Size and Weight. Though it is often stated that Basking Sharks reach a length of forty feet or more, few, if any, exceed thirty 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 twenty-nine feet in this dimension. The weight of a shark twenty-nine feet long would be about ten thousand lbs. (roughly four and a half tons), judging from the figures given about two sharks that were weighed piecemeal in America. On the other hand, an estimate of the total weight of such a fish was made at the Soay station as follows: Head, (estimated by the capacity of a small mobile crane that could only just lift it) one ton; liver (weighed), eighteen and a half hundredweight; fins (guessed), one ton; tail (guessed), half a ton; skin (guessed), one ton; meat and backbone one and a half tons; guts (guessed), half a ton; this giving a total weight of nearly six and a half tons, to which may be added half to one ton more for the contents of the stomach and intestines. 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. When Basking Sharks are seen in the water, however, the impression given to some observers is that the colour in general is a light brown rather than blackish.
Colour and Skin. It is not known whether this appearance represents the true colour of the fish in life, or whether it is produced by the layer of water between the fish and the observer. If it represents the true colour, there must be a rapid change to that described above immediately after death. 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.
Teeth. 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.
Form of Head and Snout in Young. 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 Digestive System. The skin lining the mouth and pharynx is smooth at the anterior end, but covered with papillæ behind. The papillæ 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 œsophagus, 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.
The Reproductive System. Practically nothing has hitherto deen known of the reproductive anatomy and physiology of the Basking Shark, no work having been published on the subject since the first incomplete reports over one hundred and thirty years ago.
The testes are embedded in the anterior ends of the epigonal organs, which form an investing cortex round them. The testes are divided into lobes, and these into lobules which contain many ampullæ. Testis tubules lead the spermatozoa from the ampullæ to the vasa efferentia, whence they pass through the ductuli and ductus epididymidis to the enormous ampulla ductus deferentis, where they are incorporated in spermatophores.
The ampulla ductus deferentis contains numerous transverse septa, each with an eccentric perforation; successive perforations form the lumen of the organ as a whole. Spermatic fluid enters the ampulla ductus deferentis and becomes broken up into small aggregations, which pass into the pockets between the septa. Here they are rotated by ciliary action, while the secretion from the deeper epithelial cells is laid down round them in concentric layers. The spermatophores are up to two to three centimetres or more in diameter, and consist of a translucent hyaline cortex surrounding an opaque core of spermatozoa. Several gallons of spermatophores are present in each ampulla.
The skeleton of the clasper is comparatively simple in structure, the cartilages being few in number and forming a scroll proximally and a groove distally. There is a movable style towards the distal end, armed with a sharp claw. The musculature of the clasper is reduced, the dilatator muscle being the largest. The inner surface of the clasper groove within the scroll is covered with a thick layer of glandular tissue, whose secretion is produced by the swelling, degeneration, and detachment of the superficial cells.
The siphons are long and wide sacs lying between the skin and the body-wall on the ventro-lateral surface of the abdomen. They are connected by the siphon tubes with the bases of the clasper grooves, and are invested by a thin sheet of muscle, part of m. compressor, derived from the pelvic fin. The thick epithelium lining the siphons produces a secretion by the swelling, degeneration, and detachment of the superficial cells. Nothing was found in the siphons beyond a small quantity of mucoid secretion. The siphon is probably used in some way not understood for introducing the spermatophores into the female by way of the clasper groove, but the spermatophores do not enter the siphon sacs.
The epigonal organ in both sexes is alike, and consists of a mass of lymphomyeloid tissue. Its function is hæmatopoietic, and it produces lymphocytes, leucocytes, and erythrocytes.
The ovary of the right side alone is developed; it is large and enclosed in a fibrous tunica. It consists mainly of a mass of small follicles loosely held together by a small amount of connective tissue, and is penetrated everywhere by the ramifications of a system of branching tubes which derive ultimately from a pocket on the right side of the outer surface of the ovary. The ova are discharged from the follicles when they are not more than five millimetres in diameter, and pass through the ramifying tubes to reach the exterior through the pocket. The epithelium of the discharged follicle proliferates to form a corpus luteum, the cells of which contain large quantities of lipoid material. Most of the ova, however, are not discharged, but degenerate within the follicles, forming atretic corpora lutea. Great numbers of corpora lutea atretica are present in the ovary. In an average ovary there are at least six million ova 0.5 millimetre or more in diameter, a size at which there is a considerable amount of yolk already present. The ovary is thus unlike that of other elasmobranchs, in which there are usually a few large ova, and in general appearance is more like that of an oviparous teleost. This is remarkable in view of the fact that Cetorhinus is almost certainly viviparous.
The unpaired infundibular part of the oviducts opens at the ostium abdominale, and lies shallowly embedded in the liver adjacent to the attachment of the falciform ligament. It follows a course such that the ostium abdominale is brought directly opposite, and into contact with, the pocket on the right side of the ovary. Ova thus pass from the ovary at once into the oviduct, and do not wander in the peritonea
l cavity. The paired oviducts are applied to the posterior surface of the septum transversum, and pass to the parietal wall of the abdomen, where they join the nidamentary glands. Their lumina are very narrow, not more than two to three millimetres in diameter, and their walls are thick and inelastic, so that it is impossible for an object larger than an ovum about five millimetres in diameter to pass through them, and even an ovum of this size must undergo considerable distortion.
