 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.

## Conservation

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 1990s and 2000s, confirmations of the species became very scarce. Management plans have been declared to promote effective conservations.

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 a Vulnerable species.

The endangered aspect of this shark was publicised in 2005 with a postage stamp issued by Guernsey Post.

## 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. 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, Florida and USA Gulf, and since 2008, it is subject to a target fishing and landed bycatch ban within EU waters. 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. As of 2008, efforts were under way to determine whether any sharks still live in the area and monitor their potential recovery.

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.

Two fatal attacks, both provoked, are recorded for basking sharks; during the early 1900's, a fishery for basking sharks existed off California, but was eventually shut down due to the deaths of five harpooners in 1899 and again on a group of net fisherman, as a result of the retaliatory attacks by the sharks, which lashed out and capsized the boats or dragged them underwater. Both times the sharks were provoked. Since that time, no other attacks attributable to basking sharks have been recorded as the sharks are considered otherwise harmless.

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

## Basking sharks and cryptozoology

On several occasions, "globster" corpses initially thought to be the carcasses of 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.