Basking shark

Basking shark

Overview
The basking shark is the second largest living fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, after the whale shark
Whale shark
The whale shark, Rhincodon typus, is a slow-moving filter feeding shark, the largest extant fish species. The largest confirmed individual had a length of and a weight of more than , but unconfirmed claims report considerably larger whale sharks...

. It is a cosmopolitan
Cosmopolitan distribution
In biogeography, a taxon is said to have a cosmopolitan distribution if its range extends across all or most of the world in appropriate habitats. For instance, the killer whale has a cosmopolitan distribution, extending over most of the world's oceans. Other examples include humans, the lichen...

  migratory
Fish migration
Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...

 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

, found in all the world's temperate oceans. It is a slow moving and generally harmless filter feeder
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...

 and has anatomical adaptations to filter feeding, such as a greatly enlarged mouth and highly developed gill rakers. The shape of its snout is conical and the gill slits extend around the top and bottom of its head. The gill rakers are dark and bristle-like and are used to catch plankton as water filters through the mouth and over the gills.
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Encyclopedia
The basking shark is the second largest living fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...

, after the whale shark
Whale shark
The whale shark, Rhincodon typus, is a slow-moving filter feeding shark, the largest extant fish species. The largest confirmed individual had a length of and a weight of more than , but unconfirmed claims report considerably larger whale sharks...

. It is a cosmopolitan
Cosmopolitan distribution
In biogeography, a taxon is said to have a cosmopolitan distribution if its range extends across all or most of the world in appropriate habitats. For instance, the killer whale has a cosmopolitan distribution, extending over most of the world's oceans. Other examples include humans, the lichen...

  migratory
Fish migration
Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousands of kilometres...

 species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

, found in all the world's temperate oceans. It is a slow moving and generally harmless filter feeder
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...

 and has anatomical adaptations to filter feeding, such as a greatly enlarged mouth and highly developed gill rakers. The shape of its snout is conical and the gill slits extend around the top and bottom of its head. The gill rakers are dark and bristle-like and are used to catch plankton as water filters through the mouth and over the gills. The basking shark is usually grayish-brown in colour and often seems to have a mottled appearance. 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 themselves have a single conical cusp, are curved backwards and are the same on both the upper and lower jaws.

Basking sharks are a migrating species and are believed to overwinter in deep waters. They may occur in either small schools or alone. Small schools in the Bay of Fundy
Bay of Fundy
The Bay of Fundy is a bay on the Atlantic coast of North America, on the northeast end of the Gulf of Maine between the Canadian provinces of New Brunswick and Nova Scotia, with a small portion touching the U.S. state of Maine...

 have been seen swimming nose to tail in circles in what may be a form of mating behavior. Basking sharks are not aggressive and generally harmless to people.

It has long been a commercially important
Commercial fishing
Commercial fishing is the activity of catching fish and other seafood for commercial profit, mostly from wild fisheries. It provides a large quantity of food to many countries around the world, but those who practice it as an industry must often pursue fish far into the ocean under adverse conditions...

 fish, as a source of food, shark fin
Shark finning
Shark finning refers to the removal and retention of shark fins and the discarding of the rest of the fish. Shark finning takes place at sea so the fishers only have to transport the fins.Shark finning is widespread, and largely unmanaged and unmonitored...

, animal feed, and shark liver oil
Shark liver oil
Shark liver oil is obtained from sharks that are caught for food purposes and are living in cold, deep oceans. The liver oil from sharks has been used by fishermen for centuries as a folk remedy for general health...

. Over-exploitation has reduced its populations to the point that some have apparently disappeared and others need protection.

Taxonomy


This shark is called the basking shark because it is most often observed when feeding at the surface and appears to be basking in the warmer water there. It is the only member of the family Cetorhinidae, part of the mackerel shark order Lamniformes
Lamniformes
Lamniformes is an order of sharks commonly known as mackerel sharks . It includes some of the most familiar species of sharks, such as the great white shark, as well as more unusual representatives, such as the goblin shark and the megamouth shark.Members of the order are distinguished by...

