Basking Shark Biology & Anatomy
A detailed look at basking shark biology, written by Jackie Hall
All sharks are elasmobranchs.
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, as the great white shark (the Lamniformes), however, the basking shark is in a genus of its own, the Cetorhinidae.
The dorsal fin can be up to 2 metres tall.
The notches in the dorsal, second dorsal and tail fin 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. Contrary to popular belief, a basking shark can close its mouth.
Looking at the underside of a male 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 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.
Their pectoral fins (flippers) are large and angular. The basking shark uses these rather like aeroplane wings to ‘fly’ up and down in the water column. Their caudal (tail) fin is notched and lunate (moon shaped) with a distinct lateral keel.
Adults 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.
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 four males and six 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.
From these findings Dr Matthews wrote a scientific paper on reproduction in the basking shark. Drs Matthew and Parker wrote a joint paper about basking shark general anatomy. 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.
Historical accounts give maximum sizes for basking sharks of 40-46 ft (12-13.72 m). 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 shark 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.
Dr David Sims is a prominent current basking shark researcher working mainly off the Plymouth coast. He and his team have produced many scientific papers on basking shark feeding and seasonal movements. 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 and 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.
Basking shark skin is covered with small denticles. These are small tooth-like 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 resulting in great injury to your hand.
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).
The 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 accidentally 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!
Brain and Senses
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 sense to this species. Normally sharks detect their food by seeing it, smelling it or detecting electrical signals from it with its Ampullae of Lorenzini. 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.
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.
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 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 fin due to the male biting the female’s dorsal fin during the mating embrace. Basking shark mating behaviour is far from gentle!