1911 Encyclopædia Britannica/Shark
|←Shari||1911 Encyclopædia Britannica, Volume 24
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SHARK, a Selachian fish (see Selachians), belonging to the order Plagiostomi, suborder Squali.
Sharks are almost exclusively inhabitants of the sea, but some species enter the mouths of large rivers, and one species (Carcharias gangeticus) occurs frequently high up in the large rivers of India. C. nicaraguensis of the lake of Nicaragua and the Rio San Juan appears to have taken up its residence permanently in fresh water. Sharks are most numerous between the tropics, a few only reaching the Arctic circle; it is not known how far they advance S. in the Antarctic region. Altogether some hundred and fifty different species have been described.
With regard to their habits many are littoral species, the majority pelagic, and a few are known to belong to the deep-sea fauna, having hitherto been obtained down to a depth of nearly 1000 fathoms.
|Fig. 1 — Teeth of Tope. u, Upper; l, lower. (× 2.)|
Littoral Sharks. — The littoral forms are of small size, and generally known under the name of “dog-fishes,” “hounds,” &c. Some pelagic sharks of larger size also live near the shore on certain parts of a coast, but they are attracted to it by the abundance of food, and are as frequently found in the open sea, which is their birthplace; therefore we shall refer to them when we speak of the pelagic kinds.
The majority of the littoral species live on the bottom, sometimes close inshore, and feed on small marine animals or on any animal substance. The following are deserving of special notice.
The tope (Galeus) is common on the coasts not only of England, Ireland and of S. Europe, but also of S. Africa, California, Tasmania and New Zealand. Its teeth are equal in both jaws, of rather small size, flat, triangular, with the point directed towards the one side, and with a notch and denticulations on the shorter side (fig. 1). It is of a uniform slaty-grey colour, and attains to a length of 6 ft. The female brings forth some thirty living young at one birth in May. It becomes troublesome at times to fishermen by taking their bait and driving away other fish they desire to catch. The fins of G. zyopterus of the Californian coast are much esteemed for culinary purposes by the Chinese.
|Fig. 2 — Teeth of Mustelus.|
The hounds proper (Mustelus) possess a very different dentition, the teeth being small, obtuse, numerous, arranged in several rows like pavement (fig. 2). Five or six species are known from the shores of the various temperate and subtropical seas, one (M. vulgaris) being common on the coasts of Great Britain and the United States, on the Pacific as well as the Atlantic side. It is of a uniform grey colour or sparingly spotted with white, and attains to a length of 3 or 4 ft. The young, about twelve in number, are brought forth alive in November. It is comparatively harmless and feeds on shells, crustaceans and decomposing animal substances.
|Fig. 3 — Teeth of Scyllium canicula.|
The dogfishes proper (Scyllium, Chttoscyllium, &c.) are spread over nearly all the temperate and tropical seas. Their teeth are small, in several series, with a longer pointed cusp in the middle, and generally one or two smaller ones on each side (figs. 3 and 5). They are all oviparous, their oblong egg-shells being produced at each corner into a long thread by which the egg is fastened to some fixed object. Some of the tropical species are ornamented with a pretty pattern of coloration. The two British species, the lesser and the larger spotted dogfish (Sc. canicula and Sc. catulus), belong to the most common fishes of the coast and are often confounded with each other. But the former is finely dotted with brown above, the latter having the same parts covered with larger rounded brown spots, some of which are nearly as large as the eye. As regards size, the latter exceeds somewhat the other species, attaining to a length of 4 ft. Dogfishes may become extremely troublesome by the large numbers in which they congregate at fishing stations; they are rarely used as food, except in the Mediterranean countries, in China and Japan, and in the Orkneys, where they are dried for home consumption. The black-mouthed dogfish (Pristiurus melanostomus) is rarely caught on the British coasts, and is recognized by a series of small, flat spines with which each side of the upper edge of the caudal fin is armed.
Fig. 4 — Chiloscyllium trispeculare.