. 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". The following centuries featured more attempts at naming: Squalus isodus, in 1819 by Macri; Squalus elephas, by Lesueur
Charles Alexandre Lesueur
Charles Alexandre Lesueur was a French naturalist, artist and explorer.Pictured here is the oil portrait by Charles Willson Peale of Charles-Alexandre 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 & Adriani 1953; and finally Cetorhinus maximus normani, by Siccardi 1961. Other names include bone shark, elephant shark, hoe-mother, and sun-fish.

Range and habitat


The basking shark is a coastal-pelagic shark found worldwide in boreal
Boreal ecosystem
The term boreal is usually applied to ecosystems localized in subarctic and subantarctic zones, although Austral is also used for the latter....

 to warm-temperate waters around the continental shelves. It prefers 8 to 14.5 °C (46.4 to 58.1 F) temperatures, but recently 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
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

 concentrations in the water column and is therefore often visible at the surface. They characteristically migrate with the seasons. The basking shark is found from the surface down to at least 910 metres (2,985.6 ft).

Anatomy and appearance



The largest accurately-measured specimen was trapped in a herring net in the Bay of Fundy
Bay of Fundy
The Bay of Fundy is a bay on the Atlantic coast of North America, on the northeast end of the Gulf of Maine between the Canadian provinces of New Brunswick and Nova Scotia, with a small portion touching the U.S. state of Maine...

, Canada in 1851. Its total length was 12.27 metres (40.3 ft), and it weighed an estimated 19 tonnes (18.7 LT). There are dubious reports from Norway of three basking sharks over 12 metres (39.4 ft), the largest at 13.7 metres (44.9 ft), 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 (19.7–26.2 ft) and weighs about 5.2 tonnes (5.1 LT). Some specimens still surpass 9–10 m (29.5–32.8 ), but after years of large-scale fishing, specimens of this size have become rare.

They possess the typical shark lamniform body plan and have been mistaken for great white shark
Great white shark
The great white shark, scientific name Carcharodon carcharias, also known as the great white, white pointer, white shark, or white death, is a large lamniform shark found in coastal surface waters in all major oceans. It is known for its size, with the largest individuals known to have approached...

s. The two species can be easily distinguished, however, by the basking shark's cavernous jaw, up to 1 metres (3.3 ft) in width, longer and more obvious gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

 slits that nearly encircle the head and are accompanied by well-developed gill raker
Gill raker
Gill rakers in fish are bony or cartilaginous processes that project from the branchial arch and are involved with filter feeding tiny prey. They are not to be confused with the gill filaments that compose the bony part of the gill. Rakers are usually present in two rows, projecting from both the...

s, smaller eyes, and smaller average girth. Great whites possess large, dagger-like teeth, basking shark teeth are much smaller 5–6 mm (0.196850393700787–0.236220472440945 ) and hooked; only the first 3 or 4 rows of the upper jaw and 6 or 7 rows of the lower jaw function. There are also several obvious behavioral differences between the two, since 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
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...

 may flop to one side when above the surface. Coloration 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 lamprey
Lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s or cookiecutter shark
Cookiecutter shark
The cookiecutter shark , also called the cigar shark, is a species of small dogfish shark in the family Dalatiidae. This shark occurs in warm, oceanic waters worldwide, particularly near islands, and has been recorded from as deep as . It migrates vertically up to every day, approaching the...

s. 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
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...

 regulation and long-term energy storage.

Life history



Studies in 2003 proved that basking sharks do not hibernate, showing that they are active year-round. In winter, basking sharks move to depths of up to 900 metres (2,952.8 ft) to feed on deep water plankton.

Migration


Satellite tagging confirms that basking sharks move thousands of kilometres during the winter months, seeking plankton blooms. It also found that basking sharks shed and renew their gill raker
Gill raker
Gill rakers in fish are bony or cartilaginous processes that project from the branchial arch and are involved with filter feeding tiny prey. They are not to be confused with the gill filaments that compose the bony part of the gill. Rakers are usually present in two rows, projecting from both the...

s in an ongoing process, rather than over one short period.

A 2009 study tagged 25 sharks off the coast of Cape Cod
Cape Cod
Cape Cod, often referred to locally as simply the Cape, is a cape in the easternmost portion of the state of Massachusetts, in the Northeastern United States...

, Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...

