Notes on the anatomy and biology of the Basking Shark (Cetorhinus maximus (Gunner)).
Authors: MATTHEWS L. HARRISON , PARKER H. W. 
Published: August 2010

A study of Cetorhinus maximus of the north‐eastern Atlantic was made in the Hebrides, where a number of specimens were examined and dissected.

Though it is often stated that Basking sharks reach a length of 40 feet or more, few, if any, exceed 30 feet in a straight line from the tip of the snout to the notch of the caudal fin. None of those measured in the Hebrides exceeded 29 feet in this dimension. The maximum weight appears to be a little over four tons. The general colour is very dark grey, but there is a tendency for lighter and darker areas of grey to form a pattern of longitudinal streaks. Patches of white are generally present on the ventral surface, and are sometimes of considerable extent. The placoid scales of simple type cover most of the body, their basal plates forming a complete mosaic. In some parts of the body parallel sulci which appear to correspond with lines of flexure of the skin are almost completely devoid of denticles. The teeth are simple modifications of the placoid scale and, owing to the presence of vestigial secondary cusps, are asymmetrical except near the median line. There are six rows of teeth in the upper jaw, nine in the lower. The snout in the juvenile is longer and more pointed than in the adult, but the extreme forms that have been reported may have been produced by post‐mortem shrinkage of the ampullary mass above the rostral cartilage. The sudden increase in girth at the level of the pharyngeal region, which has been described as a juvenile character, is probably entirely produced by distortion.

The skin lining the mouth and pharynx is smooth at the anterior end but covered with papillae behind. The papillae increase in size and complexity from before backwards; they are low and rounded in front, tall and conical further back, long and branched at the oesophagus into which they project as a large bunch forming a valve.

The stomach is siphonal in shape, with a large partly sacculated cardiac portion, and narrow pyloric limb. The mucosa is beset with crypts; into those of the cardiac part numerous large glands open. The semi liquid contents of the cardiac sac weigh about half a ton, and consist of disintegrating planktonic crustacea mixed with a great quantity of mucus. Removal of water at the beginning of the pyloric limb must be rapid, for this limb contains a thick, dark red paste. The distal end of the pyloric limb is expanded to form the bursa entiana before joining the duodenum. The bursa contains a clear red oil derived from the paste in the proximal part of the limb. Histological examination shows that the oil is probably absorbed by the epithelium of the bursa.

The bile and pancreatic ducts open into the duodenum, a chamber proximal to the first turn of the spiral valve, but not externally separated from the mid intestine. The valve, which is a simple spiral shelf, contains up to about fifty turns. The mucosa of the valve is covered with villi each closely beset with glands; a stratum of lymphoid tissue lies beneath the glands. The colon and rectum are comparatively short, but there is a large rectal gland lined by a thick glandular mucosa.

Full histological details are given of the various parts of the alimentary canal, and of the liver, pancreas, and spleen; and the topography of the abdominal viscera and their mesenteries is described.

The brain is small in proportion to the size of the animal, and lies in a voluminous perimeningeal space supported by innumerable fine strands of tissue. The olfactory tracts are narrow and elongated, exceeding the remainder of the brain in length. The olfactory organ is a modification of the simple type found in many smaller elasmobranchs, and 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, and leaves by a backwardly directed exhaust funnel. Details of the lateral line system are given as far as it was investigated; the majority of the ampullae of Lorenzini are concentrated into a mass which occupies the whole of the space above the rostral cartilage.

The gill arches each carry from 1000 to 1300 gill rakers up to 10 cm. long, their free ends directed towards the mouth. When the mouth is opened the rakers are erected by contraction of a complex of muscle strands connecting the bases of the rakers to the branchial cartilages: when it is shut they are returned to a position flat on the surface of the arches by the action of elastic fibres. It is suggested that plankton filtered off by the rakers is entangled in muous secreted by the epithelium at their bases, and that the mixture is squeezed out into the mouth when the rakers collapse. The details of the gill filaments are described, and the total respiratory surface in a shark 7.0 metres long is calculated to be of the order of 270 sq. metres. The functional significance of this comparatively large area is pointed out.

Among the contents of the stomach it was possible to recognize fish eggs of several species, Calanus and other copepods, and larvae of cirripedes and decapods; there were no indications of organisms larger than Calanus. The plankton‐remains in the stomach are very fragmentary, and appear to have been subjected not only to chemical disintegration but also to mechanical breakdown, possibly by a churning action of the stomach muscles and crushing movements of the gill rakers. The net weight of solid organic matter in the stomach contents was less than 30 per cent. of the total, and mucus accounted for a considerable part of it. The organic solids contain approximately 7 to 8 per cent. of the clear, red astacene‐containing oil which is separated out in the bursa entiana.

When feeding the Basking shark swims at a rate of about two knots, and calculation shows that at this speed a shark of average size would filter over two thousand tons of sea water an hour. The fish swims with the mouth widely open and the gills and pharyngeal region greatly expanded, feeding and respiration being simultaneous and almost automatic. The basking habit, in which the first dorsal fin and the tip of the tail project above the surface of the water is probably adopted when the concentration of plankton is great near the surface; it is likely that feeding also takes place when the fish are completely submerged. The basking habit is probably correlated also with the breeding behaviour of the fish.

All the sharks seen at close quarters at sea carried one or more lampreys attached to the skin, and all dead fish examined bore superficial marks caused by the suckers of lampreys. The denticles appear to form an armouring too hard for lampreys to penetrate, for no wounds attributable to them were seen.

The large parasitic copepod Dinematura producta was common on the skin of the sharks examined. Comparison of the extensive material collected with that from other hosts shows that there are at least three host‐forms of the copepod. The differences lie in the organs of prehension and adhesion; they may be ecoptypic in origin, their form being determined by the nature of the host‐skin on which the larvae settle. The parasites erode the skin of the host sufficiently to expose the basal plates of the denticles, but examination of the gut contents failed to disclose any recognizable blood corpuscles, and it is possible that the food is no more substantial than mucus.

Another parasitic copepod, Nemesis lamna, was numerous on the gills where it causes extensive though superficial damage to the filaments, the mucosa being cut up by the parasite and hypertrophied to three times its normal thickness.

Two species of cestodes of the genus Dinobothrium, one of them new to science, were found in the spiral valve, and have formed the subject of a separate report.