Biology & Behaviour
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Information
The basking shark, scientific name Cetorhinus maximus, Manx name Gobbag vooar (big mouth) is the second largest fish in the world and we, in the Isle of Man, are fortunate enough to have a large number of them coming through our coastal waters from mid May to the beginning of September. They are named basking sharks because of their habit of 'basking' at the surface of the water. They feed by filtering out plankton from the water in a similar manner to whales. Despite their huge size they are completely harmless to man. Unlike bony fish that can produce millions of young in a year basking sharks are viviparous (live bearing) and therefore breed exceedingly slowly. They are, therefore, very vulnerable to over-exploitation.
Basking sharks often bask at the surface apparently doing nothing! This one is feeding.
If you have a picture of a basking shark just basking
we would be delighted to put it here!
Picture: Pauline Oliver.
Many scientific papers mourn the fact that little is known about these gentle giants. This isn't completely true as this website draws on 37 scientific reports about basking sharks and there are many more. Drs Matthews and Harrison carried out some remarkable dissections in 1947 and their scientific papers remain the classical works on basking shark anatomy[17,18]. Until recently there were large knowledge gaps about what basking sharks did in the winter but currently Dr Sims' team at Plymouth Marine laboratory[24-31] and researchers working in New Zealand [10] are well on the way to remedying this situation. Dr Colin Speedie [33] is working with the European Basking Shark Photo-Identification Project (EBSPiP) to try to understand how far individual basking sharks move. This work is complimentary to Dr Sims teams tagging work. No one is sure where the females have their young. Hopefully the resurgence in interest about basking sharks will encourage funding to help solve the remaining mysteries surrounding these wonderful fish.
Classification
Basking sharks are, like all sharks, part of the family of fishes known as elasmobranches. The skeleton of elasmobranch fish is made of cartilage, a material not unlike the hard gristle found around mammalian joints. This might be partially calcified, as it is in the basking sharks backbone but not to the same extent as in the bony fish (teleosts). The scientific classification of sharks is a little controversial. Rather surprisingly the harmless, plankton-eating basking shark is in the same 'family', the Lamniformes, as the great white shark! However, the basking shark is in a genus of its' own, the Cetorhinidae.
Geographical distribution.
Basking sharks have a worldwide distribution in cool temperate seas of temperature 8-14°C, being found in the Pacific, Atlantic and Indian Oceans. It has only once been found in warm tropical water (off Florida) and this was probably a sick stray. They are mentioned in the scientific literature as being sighted in waters off Britain and Ireland, Florida, California, Japan, Norway, New Zealand, Canada and Novia Scotia.
General Basking Shark Biology
Basking sharks are, like all sharks, part of the family known as elasmobranches . The skeleton of elasmobranch fish is made of cartilage, a material not unlike the hard gristle found around mammalian joints. This might be partially calcified, but not to the same extent as in the bony (teleost) fishes. The harmless, plankton-eating basking shark is in the same 'family', the Lamniformes, as the great white shark! However, the basking shark is in a genus of its own, the Cetorhinidae. If you find this hard to believe the pictures below make this relationship a little more apparent!
This does look like a great white but it is actually a basking shark with its mouth
half-shut. This photograph makes it possible to see that the basking shark
might be related to a great white, however improbable that seems at
first glance! This picture was taken about half a mile
off Fleshwick, the Isle of Man.
Picture: Andreas Perethoner.
Moi? Related to a great white? I hardly think so!
Picture: Maura Mitchell.
I can look completely different from the side!
Picture: Shane Stigant.
Geographical distribution.
Basking sharks have a worldwide distribution in cool temperate seas of temperature 8-14°C, being found in the Pacific, Atlantic and Indian Oceans. It has only once been found in warm tropical water (off Florida) and this was probably a sick stray. They are mentioned in the scientific literature as being sighted in waters off Britain and Ireland, Florida, California, Japan, Norway, New Zealand, Canada and Novia Scotia.
External Features of Basking Sharks.
Most people who see basking sharks only see the fins and nose projecting above the surface, as is shown in the diagram below. Historical documents suggest that basking sharks can grow up to 13.7 metres long, as long as a double decker bus. You are very unlikely to see one longer than 11 metres today but this is still the length of single decker bus! There is a huge amount of debate about basking shark maximum size [15,18] (see the next section for more on this).
Cartoon by Derek Pitman
The dorsal fin can be up to 2 metres tall. Note the notches in the dorsal, second dorsal and tail fin. These are quite characteristic of the basking shark as is the bulbous nose. The pectoral fins are very large. They enable the fish to move up and down in the water column. Looking at the underside of the shark, between the pelvic and anal fins, you can see the claspers. These are the male sexual organs, used to introduce sperm into the female. Basking sharks often have one or more long thin fish called lampreys attached to this area. The five gill slits almost completely surround the head.
Imagine trying to study the anatomy of such a huge animal as a basking shark! Dissecting an animal of such huge proportions would seem to be impossible. However, in 1947 two scientists, Drs Matthews and Parker, accepted the kind invitation of Major Gavin Maxwell, of “Ring of Bright Water” fame, to visit his basking shark fishery on the Scottish Isle of Soay, off Skye. They were able to observe the sharks from Gavin Maxwells’ hunting craft and to dissect 4 males and 6 females on shore. These massive dissections were performed on the landing slips using the very tools that were normally used to butcher the animals. Pathology specimens were fixed in formalin and taken back to their laboratories for later study. Dr Matthews wrote a scientific paper on reproduction in the basking shark[17]. Drs Matthew and Parker wrote a joint paper about basking shark general anatomy[18]. More than 50 years later these two scientific papers are still the definitive works on basking shark anatomy and this section draws heavily upon them.