, and indicated that at least some individuals migrate south in the winter. Remaining at depths between 200 metres (656.2 ft) and 1000 metres (3,280.8 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
Amazon River
The Amazon of South America is the second longest river in the world and by far the largest by waterflow with an average discharge greater than the next seven largest rivers combined...

. It is unknown why they undertake this journey. Lead author Gregory Skomal of the Massachusetts Division of Marine Fisheries, suspects it may be related to reproduction.

They are slow-moving sharks (feeding at about 2 knots (1.1 m/s)) and do not evade approaching boats (unlike great white sharks). They are harmless to humans if left alone and are not attracted to chum.

Even though the basking shark is large and slow, it can breach, jumping entirely out of the water. This behaviour could be an attempt to dislodge parasites or commensals
Commensalism
In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...

. Such interpretations are speculative however, and difficult to falsify; breaching in large marine organisms such as whales and sharks might equally well be intraspecific
Intraspecific competition
Intraspecific competition is a particular form of competition in which members of the same species vie for the same resource in an ecosystem...

 threat
Threat display
Threat display is a type of display behaviour aiming at intimidation of a potential enemy. It may be directed at a rival of the same species , or at a potential threat from a different species....

 display
Display (zoology)
Display is a form of animal behaviour, linked to survival of the species in various ways. One example of display used by some species can be found in the form of courtship, with the male usually having a striking feature that is distinguished by colour, shape or size, used to attract a female...

 of size and strength.

Interactions



Basking sharks are social animals and form sex-segregated schools, usually in small numbers (3 or 4) 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 conspecifics. Females are thought to seek shallow water to give birth.

Predators


While basking sharks have few if any predators, white sharks have been reported to scavenge on the remains of these sharks. Observers have long suspected that killer whales, also known as orcas, actively pursue and feed on basking sharks; yet, this is based upon the account of only one person, a West Cornwall fisherman who claims to have witnessed a frenzied attack by a killer whale on a large basking shark off Porthcurno more than 50 years ago.

Lamprey
Lamprey
Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

s are often seen attached to them, although it is unlikely that they are able to cut through the shark's thick skin.

Diet



The basking shark is a passive filter feeder
Filter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...

, filtering zooplankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

, small fish and invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

s from up to 2000 short tons (1,785.7 LT) 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
Megamouth shark
The megamouth shark, Megachasma pelagios, is an extremely rare species of deepwater shark. Since its discovery in 1976, only a few megamouth sharks have been seen, with 53 specimens known to have been caught or sighted as of 2011, including three recordings on film...

 and whale shark
Whale shark
The whale shark, Rhincodon typus, is a slow-moving filter feeding shark, the largest extant fish species. The largest confirmed individual had a length of and a weight of more than , but unconfirmed claims report considerably larger whale sharks...

, the basking shark does not appear to actively seek quarry, but it does possess large olfactory bulb
Olfactory bulb
The olfactory bulb is a structure of the vertebrate forebrain involved in olfaction, the perception of odors.-Anatomy:In most vertebrates, the olfactory bulb is the most rostral part of the brain. In humans, however, the olfactory bulb is on the inferior side of the brain...

s that may guide it. It relies only on the water that it pushes through its gills by swimming; the megamouth shark and whale shark can suck or pump water through their gills.

Reproduction


Basking sharks are ovoviviparous
Ovoviviparity
Ovoviviparity, ovovivipary, or ovivipary, is a mode of reproduction in animals in which embryos develop inside eggs that are retained within the mother's body until they are ready to hatch...

: the developing embryos first rely on a yolk sac, and there is no placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...

l connection. Their seemingly useless teeth may play a role before birth in helping them feed on the mother's unfertilized ova
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...

 (a behaviour known as oophagy
Oophagy
Oophagy , literally "egg eating", is the practice of embryos feeding on eggs produced by the ovary while still inside the mother's uterus. The word oophagy is formed from the classical Greek ᾠόν and classical Greek φᾱγεῖν ....

). In females, only the right ovary
Ovary
The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

 appears to function.

Gestation
Gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....

 is thought to span over a year (perhaps 2 or 3 years), with a small though unknown number of young born fully developed at 1.5–2 m (4.9–6.6 ). Only one pregnant female is known to have been caught; she was carrying 6 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 not known but is thought to be between the ages of 6 and 13 and at a length of 4.6–6 m (15.1–19.7 ). Breeding frequency is also unknown, but is thought to be 2 to 4 years.

