Tuatara: Volume 6, Issue 1, January 1956
The Diversity of the Shark World
The Diversity of the Shark World
It is customary for most people to think of sharks only in the terms used by many writers of adventure fiction. In other words, all sharks are sleek ravening monsters of the sea, differing little from each other in appearance, but characteristically with gaping maws and hideous teeth ever ready to seize a human victim should the opportunity present itself. A concept such as this does poor justice to sharks as a whole, for although they do not number many species — only some 250 are known throughout the world — they show a diversity of form, habitat, diet and development equal to that of many other and better known animal groups. Within the confines of their marine environment they invade many niches, and exhibit a variation in structure enabling them to pursue many more ways of life than the fiction writer would have us believe.
In discussing the diversity of a group of animals, it is perhaps useful to begin by giving an account of the sizes reached by the smallest and largest members, for in this way comparisons can be made with other known animals or objects which will then serve as standards. The size variation of sharks is striking. In terms of overall length, the largest species known, the Whale Shark, is at least thirty times longer than the smallest, and reaches certainly to 45 feet and possibly to 60 feet. In contrast the smallest species of which there are several amongst the Cat Sharks and some of the Smooth Dogfishes, mature at lengths of 1 foot 6 inches, and never grow much beyond that. The range between these two extremes is well represented by other species, not only at the small end of the scale, but also by several species of large sharks. Thus the Basking Shark which is known to reach 40 feet and possibly more, and the White Pointer of which the largest specimen measured was 36½ feet long, indicate that the Whale Shark is not an isolated ‘giant’ species but simply the largest known living member of the well graduated series of fishes which we know as sharks. Some fossil sharks apparently grew to an even larger size, for fossil teeth up to six inches long, and from a species similar to if not identical with the White Pointer, have been found in various parts of the world. The longest teeth in the 36½ feet White Pointer mentioned above were only about two inches long, so that it has been assumed that the fossil White Pointers grew to 80 or 90 feet.
The maximum weight attained by the larger sharks is not inconsiderable even though it far from parallels the weight of the other large marine vertebrates, the whales. Unfortunately there are no records of the weight of the largest sharks known, but a specimen of Whale Shark 38 feet long page 14 was estimated to weigh just less than 27,000 pounds. In comparison a Humpback Whale of the same length would weigh more than twice as much.
Diversity in the body form of various sharks is on a comparable scale to that of their size, and the differences are sufficiently marked to separate them into seven major groups each of which has its own distinctive combination of characters. Three of the groups, the Six- and Seven-Gilled Sharks, the Galeoid Sharks and the Spiny Dogfishes, are superficially rather similar, with a ‘typical’ shark-like appearance, but differ from each other and from the other groups in such fundamental features as the number of gill-openings, the presence or absence of various fins, fin-spines and so on. As these features do not greatly alter the overall appearance, they do not call for attention here. An illustration of a Galeoid Shark is given (Fig. 1) for comparison with members of the other groups mentioned below.
The four remaining groups of sharks show a remarkable diversity of form. The Port Jackson or Bullhead Sharks (Fig. 2) which have a massive head, with prominent ridges above the eyes, an almost terminal mouth, and such a characteristic profile as to result in the common name ‘Bullhead Shark’. are the least modified when compared with the ‘typical’ sharks, though there is no doubt about their distinction.
The Frill-Gilled Sharks (Fig. 3) include only one species which is so far known only from deep water off Japan and the eastern North Atlantic, and are eel-like sharks with long, thin bodies and a terminal mouth. The single dorsal fin is set well back along the body, as are the pelvic fins, thereby enhancing the eel-like appearance.
The Saw Sharks (Fig. 4) are rather small sharks, of tropical and sub-tropical waters. They show a remarkable development of the snout which extends forward as a long blade-like process armed with a row of tooth-like structures on each side, and with a pair of barbels on its under-surface about halfway along its length. It is worthy of note that this feature is paralleled in the Skates and Rays, which are closely related to the Sharks, by the Saw-Fishes which grow to a large size — 20 feet or more — and have similar blade-like snouts armed with heavier teeth, but lacking the barbels.