Physical characteristics of basking sharks.
Picture: Maura Mitchell.
Basking sharks have five pairs of very large gill slits that almost completely surround the head. They have a large, hoop-like sub-terminal mouth with hundreds of tiny, vestigial teeth.
Picture: Maura Mitchell.
The pink gill rakers, used for filtering their planktonic shrimp food from the water, are visible if you are too close! Note the small eye.
Picture: Maura Mitchell.
Their pectoral (side front) fins are large and angular. The basking shark uses these rather like aeroplane wings to 'fly' up and down in the water column. Their dorsal (back) fins are very large, up to 2 m tall and angular (see photograph below). Their caudal (tail) fin is notched and lunate (moon shaped) with a distinct lateral keel. (See photograph at top of this page ).
Picture: Maura Mitchell.
Adults such as the one in the photograph above have a characteristic bulbous nose. Juvenile basking sharks are smaller and more slender with a nose described as a “pointed prominence or beak”. The snout makes the transition from the juvenile to the adult form at about 12-16 ft (about 3.6 - 4.8m). Smell seems to be very important to the basking shark (see section on basking shark brains further down).
Picture: Pauline Oliver.
Sometimes, when the basking shark is feeding or basking at the surface, three points are visible, the pointed nose, the dorsal fin and the caudal or tail fin. Note how the large dorsal fin has flopped over in this shark. This is more common in large specimens.
Size, weight and sexual maturity.
Historical accounts give maximum sizes for basking sharks of 40-46 ft (12-13.72 m)[15]. It is notoriously difficult to estimate the length of a fish when you see it in the water because of the magnifying effect of the water. Dr Matthews found how easy it is to over-estimate the size of a basking shark when it is still in the water. His estimate of a shark when it was along side the boat turned out to be surprisingly exaggerated when they landed the animal. Some down-to-earth facts come from Anthony Watkins who was a very experienced basking shark fisherman operating off Scotland and Ireland. In his book ‘The Sea My Hunting Ground’, published in 1958, he discusses the issues of length and weight of basking sharks. He says that the biggest basking sharks his fishery caught were about 29ft (8.7m). This is about the length of a large bus. He estimated the weight of a 29 foot (8.7m) long basking shark by extrapolating measurements obtained from a small basking shark he weighed whole. Watkins reasoned that as the weight of a fish varies in proportion to the cube of its length if you know the weight and the length of one fish of any species you can calculate the weight of any fish of the same species from its length. On this basis he calculated that a 29 foot one probably weighed about 6.2 tons. Given that mature fish tend to be bulkier than young ones this might be a minimum estimate. It seems reasonable to estimate a weight of about 7 tonnes for an 8.7m basking shark. This is about the weight of two fully grown elephants.
Several observers on the isle of Man have estimated basking shark lengths there to be up to 12meters! Keep sending your basking shark sighting forms in and the picture of basking shark size distribution on the island should become clearer.
Dr David Sims, is a prominent current basking shark researcher. He is working mainly off the Plymouth coast. He and his team have produced many scientific papers on basking shark feeding and seasonal movements[24-31]. Using the scientific literature and his own experience Dr Sims estimated the ages of sharks at different lengths. Newborn sharks are about 1.5m–2 m long. The young-of-the-year sharks that appear inshore in June and July are 2-3 m. Basking sharks reach 5m by age 3-4 years and 10m by approximately 8 to 15 years. It is not known how long basking sharks live but it may be 30-50 years, so the possibility for bigger basking sharks does exist.
Basking sharks are immature up to 6m in length and become sexually mature adults when they reach 6-9m.
Skin
“Basking sharks are dark grey, almost black, with an inconsistent amount of light grey or white in the mid ventral line”. Colour variation occurs amongst individuals. The colour is very patchy and has been described as being not unlike mackerel skin markings on a grand scale.
Picture: Maura Mitchell.
The skin may have scars where lampreys have attached themselves. These white scars are typically 2-3 inches (5-8cm) long and half as wide. The shark in the picture above has some white scars behind its dorsal fin. Basking sharks often have scars and bleeding regions around the cloaca (external genital region). These are the result of their very rough skins rubbing together during mating. There are often white scars around or on the dorsal (back) fin due to the male biting the female's dorsal fin during the mating embrace. Basking shark mating behaviour is far from gentle!
Basking shark skin is covered with small denticles. These are small tooth-like placoid scales arranged with their points directed posteriorly, so that the skin feels smooth to a hand passed over it from front to back, but exceedingly rough in the reverse direction[18]. This account by Drs Matthews and Parker is at odds with an account from Florida, which examined the skin from 2 specimens. One of the sharks had denticles all pointing in the same direction but the ‘Sarasota’ specimen had skin with denticles pointing randomly. It was exceedingly rough when stroked in any direction. The scientific paper which reports this type of basking shark skin has 2 photographs of the Sarasota basking shark’s skin with sharp denticles pointing in all directions . This would be very unpleasant to rub against! It appears that basking shark skin becomes very rough once tanned and made into leather. It was traditionally used to sole fishermens boots as it provided such good grip on slippery surfaces. If the 'Sarasota' specimen had been dead for a while it is possible that the skin denticles could have become dry and disordered. Basking sharks skin is criss-crossed with varying patterns of 2 mm-deep skin creases that are bare of denticles. These creases correspond with lines of flexure (where the skin bends).