Importance to humans


Historically, the basking shark has been a staple of fisheries because of its slow swimming speed, unaggressive nature and previously abundant numbers. Commercially it was put to many uses: the flesh for food and fishmeal, the hide for leather
Leather
Leather is a durable and flexible material created via the tanning of putrescible animal rawhide and skin, primarily cattlehide. It can be produced through different manufacturing processes, ranging from cottage industry to heavy industry.-Forms:...

, and its large liver
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...

 (which has a high squalene
Squalene
Squalene is a natural organic compound originally obtained for commercial purposes primarily from shark liver oil, though plant sources are used as well, including amaranth seed, rice bran, wheat germ, and olives. All plants and animals produce squalene, including humans...

 content) for oil
Oil
An oil is any substance that is liquid at ambient temperatures and does not mix with water but may mix with other oils and organic solvents. This general definition includes vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils....

. It is currently fished mainly for its fins (for shark fin soup
Shark fin soup
Shark fin soup is a popular soup item of Chinese cuisine usually served at special occasions such as weddings and banquets, or as a luxury item in Chinese culture. The shark fins provide texture while the taste comes from the other soup ingredients.There is controversy over the practice of shark...

). Parts (such as cartilage
Shark cartilage
Shark cartilage is a dietary supplement made from the dried and powdered cartilage of a shark; that is, from the tough material that composes a shark's skeleton. Shark cartilage is claimed to combat and/or prevent a variety of illnesses, most notably cancer. It is often marketed under the names...

) are also used in traditional Chinese medicine
Traditional Chinese medicine
Traditional Chinese Medicine refers to a broad range of medicine practices sharing common theoretical concepts which have been developed in China and are based on a tradition of more than 2,000 years, including various forms of herbal medicine, acupuncture, massage , exercise , and dietary therapy...

 and as an aphrodisiac
Aphrodisiac
An aphrodisiac is a substance that increases sexual desire. The name comes from Aphrodite, the Greek goddess of sexuality and love. Throughout history, many foods, drinks, and behaviors have had a reputation for making sex more attainable and/or pleasurable...

 in Japan, further adding to demand.

As a result of rapidly declining numbers, the basking shark has been protected and trade in its products restricted in many countries. It is fully protected in the UK, Ireland, Malta, Florida and USA Gulf and Atlantic waters. Targeted fishing for basking sharks is illegal in New Zealand. Once considered a nuisance along the Canadian Pacific coast
British Columbia Coast
The British Columbia Coast or BC Coast is Canada's western continental coastline on the Pacific Ocean. The usage is synonymous with the term West Coast of Canada....

, basking sharks were the target of a government eradication program there from 1945 to 1970. , efforts are underway 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.

Basking sharks and cryptozoology



On several occasions, "globster
Globster
A globster, or blob, is an unidentified organic mass that washes up on the shoreline of an ocean or other body of water. The term was coined by Ivan T. Sanderson in 1962 to describe the Tasmanian carcass of 1960, which was said to have "no visible eyes, no defined head, and no apparent bone...

" corpses initially thought to be sea serpent
Sea serpent
A sea serpent or sea dragon is a type of sea monster either wholly or partly serpentine.Sightings of sea serpents have been reported for hundreds of years, and continue to be claimed today. Cryptozoologist Bruce Champagne identified more than 1,200 purported sea serpent sightings...

s or plesiosaur
Plesiosaur
Plesiosauroidea is an extinct clade of carnivorous plesiosaur marine reptiles. Plesiosauroids, are known from the Jurassic and Cretaceous Periods...

s have later been identified as likely to be the decomposing carcasses of basking sharks, as in the Stronsay beast
Stronsay Beast
The Stronsay beast was a large, dead carcass or globster that washed ashore on the island of Stronsay , in the Orkney Islands, after a storm on September 25, 1808...

 and the Zuiyo Maru
Zuiyo Maru
The Zuiyo-maru carcass is a creature initially claimed to be a prehistoric plesiosaur that was caught by the Japanese fishing trawler off the coast of New Zealand in 1977...

cases.

See also



External links