The nidamentary gland is comparatively small, and shows no subdivision into albumen and shell-secreting parts; no stored spermatozoa were found in it. An elongated narrow isthmus leads from the nidamentary gland to the enormous uterus. The greater part of the uterus is lined by innumerable villus-like trophonemata. These are based upon low longitudinal ridges, and may be regarded as strap-like prolongations of their free edges. Each trophonema is supported by a central core of connective tissue and is richly vascularised; no separate glands are present upon it, but the epithelial cells increase greatly in number, and become swollen with secretion, perhaps trophic in function, which they release by becoming detached and disintegrating. In many places the proliferation of epithelial cells is so great that a solid mass of swollen cells, in which the trophonemata are partly or wholly buried, results. Numerous vesicles up to four or five millimetres in diameter, and containing a clear fluid, may be present in the solid cell-mass. It is possible that their secretion may have a solvent action on the cortex of the spermatophores.
The lateral walls of the common vagina bear thick pads of dense fibrous tissue; in adult fish these pads bear scars or lacerations caused by the claw on the clasper of the male. The incidence of the lacerations shows that one clasper only is inserted at a time. A small but distinct hymen marks the lower limit of the common vagina.
The majority of the sharks seen basking at the surface of the inshore waters of the west coasts are non-pregnant females, and pairing certainly takes place during the late spring and early summer; it may possibly also take place at other times of the year. The basking habit, however, is probably in some way connected with the sexual behaviour-pattern, as is the annual appearance of the fish near the coast. Cetorhinus shows the paradox of having an ovary containing a vast number of small ova and a large uterus thickly lined with trophonemata, the first suggesting that reproduction is by spawning, as in teleost fish, but the second showing that it is undoubtedly viviparous. There is no record of a female fish containing recognisable embryos ever having been examined in modern times. It is evident, therefore, that the female fish, after being inseminated, and before any embryo is recognisable, must refrain from basking, and either swim nearer the bottom or leave inshore waters, or both.
It is impossible to do more than guess at the probable length of gestation. The eggs of small oviparous sharks such as dogfish take as much as a year to hatch, and in some of the small viviparous sharks gestation lasts eighteen months, or even two years. It is therefore probable that gestation in the Basking Shark is lengthy, especially in view of the small size of the egg when it is released from the ovary; and a gestation of two years or more is probable. It is also possible that many eggs are released at ovulation but that only a few, or perhaps a single one, gives rise to a new shark, the successful embryo developing at the expense of the others.
No young Basking Shark less than about six feet in length has ever been recorded, and it may therefore be provisionally assumed that sharks of this length are the young of the year. A consideration of the lengths of immature sharks and of the months in which they have been recorded leads to the tentative conclusion that sexual maturity is not attained until at least the third year of life, or perhaps the fourth, when the fish have reached an overall length of about twenty-three feet. Thereafter growth continues for another two years, until the maximum overall length of about twenty-nine feet is attained.
Longevity. If the fish are four years old at their first breeding season, and gestation lasts two years, the life-span of the individual must be long if the population is not to decrease in numbers. Each female fish must leave at least one daughter that survives long enough to reproduce, and some of them must also leave male offspring: breeding is probably promiscuous, and it would therefore not be necessary, in theory at any rate, for each female to leave a son. But in slowly maturing animals the hazards of life before reaching breeding age are great, and more than this minimum of offspring must be produced if enough are to reach maturity and to breed a further generation. Each female shark must produce probably at least three offspring, and possibly many more, if the population is to be maintained. Further, it is probable that only one young shark is produced at each birth, and the age necessary to produce three offspring therefore is 4+2+2+2 = 10 years. This is probably an under-estimate, and it is quite possible that Basking Sharks commonly reach their quarter century.
The Brain and Behaviour. 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-like cobweb. The brain is little more than three times the length of that in quite small sharks, such as dogfish, and its small size is possibly correlated with the generally slow movements of the fish and their apparent insensitivity to pain when struck by a harpoon or otherwise injured. There is, of course, a slight reaction if the fish is disturbed, but in common with other elasmobranchs there seems to be little reaction to injuries that would cause great pain in higher animals, and the fish seem to be immune to “surgical shock.” It is well known that the brain in some lower animals can be destroyed, but that many of the bodily activities are unimpaired by the operation, a large number of the bodily functions being controlled by the spinal cord; an animal on which this operation has been performed is known as a “spinal animal.” The comparison of the Basking Shark, with its tiny brain and sluggish habits, with a spinal animal needs no stressing. 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. In the lateral-line system the majority of the ampullæ of Lorenzini are concentrated into a mass which occupies the whole of the space above the rostral cartilage.
Feeding. The gill arches each carry from one thousand to thirteen thousand gill-rakers up to ten millimetres 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 mucus secreted by the epithelium at their bases, and that the mixture is squeezed out into the mouth when the rakers collapse. The total respiratory surface of the gill filaments in a shark seven metres long is calculated to be of the order of two hundred and seventy square metres.
Among the contents of the stomach it was possible to recognise fish eggs of several species, Calanus and other copepods, and larvæ 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 thirty per cent of the total, and mucus accounted for a considerable part of it. The organic solids contain approximately seven to eight 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.
Parasites. The large parasitic copepod, Dinematura producta (photographs 70 and 71), 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 larvæ 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 recognisable blood corpuscles, and it is possible that the food is no more substantial than mucus.