Basking shark wrinkles! The skin in the gill region needs
to be able to expand a lot during feeding.
Picture Maura Mitchell.
Basking shark skin is plentifully supplied with mucous. During life the mucous forms an even film over the entire surface of the skin below the surface of the denticles. Scuba divers who have accidently come into contact with this mucous say that it is thick and black, rather like axle grease, very fishy-smelling and rather hard to clean off one’s wetsuit!
Basking sharks suffer from several external parasites typically seen around the shoulder, flank and vent. The largest is the lamprey (Petromyzon marinus) an eel-like primitive fish that has a sucker-mouth that, when attached to a soft skinned whale, might feed from the flesh. It appears that this does not happen in basking sharks since its jaws cannot penetrate the thick denticle-covered skin. The lamprey probably just uses the shark as a mode of transport. Lamprey are often seen clinging to the area around the vent in groups of up to 8.
This shark has a lamprey attached near its ventral fin.
Picture Maura Mitchell.
Drs Matthews and Parker found that basking shark skin was often infested with three species of parasitic shrimp-like copepods, more commonly known as fish lice.
Fish lice tend to attach themselves behind the basking shark gills, as in this amazing
photograph. The photographer was in a kayak, letting the fish come to him.
The camera was put into the water next to the kayak and the shark (as you can see)
swam very close. Niarbyl 2003.
Picture: Shane Stigant.
Basking Shark Brains.
Basking shark brains are very small, especially when compared to the size of the animal. The basking shark needs to locate its preferred planktonic food and the brain provides some evidence that smell is the most importance sence to this species. Normally sharks detect their food by seeing it, smelling it or by detecting electrical signals from it with its Ampullae of Lorenzini (of which more later). The tiny eyes are probably not directly important in locating its food but it is probable that they are important as light detectors, helping the basking sharks to detect changing light levels when they are moving up and down through the water column trying to locate food. Drs Matthews and Parker found that the brains olfactory tract, where the sense of smell is processed, is 15cm long whereas the whole of the rest of the brain is only 10.5 cm long! If basking sharks allocate that much relative brain power to smell it would seems to indicate that smell is VERY important to basking sharks! Interestingly, the basking sharks brain cavity is much larger than its brain. Drs Matthews and Parker found that the small brain was held in place in the large brain cavity by fine strands of tissue! Its predatory relatives have proportionally larger brains. Obviously the basking shark does not need a large brain like its predatory cousins and its small brain with a relatively large portion for smelling out its food source is all it needs to survive.
Feeding
What do basking sharks eat?
Basking sharks eat plankton. Plankton is composed of very small animals (zooplankton) and microscopic plants (phytoplankton). Their preferred food is zooplankton, which is a rich mixture of shrimps, very small fish, animal eggs and larvae. When scientists have examined the pink soup-like stomach contents of basking sharks they have found a macerated mass of zooplankton, small fish, fish eggs and, in one case, an eel. We are used to seeing basking sharks feed at the surface but plankton occurs throughout the water column and as pelagic deep-sea shrimps have been found in basking shark stomachs in Japan, it is apparent that they feed at depth as well as at the surface[19]. Dr David Sims and his team of scientists at Plymouth University have been studying the basking shark since 1996. They are particularly interested in their feeding behaviour. This section draws heavily on their work.
This basking shark is feeding on planktonic shrimp. The camera flash reflects off
tiny particles such as plankton and shows them as little
flecks in the water. Niarbyl, Isle of Man 2003.
Picture: Shane Stigant.
Scientists had assumed that basking sharks fed indescriminately but Dr Sims and his team found that basking sharks foraged preferentially and for longer in areas where their favourite shrimp prey species, Calanus helgolandicus, was more abundant than other species in the plankton. C.helgolandicus is a species of small (about 1-2mm long) shrimp called a calenoid copepod. It is about the same size as as half a gain of uncooked rice. When the scientists looked at the type of plankton present where basking sharks were feeding they found more and bigger plankton, with more of their preferred prey species, C.helgolandicus, than they did when sampling plankton where sharks were just swimming, not feeding.
The huge basking shark feeds on zooplankton such as these planktonic shrimp.
These are photographed through a microscope, magnified
about 15 times bigger than real life. They are actually
about half the size of an uncooked rice grain.
Picture: Marine Conservation Society.
How do basking sharks feed?
There are few more dramatic sights than a 7m long feeding basking shark coming towards you! When feeding at the surface the basking shark swims slowly along with the mouth wide open.
This wonderful photograph, taken off Niarbyl, the Isle of Man, shows the
five gill slits rather well! The gullet is closed so that water
does not pour into the stomach.
Picture: Shane Stigant.
The plankton-laden water flows passively through the mouth and out through the 5 gill slits where the plankton is filtered out by its pinkish, filamentous, brush-like gill rakers. The basking shark uses its gills to 'breath' extracting oxygen and releasing carbon dioxide through them. The basking shark gill slits also house the brush-like gill rackers that sieve the plankton from the water.