The tiger-shark (Stegostoma tigrinum) is one of the commonest and handsomest sharks in the Indian Ocean. The ground colour is a brownish-yellow, ornamented with black or brown transverse bands or rounded spots. It is a littoral species, but adult specimens, which are from 10 to 15 ft. long, are met far from land. It is easily recognized by its enormously long bladelike tail, which is half as long as the whole fish. The teeth are small, trilobed, in many series. The fourth and fifth gill-openings are close together.
|Fig. 5 — Confluent Nasal and Buccal Cavities of the same fish.|
The genus Crossorhinus, of which three species are known from the coasts of Australia and Japan, is remarkable as the only instance in this group of fishes in which the integuments give a “celative” rather than a “protective” resemblance to their surroundings. Skinny frond-like appendages are developed near the angle of the mouth, or form a wreath round the side of the head, and the irregular and varied coloration of the whole body closely assimilates that of a rock covered with short vegetable and coralline growth. The species of Crossorhinus grow to a length of 10 ft.
The so-called Port Jackson shark (Heterodontus = Cestracion) is likewise a littoral form. Besides the common species (H. philippi), three other closely allied kinds from the Indo-Pacific are known. This genus, which is the only existing type of a separate family, is one of special interest, as similar forms occur in Primary and Secondary strata. The jaws are armed with small obtuse teeth in front, which in young individuals are pointed, and provided with from three to five cusps. The lateral teeth are larger, pad-like, twice as broad as long and arranged in oblique series (fig. 7). The fossil forms far exceeded in size the living, which scarcely attain to a length of 5 ft. The shells of their eggs are found thrown ashore like those of our dogfishes. The shell is pyriform, with two broad lamellar ridges each wound edgewise five times round it (fig. 8).
Fig. 6 — Heterodontus galeatus.
The spiny or piked dogfish (Acanthias) inhabits the temperate seas of both the N. and S. hemispheres. For some part of the year it lives in deeper water than the sharks already noticed, but at uncertain irregular times it appears at the surface and close inshore in almost incredible numbers. Couch says that he has heard of 20,000 having been taken in a seine at one time; and in March 1858 the newspapers reported a prodigious shoal reaching W. to Uig, whence it extended from 20 to 30 m. seaward, and in an unbroken phalanx E. to Moray, Banff and Aberdeen. These fishes are distinguished by each of the two dorsal fins being armed in front by an acute spine. They do not possess an anal fin. Their teeth are rather small, placed in a single series, with the point so much turned aside that the inner margin of the tooth forms the cutting edge (fig. 9). The spiny dogfish are of a greyish colour, with some whitish spots in young specimens, and attain to a length of 2 or 3 ft. They are viviparous, the young being produced throughout the summer months.
Fig. 7 — Upper Jaw of Port Jackson Shark (Heterodontus philippi). (× ½.)
Finally, we have to notice among the littoral sharks the “angel-fish” or “monk-fish” (Rhina, squatina), which, by its broad flat head and expanded pectoral fins, approaches in general appearance the rays. It occurs in the temperate seas of the S. as well as the N. hemisphere, and is not uncommon on sandy parts of the coast of England and Ireland. It does not seem to exceed a length of 5 ft., and is too rare to do much injury to other fish. It is said to produce about twenty young at a birth.
Fig. 8 — Egg-shell of same fish (× ½). I., External view; II., section; a and b, the two spiral ridges; c, cavity for the ovum.
|Fig. 9 — Teeth of Acanthias vulgaris.|
Pelagic Sharks. — All these are of large size, and some are surpassed in bulk and length only by the larger kinds of cetaceans. Those armed with powerful cutting teeth are dangerous to man, whilst others, which are provided with numerous but very small teeth, feed on small fishes only or marine invertebrates, and are of a timid disposition, which causes them to retire into the solitudes of the open sea. On this account we know very little of their life. All pelagic sharks have a wide geographical range, and nearly all seem to be viviparous.
