General 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!

SharkBiology1

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

SharkBiology2

Moi? Related to a great white? I hardly think so!
Picture: Maura Mitchell.

SharkBiology3

I can look completely different from the side! Picture: Shane Stigant.

Find out more ….

Click on the links below to find out about geographical distribution, external features, physical characteristics of basking sharks, size, weight and sexual maturity, skin and basking shark brains!

  • 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
    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 Historical documents suggest that basking sharks can grow up to 13.7 metres long, as long as a double decker bus.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 section below for more on this).

    Shark-BUS

    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.

    baskerdiagram_colour
    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
    mm0008
    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
    mm0005
    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
    mm0001
    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 ).
    mm0020
    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 Maura Mitchell
    pauline_03
    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. Picture Pauline Oliver
  • 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.

    mm0011
    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 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.

    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.

    mm0018 Basking shark wrinkles! The skin in the gill region needs to be able to expand a lot during feeding. Picture Maura Mitchell.
    mm0013

    This shark has a lamprey attached near its ventral fin. Picture Maura Mitchell.

    03430030
    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.