This close-up shows the pink gills in the gill slits. The photographer
did not chase or harrass the shark but waited patiently
in the water until it swam past.
Picture: Maura Mitchell.
A basking shark typically swims with its mouth open for 30 to 60 seconds before closing its mouth and swallowing. Divers have reported that if the water is clear, indicating low plankton concentrations, the swallowing rate is slower, occurring every few minutes[. When the shark closes its mouth to swallow, it gulps 3-5 times in a characteristic fashion which Maura Mitchell, an Isle of Man scuba diver, describes as ‘Kermit-like”. For those of you who are not familiar with Kermit he was a frog puppet from the Muppet show, popular in the 1980s!
A basking shark with its mouth closed, swallowing.
Picture: Maura Mitchell.
This type of feeding is called obligate ram filter-feeding. The other large filter-feeding sharks, the whale shark (Rhincodon typus) and the megamouth shark (Megachasma pelagios) feed by a different method, suction-feeding[7]. This means that they don’t have to swim forward fast through their prey in the same way as the ram-feeding basking sharks.
Dr Sims found that the basking shark typically swims slower, at 0.85meters per second (1.9miles per hour) when feeding as compared to 1.08 meters per second (2.4 miles per hour) when swimming without feeding. This was not anticipated by mathematical models that predict fish swimming speeds. Bony teleost fish such as the anchovy swim faster when feeding than when not feeding. Dr Sims concluded that this was because bony fish have just one gill slit opening and filtration apparatus for the water to be forced through, whereas the basking shark has five. As you can imagine this must create considerable drag, thus reducing the swim speed. Do not imagine that 2.4 miles per hour is the top speed of basking sharks. This is just a cruising speed. They are capable of generating huge amounts of power as can be seen when they breach (leap clear of the water during courtship displays).
The anatomy of a basking sharks mouth and feeding apparatus.
Drs Matthews and Parker describe the mouth of an average size basking shark as being 70 cm from the central mid-lip to the angle of the gape and rather more across the mouth from side to side. The palate was 1m long, the tongue 0.8 m long. Up to 1,500 small teeth, approximatly 5mm by 3mm are present on 4cm wide tooth cartilage which looks rather like a pair of thin lips! There are six rows of teeth on the upper jaw, nine on the lower. These teeth are not known to be used for feeding but they are certainly important for grasping mating partners.
If you look closely at the mouth edge of this shark you can see the thin, lip-like area that has
about 1,500 very small teeth on it, used only for holding on during mating.
Picture: Shane Stigant.
In the mouth, near the rakers, are cells that secrete huge quantities of mucous. Drs Matthews and Parker suggested that plankton filtered out by the rakers become entangled in the mucous. When the mouth is closed the rakers collapse, squeezing the plankton-laden mucous into the mouth where it can be swallowed.
Where basking sharks feed.
Dr David Sims and his team are interested in where and why basking sharks choose to feed where they do. They have found that basking sharks prefer to feed at current ‘fronts’ where two water masses of different temperature meet. When the sea is calm less mixing occurs and the water stratifies into different layers, typically warmer on top, cooler below. This may result in the plankton experiencing low nutrient levels. Therefore plankton levels are higher where waters of different temperatures mix, such as at a thermal front. An excellent definition of a thermal front is given by a scientist named Le Fevre “A thermal front is a region characterised by a larger than average horizontal gradient in water temperature which forms a boundary between warm, stratified and cold, mixed waters”.
Basking sharks accumulate at tidal fronts where their microplantonic food gathers.
Macroplankton such as this lions-mane jellyfish appear at the same places.
Picture: Maura Mitchell.
These ‘fronts’ can be visually identified as narrow ribbons of still water occurring next to an area with small ripples. These fronts may have an oily looking slick of fish eggs floating on the surface. There might be large quantities of floating seaweed debris and Aurelia jellyfish (moon jellies). This has been described as looking like an in-the-water strandline. Plankton concentrations are higher here than in the general water body.
How dense does the plankton have to be for basking sharks to feed?
Dr Sims has shown that basking sharks do not feed when the plankton concentration is less than 1 gram of plankton per cubic meter of water, presumably because it is energetically not worthwhile so to do[24]. To help you visualise how much food is in 1gram of plankton per cubic meter; it has been calculated[24] that 0.62 gram of plankton per cubic meter equates to about 400 copepod shrimps, so 1 gram of plankton per cubic meter of water equates to approximately 650 planktonic shrimp.
This basking shark is feeding off Niarbyl, on the West coast of the
Isle of Man. The water is quite clear and plankton
concentrations are probably low.
Picture: Shane Stigant.
The higher the plankton concentration, the longer the sharks feed. When the plankton reach concentrations of 3 grams of per cubic meter of water the sharks will feed for up to two and a half times longer than when it’s at 1 gram. When the basking sharks find a good place to feed they are seen to stay in the area of high plankton concentrations by zigzagging backwards and forwards in approximately 50m ‘legs’. The technical term for this is area restricted searching, more commonly known as ARS. Dr Sims found that the sharks followed particularly productive frontal areas for up to 20km over two days.
When basking sharks find a good patch of plankton they make the most of
it by 'zigzaging' through the water.
Picture: Pauline Oliver.
How do they locate high concentrations of their favourite food?