Of the more remarkable forms which we propose to notice here the genus most abundantly represented in species and individuals is Carcharias, now split up by many authors into several separate genera. Perhaps nine-tenths of the sharks of which we read in books of travel belong to this genus. Between thirty and forty species have been distinguished, all of which are found in tropical seas. They are the sharks which so readily attach themselves to sailing vessels, following them for weeks. Others affect more the neighbourhood of land. One of the most common species is the blue shark (Carcharias glaucus), of which specimens (4 to 6 ft. long) are frequently caught on the S. coasts of England and Ireland. Other species of Carcharias attain a length of 30 ft. The mouth of all is armed with a series of large flat triangular teeth, which have a sharp, smooth or serrated edge (fig. 10).
Fig. 10 — Dentition of the Blue Shark (Carcharias glaucus). The single teeth are of the natural size.
Galeocerdo is likewise a large shark very dangerous to man, differing from the preceding chiefly by having the outer side of its teeth deeply notched. It has long been known to occur in the N. Atlantic, close to the Arctic Ocean (G. arcticus), but its existence in other parts has been ascertained within a recent period; in fact, it seems to be one of the most common and dangerous sharks of the Indo-Pacific, the British Museum having obtained specimens from Mauritius, Kurrachee, Madras and the W. coast of Australia.
|Fig. 11 — Upper and Lower Tooth of Lamna.|
Hammerheaded sharks (Sphyrna = Zygaena) are sharks in which the anterior portion of the head is produced into a lobe on each side, the extremity of which is occupied by the eye. The relation of this unique configuration of the head to the economy of the fish is unknown. Otherwise these sharks resemble Carcharias, and are equally formidable, but seem to be more stationary in their habits. They occur in all tropical and subtropical seas, even in the Mediterranean, where S. Zygaena is by no means rare. In the Indian Ocean it is common, and Cantor states that specimens may be often seen ascending from the clear blue depths of the ocean like a great cloud.
|Fig. 12 — Tooth of Carcharodon rondeletii.|
The porbeagles (Lamna) differ from the preceding sharks in their dentition and are not dangerous to man; at least there is no instance known of a person having been attacked by the species common on the British coast (L. cornubica). This is referred to in the works of older British authors as “Beaumaris shark.” The short and stout form of its body contrasts strikingly with its much attenuated tail, which, however, is strengthened by a keel on each side and terminates in a large and powerful caudal fin. The snout is pointed, and the jaws are armed with strong lanceolate teeth, each of which bears a small cusp on each side of the base (see fig. 11). The teeth are not adapted for cutting, like the flat triangular teeth of man-eating sharks, but rather for seizing and holding the prey, which consists chiefly of various kinds of fishes and cephalopods. In the upper jaw there are from thirteen to sixteen teeth on each side, the third being remarkable for its small size; in the lower jaw from twelve to fourteen. The gill-openings are very wide. The porbeagle attains to a length of 10 or 12 ft., and is a pelagic fish, not rare in the N. Atlantic and Mediterranean, and frequently wandering to the British and more rarely to the American shores. This species is widely distributed over the N. of the Atlantic and Pacific Oceans. Other closely allied species (L. spallanzanii, L. glauca) are known to occur in the S. Atlantic, from the Mediterranean to the Cape of Good Hope.
To the genus Carcharodon particular interest is attached, because the single still existing species is the most formidable of all sharks, as were those which preceded it in Tertiary times. The existing species (C. rondeletii) occurs in almost all tropical and subtropical seas, but seems to be verging towards extinction. It is known to attain to a length of 40 ft. The tooth figured here of the natural size (fig. 12) is taken from a jaw much shrunk in drying, but still 20 in. wide in its transverse diameter, and taken from a specimen 36½ ft. long. The extinct species must have been still more gigantic in bulk, probably reaching a length of 90 ft., as we may judge from teeth which are found in the crag or which were dredged up from the Pacific Ocean by the “Challenger” expedition, and which are 4 in. wide at the base and 5 in. long measured along their lateral margin. In some Tertiary strata these teeth are extremely abundant, so much so that for instance, in Florida the strata in which they occur are quarried to obtain the fossil remains for export to England, where they are converted into artificial manure.
Fig. 13 — Basking Shark.