It used to be thought that basking sharks were indiscriminate filter feeders[18]. It was a commonly held belief that they ate pretty much any plankton that happened to be in their path. This was not an unreasonable assumption as it is hard to understand how they could select which planktonic species to filter from the seawater! Dr Sims and his team have, however, shown that basking sharks elect to feed in waters which contain higher concentrations of their preferred prey species of planktonic shrimp. It is not known how sharks locate high concentrations of plankton and their favourite prey. Their eyes are very small and it is not apparent that they are important in prey selection. Dr Sims and his team hypothesize that they might smell the dimethyl sulpide (DMS) given off by the plant plankton (phytoplankton) when it is being grazed by zooplankton. Drs Matthews and Parker state that “The olfactory organ…..is arranged so that a continuous stream of water enters at a scoop like funnel, passes over the nasal mucosa spread out on a number of plates”. So they have a very specialised ‘nose’ to ensure that they are continuously sampling the smell in the water. If you have read the section on basking shark brains you may recall that the brain region responsible for smell (olfaction) was bigger than the whole of the rest of the brain put together, so smell is obviously very important to them. The other theory that Dr Sims and his team propose is that the basking sharks can detect the muscle activity of their prey using their electroreceptors. Sharks are able to detect electrical activity in the water using their sensory pits, which are called Ampullae of Lorenzini. Drs Matthews and Parker describe how a large mass of these sensory pits fills the whole space above the nose cartilage and under the basking shark’s eye. Clearly electrical detection is very important to the basking shark.
It is known that basking sharks feed at considerable depth as deep-sea shrimp have been found in their stomachs. Dr Sims and his team think that basking sharks undertake deep dives through the various stratified water layers to sample the food potential (smell and electrical signals) of each layer. This enables them to detect whether any sampled water layer contains a food source worth exploiting. The stratification (layering) that occurs between waters of different salinity or temperature would otherwise mean that the sharks would be unable to detect food signals from one water layer whilst they were in another.
How far do basking sharks travel to find food (and mates)?
It has been known for a long time that basking sharks undertake seasonal migrations from wherever they spend the winter months to their summer feeding and courtship grounds. The few studies which have tracked basking sharks haven’t done it for long enough to be able to see how far they do travel. Dr Sims and his team tagged 5 basking sharks and found that they undertake long journeys both horizontally (up to 3400km) and vertically (up to 750m). Shark Bay Films have made a film called ‘E.Mail from a Shark’ about this innovative research. See the references and contacts section for how to order it. The tags, which cost £3000 each, are shot through the base of the shark’s dorsal fin. This is done with a harpoon fired from the surface. The placement of the tag has no lasting effect on a shark’s health. The tags contain a mini-computer data logger. This collects information about where the shark has been. It also records water temperature and light levels. They are designed to release themselves from the shark after a pre-set period. They then pop up to the surface and send the data information via a satellite to Dr Sims e.mail! The sharks' tags stayed attached for between 2 and 7 months, covering summer, autumn and winter. One shark travelled 500km in just 10 days with a directed movement towards the continental shelf edge. Another shark, a sub-adult, travelled 1900km in 76 days, tracking the plankton along the shelf edge from the English Channel to Scotland. They found that the sharks, which were tagged off the southern coast of Britain, had huge ranges extending from the sea off France to the Scottish North-Western Isles. They correlated this information with data about plankton levels and found that the sharks moved to, or stayed in, the areas with the highest plankton levels. These areas tended to be around continental shelf and shelf-edge habitats. Dr Sims concluded that the basking sharks he tagged around Britain were ‘local’ British sharks, not seasonal migrants from foreign shores moving into inshore British waters.
Anatomy of a basking shark stomach.
The stomach, as you might expect, is enormous. Drs Matthews and Parker dissected a 7.03 m long basking shark with a stomach 1.8m long and 0.6m wide. In his paper on the reproduction of the basking shark Dr Matthews describes the devastating results of accidentally spilling the “better part of a ton of semi-digested plankton” over his dissection!
The stomach can be described in two halves; first the cardiac sac with contents of approximately half a ton of plankton mixed with mucous. By the time that all the mashed and churned plankton has reached the pyloric limb of the stomach, much of the water has been removed.
Basking shark liver oil.
Oil “digestion”.
It is at the next point that the digestive process becomes very different from that found in mammals. Anyone who has observed planktonic shrimp down a microscope will notice that the shrimp contains a shiny globule of oil. Drs Matthews and Parker came to the conclusion that this oil was forced out of the food mass by pressure at the end of the stomach. The oil is then diverted into an organ called the bursa entiana from where it is probably absorbed through the epithelium (cell lining). This could not be more different from the mammalian digestive process. Drs Matthews and Parker were clearly puzzled and they say “it is not obvious how the oil is separated from the rest of the food; nor how the solid matter can pass from the pyloric limb through the bursa to the duodenum without the oil accompanying it. It is remarkable that oil should be absorbed in the gut, proximal to the pylorus, before the bile and pancreatic secretion have acted upon the food”.
Why is basking shark liver so rich in oil: for energy or for buoyancy?
This oil is vitally important to the shark. A four-tonne basking shark will have a one tonne liver full of oil! It was this liver, so rich in commercially valuable oils, that caused the basking shark to be hunted so enthusiastically. The liver is divided into two lobes that extend the whole length of the abdominal cavity. The basking shark does not have solid fat stored under the skin like marine mammals. Its lipid reserves are all stored as oils in the massive liver.
It used to be thought that shark-liver oil was an energy reserve for times of starvation but research performed on many shark species indicates that its main purpose is to keep the shark neutrally buoyant in the water. The teleost bony fishes control their buoyancy with air-filled swim bladders but sharks do not have these. Oils are less dense than seawater, allowing the animal to remain neutrally buoyant. Basking shark liver oil is so effective in this role that the net weight of a 2 tonne basking shark supported in seawater is likely to be just a few kilograms. Sharks control their position in the water by tilting their pectoral fins like aeroplane wings whereas the bony fish change the amount of gas in their swim bladders.
Reproduction
Quite a lot is known about basking shark courtship but very little is certain about their gestation or where they give birth. No scientist has ever dissected a pregnant basking shark and only one live birth of six 1.5-2m long pups has been observed. The terminology surrounding basking shark reproduction can be quite confusing but this should clarify it-
- They are live bearing (this is called being viviparous).
- The young hatch from eggs inside the uterus (this is called being oviparous).
- The developing embryos nourish themselves by eating eggs in the uterus (they are oophagous).
Picture: Pauline Oliver.
Courting basking sharks often congregate in groups of 2 to 12 off the West coast of the Isle of Man. You may be lucky enough to see courting behaviours such as paired swimming, following nose to tail, close following, or, if you're really lucky, breaching (leaping out of the water).
Basking Shark Courtship Behaviour, and Mating.
It is wonderful that something as exotic as basking shark courtship behaviour can be seen from the shore on the Isle of Man. You will find an eyewitness account in the Isle of Man basking shark stories section of this website.
Dr David Sims team has provided a marvellous up to date account of basking shark courtship behaviour . Dr Sims’ team based their scientific paper on the courtship behaviour of basking sharks on observations of 25 separate episodes that occurred over a 5-year period. They concluded that courting behaviours occur along oceanographic fronts off Plymouth from May to July while the basking sharks are feeding off the rich plankton found at these fronts. The same seems to be true off the Isle of Man coast. Another group of scientists, travelling in a Coastguard helicopter off the coast of Novia Scotia, had a wonderful bird’s-eye view of 13 basking sharks engaged in courtship behaviour and of a possible mating. The sighting only lasted 5 minutes but they had video footage and photographs that they were able to analyse later.
When do basking sharks become mature?
Previous to work done by Dr David Sims and his team it was thought that the female did not become sexually mature until 7-9m[17,21] and males were estimated to become mature at 4.6-6.1 m[17]. Dr Sims and his team found that sharks exhibited sexual courtship behaviour when over 5m in length. Smaller individuals who were present did not engage in such behaviour.
What is basking shark courtship behaviour?
Dr Sims describes basking sharks nose-to-tail following, close following, approaching one another's flank, close swimming, parallel swimming, echelon swimming (where sharks follow one another but are slightly offset, not in a straight line) and breaching (leaping out of the water). They describe how only large basking sharks in groups of 3 or more breach repeatedly, leaping clear of the water. They propose that breaching is being used as male-male competitive behaviour during courtship displays. However, the only shark that they were able to definitely sex was a large female that breached 7 m from their boat, which must have been interesting! They propose the alternate idea that female basking sharks breach to signal their readiness for mating. The scientists in Novia Scotia describe similar courtship behaviour to Dr Sims’ team. They also mention that the basking sharks swam faster than usual and exhibited circling behaviour by up to 10 sharks at a time.
The Manx Basking Shark Watch Courtship Study 2008-2010
When the Manx Basking Shark Watch (MBSW) public sighting scheme began in 2005 it soon became apparent that there is a lot of social swimming, putative courtship behaviour, in Manx waters. Much of it is located in June and July off the West coast of the Isle of Man. MBSW has undertaken to study this behaviour. Here are some excellent photographs taken by local people who have observed these behaviours. IF you see any of these behaviours please leave the area as not only are you likely to disturb this important part of this endangered animals life cycle but it is also possible that one might breach, landing on your boat. this would be bad for you and bad press for the basking shark, which is only doing what comes naturally!
These basking sharks are close nose-to-tail following. We see a lot of this behaviour. Sometimes there are many animals in a line: Photo by Mal Kelly.
This photo was taken by Dr Fiona Gell of DAFF. It shows two sharks, one shark swimming parallel to the other but slightly behind the other. They often slip from parallel swimming to this and back again.
This photo by Adrain Corkill shows two sharks that are swimming in circles after one another.
This great photo of courting circling basking sharks was taken by Adrian Corkill.
It would be easy to assume that this group of sharks is feeding in a circle because they have found a good spot of plankton. However, if you look closely you will see that some of them are VERY close together. If they stay this close for more than a minute or so assume that this is an intense courtship event and leave the area. At the very least drop back 100m or more to watch. Do this very carefully and slowly as there may be many more sharks feeding below your boat.
This great basking shark courtship photo was taken by Dermot Shimmin. Three sharks are swimming in VERY tight formation. Two are parallel swimming with the third shark forming a circle with the other two by nose to tail following.
This photo by Dr Fiona gell shows two basking sharks VERY close together indeed. These sharks had previously been identified as a male and a female. Unfortunately we did not get any underwater photographs of this. Colin, the male, was tagged with an archival MK10 PAT tag shortly after this event.