The fox-shark or thresher (Alopecias vulpes), of which every year specimens are captured on the British coast, but which is common in the N. and S. hemispheres, is readily recognized by its extremely slender tail, the length of which exceeds that of the remainder of the body. Its teeth are small, flat, triangular and without serrature. It follows the shoals of herrings, pilchards and sprats in their migrations, destroying incredible numbers and frequently injuring the nets. When feeding it uses the long tail in splashing the surface of the water, whilst it swims in gradually decreasing circles round a shoal of fishes which are thus kept crowded together. Sometimes two threshers may be seen working together. Statements that it has been seen to attack whales and other large cetaceans rest upon erroneous observations; its dentition is much too weak to bite through their skin. The thresher attains to a length of 15 ft., the tail included.
Fig. 14 — Greenland Shark (Laemargus borealis).
The basking shark (Selache maxima), sometimes erroneously called “sunfish,” is the largest fish of the N. Atlantic, growing to a length of more than 30 ft. Though best known from the N. of the Atlantic or Pacific oceans, this species has also been recorded from the Australian seas. The mouth is of an extraordinary width, and, like the gill-cavity, capable of great expansion, so as to enable the fish to take at one gulp an enormous quantity of the small fish and other marine creatures on which it subsists. Also the gill-clefts are of great width, and the internal opening of each is guarded by a kind of strainer, formed by the enormously elongated gill-rakers, which serves to prevent the food organisms from passing out through the clefts. The teeth are very small, numerous, arranged in several series, conical and probably without use in feeding. This shark is therefore quite harmless if not attacked. Off the W. coast of Ireland it was at one time hunted for the sake of the oil from the liver, one fish yielding from a ton to a ton and a half. Its capture is not unattended with danger, as one blow from the tail is sufficient to stave in the sides of a large boat. The basking shark is gregarious, and may be seen in calm weather lying with the upper part of the back raised above the surface of the water, a habit which it has in common with the true sunfish (Orthagoriscus) , and from which it has derived its name.
A shark similar in many points to the basking shark, and an inhabitant of the Indo-Pacific Ocean, is Rhinodon typicus. So far as our present knowledge goes, it is the largest of all sharks, as it is known to exceed a length of 50 ft., but it is stated to attain that of 70. The captures of only a few specimens are on record, at the Cape of Good Hope and near the Seychelles, where it is known as the “chagrin.” The snout is extremely short, broad and flat, with the mouth and nostrils placed at its extremity; the gill-openings very wide, and the eye very small. The teeth are extremely small and numerous, conical in shape. No opportunity should be lost of obtaining exact information on this shark. The same applies to the allied Micristodus punctatus recorded from off the W. coast of America.
|Fig. 15 — Dentition of Greenland Shark.|
The Greenland shark (Laemargus borealis) belongs to the same family as the spiked dogfish, but grows to a much larger size, specimens 26 ft. long having been met with. The two dorsal fins are small and destitute of spines. The teeth (fig. 15) in the upper jaw are small, narrow, conical in shape; those of the lower flat, arranged in several series, one on the top of the other, so that only the uppermost forms the sharp dental edge of the jaw. The points of these lower teeth are so much turned aside that the inner margin only enters the dental edge. The Greenland shark is an inhabitant of the Arctic regions, sometimes straying to the latitudes of Great Britain and of Cape Cod in the W. Atlantic; it is one of the greatest enemies of the whale, which is often found with large pieces bitten out of the tail by this shark. Its voracity is so great that, as Scoresby tells us, whilst engaged in feeding on the carcase of a whale it will allow itself to be stabbed with a lance or knife without being driven away.