This wonderful photograph was taken by Adrian Corkill off Bradda Head. It shows in a courtship display, two basking sharks parallel swimming. Note exactly HOW close they are to one another. Sharks swimming parallel to one another but far apart are not courting like these are. You may see parallel swimming sharks get very agitated and start tail thrashing at this stage. Again, leave the area asap!
Again, these two sharks are close parallel swimming. Photo by Mal Kelly. Sometimes they are almost stationary in the water when they do this, then one starts tail thrashing.
On a lighter note this cartoon by Derek Pitman illustrates the problems of basking shark group behaviour. Although it is very nice seeing lots of sharks together it can become very tricky driving your boat. If you see one basking shark assume that there are many more around. there often are. It means that you must drive very very slowly with a look-out watching for sharks on or just below the surface. Remember it is against the law to harm a shark by careless driving.
Basking shark mating.
There has only been one scientific account of a possible basking shark mating but we have our own possible Isle of Man mating sighting documented in our 'Isle of Man Basking Shark Stories' section on this website! The MBSW scientific investigation of basking shark courtship hopes to film mating one day.
The scientists in the Novia Scotia helicopter saw possible mating behaviour where 2 sharks body curves matched one another. They saw one shark holding on to another by biting down on its pectoral fin. They also observed white-coloured marks on the dorsal area of the pectoral fins and behind the dorsal fin. They interpreted these as being bite marks made by males on females during mating. Although basking sharks are filter feeders they do have an impressive array of small teeth (see the section of feeding for more details). It would seem likely that these teeth are primarily used for holding on to their sexual partner during mating, as there is no evidence of basking sharks using teeth for catching any food items. When Dr Matthews dissected recently mated basking sharks he found many abrasions on the genital cloaca regions of sharks that were known to have been recently mated. He surmised that the claw on the males’ claspers caused abrasions on and in the females and that abrasions on both sexes were caused by the abrasion of the sharp sharkskin one against the other. Evidently basking sharks are not gentle with one another! We often see rubbed noses on mature basking sharks. This is probably caused by sharks rubbing against one another during courtship.
It is possible to identify the sex of a basking shark if you can see its ventral region (underneath). Dr Matthews found that the male has two long scroll-like claspers, which are used to introduce about 4 gallons of sperm into the female during mating. He dissected one female that had recently mated, which is how he knew how much sperm was used. He measured one clasper as being 1.09m long. It would seem that everything about basking sharks is done on a grand scale!
How do basking sharks nourish their unborn young?
Is not clear how the basking shark mother nourishes her young so it is worth explaining some of the fascinating different methods that other shark species use to nourish their live-born young[9]. Sharks are a very diverse group and they exhibit many different forms of reproduction. Some sharks lay eggs, others bear live young that are nourished inside the mother by egg yolk only. Others provide egg yolk and a rich milk-like substance called uterine milk. Some shark species are oophagous, the mother ‘lays’ many eggs into her uterus and her developing young eat them. Other shark species cannibalise their womb-mates; this is called adelophagy and can do little to improve the reputation of sharks! Some shark species exhibit a combination of strategies, the live-born young having been nourished by egg-yolk, intrauterine milk and by eating eggs whilst in the uterus. Finally, some sharks have placental structures that directly nourish their young.
When Dr Matthews dissected 6 female basking sharks he found that only the right ovaries were functional. He didn’t understand why the half-metre across ovary had 6 million eggs ready to be spawned when a female would only have a few live births in her lifetime. It now appears that this is because the eggs are used to feed the developing young. This is probably a rich eggy soup like some other shark species. This may explain the curious structure of the basking sharks nose (see section below with photographs and links to research suggesting this). The grooved nose may act like a straw helping the unborn shark to suck up the soup.
Dr Matthews also found that the uterus was lined with structures called trophonemata or ‘feeding threads’. Trophonemata are associated with shark species such as the butterfly ray in which these long nutritive threads feed the developing sharks directly into their oesophagus. It is possible that basking sharks adopt more than one strategy. Basking sharks might use a combination of nutrition from oophagagy (eating eggs) AND trophonemata (feeding threads) to nourish their unborn, developing young. As discussed before, other species of sharks use a combination of methods to nourish their unborn young but until a pregnant basking shark is examined this will remain a matter for conjecture.
How long is a basking shark pregnancy?
It is not clear how long the gestation period of the basking shark is. Gestation in some other viviparous (live-bearing) sharks is known to be about 22 months. There have been various suggestions about how long basking shark gestations are. Some suggest 20 months others basing their estimate on an analysis of the age/length data suggest a gestation period of 3.5 years, another equally prominent scientist concluded that a 1-year gestation was more probable. The conclusion? It is not known how long a developing basking shark spends in its mothers uterus!
Where are basking sharks born and how big is a newborn basking shark? There are only three accounts of female sharks being found with live young in them[22,17,36], and one record of a basking shark birth. Given how many basking sharks have been butchered for their livers the lack of stories about pregnant basking sharks suggests that they might stay in deeper water or somewhere that is not normally targeted by fisheries. Dr Sims and his team reasoned that as young-of-the-year, (size 2-3m) do not appear inshore until mid-June that it was probable that young of 1.5m length were born offshore in deep water in May and June.
The oldest record of a pregnant basking shark female is by a T. Pennant writing in 1769 who wrote “ They are viviparous, a young one about a foot in length being found in the belly of a fish of this kind”. Dr Matthews also describes how a fisherman told him that when he opened a female he found a young one 6 foot long inside. A Norwegian fisherman described catching a female basking shark that gave birth to 5 live young and 1 stillborn which he estimated to be between 1.5 and 2m long.
When Dr Matthews[17] dissected basking shark females he found that they had a 1-2m long uterus. The uterus size tallied with the very few accounts of the size of basking shark young found inside female sharks. The smallest newborn basking sharks are about this size so this is quite plausible. We have been extremely lucky to get a picture of a newborn basking shark that was seen off the West coast of the Isle of Man during July 2005. The picture shows the remarkable snub nose, a bit like a little pig.
This snubby pig-like nose with the underset lower jaw is so unlike the adult that
unless it was so closely associated with the group of adults and so clearly
feeding on plankton with them you would think it was a different species!
Picture Shane Stigant July 17th 2005 Four miles off Dalby.
Armelle Jung of the French shark research Team APECS has very kindly let us use their picture of a very young basking shark that was taken off France. Note the similarities with the Manx picture.
There are some excellent photographs of a 2.9m long male basking shark at
http://homepage.mac.com/mollet/Cm/CSteinbergImages/BaskingShark_js_ss.html
These photographs of a small dead basking shark clearly show the same features in the 'nose' area. A most interesting feature is a small 'nostril-like' hole below the pointed peak of the nose area.
According to www.elasmo-research.org/education/ecology/rocky-basker.html this is a physical adaptation to help them suck up the 'eggy soup' that nourishes them in their mother's womb. The following text is pasted and copied from their site. Their copyright applies-
"Juvenile Basking Sharks offer yet another mystery: they have a peculiar elongated snout tip that looks remarkably like a wavy carrot. In some individual Baskers the carrot-tip waves upward, in others downward. This peculiar snout shape disappears by the time a Basking Shark reaches a length of about 16 feet (5 metres). A recent study examined the rostral (snout) cartilages of an 8.5-foot (2.6-metre) female Basker from Japanese waters. It revealed that the undersurface of the snout of a juvenile Basker has a groove that is continuous with the palate. This groove becomes very faint in specimens longer than about 10 feet (3 metres) and disappears completely by a length of 30 feet (8.5 metres). This grooved, elongated snout may help channel food into the mouth of very young Basking Sharks. Like other lamnoid sharks, Baskers supplement their diet during fetal development by imbibing a thick ‘soup’ containing thousands upon thousands of tiny, unfertilized eggs produced by the mother. The grooved, elongated snout may help channel these eggs into the mouths of fetal Baskers as they develop in their mother’s womb. Therefore, the peculiar snout of near-term and new-born Basking Sharks may help support their energy needs during their period of most rapid growth."
Do basking sharks hibernate?
Until recently it was thought that basking sharks hibernate in the winter. There were two main reasons for this hypothesis. The first reason was that basking sharks apparently disappeared for most of the year. People normally see them in the late spring and summer. Sightings at other times of year are very much rarer. There has been one sighting of a basking shark off the west coast of the Isle of Man this February (2005) so winter sightings are not unknown, just rare. The second major reason for thinking that basking sharks hibernate was that some winter-caught basking sharks have shed their gill rakers. This obviously renders them incapable of filter feeding. This led to the general belief that basking sharks probably hibernate on the bottom in deep water whilst their rakers grow back and until the plankton levels came back to levels that made it worthwhile for them to feed. It was thought that they lived off the huge store of oil in their liver. A 4-ton basking shark may have a one ton liver, which is almost totally composed of stored oil. The winter hibernation theory was not unreasonable as the first spring sightings of basking sharks are normally near deep water and winter-caught basking sharks were caught in deep water.
Basking sharks are sometimes found to be without gill rakers when they are caught in the
winter. This, combined with the fact that they are not normally seen at the surface
in winter lead to the theory that they hibernated on the sea bottom and didn't feed
in the winter. This shark was photographed off Niarbyl, the Isle of Man.
Picture: Shane Stigant.
The studies that disprove the basking shark hibernation theory.
A couple of studies by Drs Francis and Duffy in New Zealand and Dr Sims and his team in Plymouth, England, have laid the hibernation theory to rest. Drs Francis and Duffy were looking at incidental trawl-caught basking sharks caught in fishing nets off New Zealand. They found that 203 basking sharks were accidentally caught over a 13-year period. They found that many basking sharks were caught in winter, particularly on the West Coast of New Zealand. Although it was not possible to determine the exact depths that the fish were caught there was no doubt that some of the sharks were caught in midwater trawls. They concluded that hibernating sharks were unlikely to hover in midwater and that these sharks were not hibernating. The study by Dr Sims and his team showed that the basking sharks which were tagged off Plymouth did not hibernate but actually travelled huge horizontal distances of up to 3400 km, actively diving in all water depths from the surface down to 750m deep during summer, autumn and winter. This study clearly showed that the basking sharks were not lying on the bottom in the winter but were very actively tracking the areas with the highest quantity of plankton in order to feed there.
As winter basking sharks have been shown to travel actively, to move to the areas with the highest plankton levels, and to be caught apparently actively swimming in midwater, the hibernation theory can be assumed to be disproved: basking sharks do NOT hibernate. It now seems more probable that basking sharks shed their gill rakers at irregular intervals in the winter in order to renew worn feeding apparatus rather than not needing them to feed because they are hibernating.
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