The spinous shark (Echinorhinus spinosus) is readily recognized by the short bulky form of its body, its short tail, and the large round bony tubercles which are scattered all over its body, each of which is raised in the middle into a pointed conical spine. While most frequently recorded from the E. Atlantic, specimens have also been obtained from the coasts of N. America and of New Zealand. It always lives on the bottom, and probably descends to some depth. It does not seem to exceed a length of 10 ft.
|Fig. 16 — Chlamydoselachus anguineus.|
Bathybial Sharks. — Sharks do not appear to have yet reached the greatest depths of the ocean; and so far as we know at present we have to fix the limit of their vertical distribution at 1000 fathoms. Those which we find to have reached or to pass the 100 fathoms line belong to generic types which, if they include littoral species, are ground-sharks — as we generally find the bottom-feeders of our littoral fauna much more strongly represented in the deep sea than the surface swimmers. All belong to two families only, the Scylliidae and Spinacidae, the littoral members of which live for the greater part habitually on the bottom and probably frequently reach to the 100 fathoms line. Distinctly bathybial species are two small dogfishes — Spinax granulatus from 120 fathoms, and Scyllium canescens from 400 fathoms, both on the S.W. coast of S. America; also Centroscyllium granulatum from 340 fathoms in the S. Ocean, whose congener from the coast of Greenland probably descends to a similar depth. The shark which reaches the greatest depth recorded hitherto appears to be Scylliorhinus indicus obtained by the Valdivia expedition from a depth of nearly 1000 fathoms in the W. Indian Ocean. It belongs to the genus Centrophorus, of which some ten species are known, all from deep water in the N. Atlantic, Mediterranean, the Molucca and Japanese seas. The Japanese species were discovered by the naturalists of the “Challenger” on the Hyalonema ground off Inosima in 345 fathoms. Dr E. P. Wright found C. coelolepis at a still greater depth on the coast of Portugal. The fishermen of Sétubal fish for these sharks in 400 or 500 fathoms, with a line of some 600 fathoms in length. “The sharks caught were from 3 to 4 ft. long, and when they were hauled into the boat fell down into it like so many dead pigs”; in fact, on being rapidly withdrawn from the great pressure under which they lived they were killed, like other deep-sea fishes in similar circumstances. It is noteworthy that the organization of none of these deep-sea sharks has undergone such a modification as would lead us to infer that they are inhabitants of great depths.
One of the most interesting types of the division of sharks is the small family of Notidanidae, which is externally distinguished by the presence of a single dorsal fin only, without spine and opposite to the anal, and by having six or seven wide branchial openings. They represent an ancient type, the presence of which in Jurassic formations is shown by teeth extremely similar to those of the living species. Their skeleton is notochordal. Only four species are known, of which one (Notidanus griseus) has now and then strayed N. to the English coast. Allied to the Notidanidae are the Chlamydoselachidae or frilled sharks, represented so far as is known by a single living species, C. anguineus Garman (fig. 16), which occurs frequently in deep water off the coast of Japan and as isolated specimens off the coasts of New South Wales, Madeira and Norway. A fossil species has been described from the Pliocene of N. Italy. It resembles a conger in shape, and differs from the Notidani proper by its elongated body, wide nearly terminal mouth, extremely wide gill-openings and peculiarly formed teeth. The teeth are similar in both jaws, each composed of three slender curved cusps separated by a pair of minute intermediate points, and with a broad base directed backwards.
A few words may be added with reference to the economic uses of this group of fishes. As mentioned above, some of the smaller dogfishes are eaten at certain seasons by the captors, and by the poorer classes of the population. An inferior kind of oil, chiefly used for the adulteration of cod-liver oil, is extracted on some of the N. fishing-stations from the liver of the spiked dogfishes, and occasionally of the larger sharks. Cabinet-makers make extensive use of shark's-skin under the name of “shagreen” for smoothing or polishing wood. This shagreen is obtained from species (such as our dogfishes) whose skin is covered with small, pointed, closely-set, calcined papillae, whilst very rough skins, in which the papillae are large or blunt, are useless for this purpose. The dried fins of sharks (and of rays) form in India and China an important article of trade, the Chinese preparing gelatin from them, and using the better sort for culinary purposes. They are assorted in two kinds, viz. “white” and “black.” The former consists exclusively of the dorsal fins, which are reputed to yield more gelatin than the other fins. The pectoral, ventral and anal fins constitute the “black” sort; the caudal are not used. (A. C. G.; J. G. K.)