The Angel Sharks (Fig. 5) are small sharks which superficially have a very strong resemblance to skates and rays and particularly to the Fiddler Rays. In other words their heads and bodies are greatly flattened, the paired fins are extended in size, the dorsal fins are placed well back and near to the tail, and the hinder part of the body is sharply marked off from the flattened front part. The resemblances shared in these features by the Angel Sharks and the Fiddler Rays are so great as to necessitate the distinction of the Angel Sharks only by such characters as the lack of fusion of the front margins of the pectoral fins along the sides of the head, and the at least partly lateral position of the gill-openings.
The diversity of form that is seen in the above brief examination of the main groups of sharks by no means exhausts all that occurs in sharks as a whole. Within some of the main groups an almost equal diversity is known, page 15 particularly in the Galeoid Sharks which were not considered in the preceding account because the majority of them are ‘typical’ sharks. Two examples of Galeoid Sharks, the Thresher Shark and the Hammerhead, may be selected as outstanding examples.
The Thresher Shark (Fig. 6) differs from all others in that the tail is long and sickle-shaped, its length equal to that of the body. If we add to this the fact that the body is very streamlined, and that in one kind of Thresher Shark the eyes are extremely large, we get altogether a very unusual-looking fish. The Thresher Shark feeds chiefly on schooling fishes, and is reputed to occasionally use the peculiar extended tail as a flail to stun its prey. Other accounts repute that the Thresher will swim around a school of fish while thrashing its tail in the water, thus frightening the fish and keeping them together until it can get amongst them.
The Hammerhead Shark (Fig. 7) when seen in side view has nothing extraordinary about its appearance. The same cannot be said when it is viewed from above or below, for the sides of the head in the region of the eyes are so much expanded that the head resembles a double-ended hammer. The eyes are placed at the outer ends of these ‘hammer’ processes, and hence are literally out on stalks.
Other differences in the shape and appearance of sharks may also be seen in the Galeoids. The Mako Sharks (Fig. 8) which are a well-known big-game fish in New Zealand as well as in other parts of the world, are so streamlined as to be truly torpedo-shaped. The head of a Mako is sharply pointed and thus contrasts strongly with that of the Tiger Shark (Fig. 9) which is almost square-cut. One of the Smooth Dogfishes recently discovered in New Zealand waters has such an attenuate body as to be almost eel-like, though its head is flat and shovel-shaped. Colour differences are also prominent in many Galeoids. The majority of sharks are dark coloured above and lighter below, the dark colour usually being grey, blue or brown, and the lighter colour white or cream. As an example of this the Blue Shark is a brilliant indigo blue above and snow white below. However, many of the Cat Sharks and Carpet Sharks have a pattern superimposed on their backs and sides, and in young specimens of these species the resultant effect is very attractive. The patterning may take the form of spots or blotches, checker-board squares, or chain-like markings. The Tiger Shark is similarly patterned with rather irregular transverse stripes, though these disappear in older animals. It may be added here that in a few sharks, notably some of the deep-sea Spiny Dogfishes, there are luminescent organs in the skin.
Although most sharks are marine, at least a few species are found far upstream in large fresh-water rivers such as the Ganges, while one species is landlocked in Lake Nicaragua. More sharks occur in sub-tropical and tropical waters than elsewhere, and so far only a few species of the genus Somniosus, the Greenland Shark, are known from polar waters. One specimen of Somniosus was washed ashore at Macquarie Is. early this century and is our only record of the genus in southern waters. However, page 16 Greenland Sharks are relatively abundant in the Arctic, both in the North Pacific and the Atlantic, and have been observed and caught amongst the pack-ice in water as cold as −0.6° C. Many sharks are oceanic, particularly the larger species, and are often seen at the surface, while others live near or on the bottom in either deep or shallow water. The deep-water sharks living on or near the bottom are for the most part the Spiny Dogfishes, and are characteristically a uniform dark colour, which may be grey, brown or even black. They include some bizarre members such as Oxynotus (Fig. 10) which has high, enlarged dorsal fins, and several other species with flattened, extended, shovel-like snouts. The scales or dermal denticles on these deep-water Spiny Dogfishes are usually very large, and range in shape from bristle-like structures to three-pronged or rounded blades which are variously sculptured on their outer surface. A similar variation in denticle shape and structure occurs in other sharks also, though it is less obvious because of the commonly minute size of the denticles. The greatest depth in which any shark is known to have been caught is 1,500 fathoms; conversely many Carpet Sharks occur on weedy or rocky shores in water only a few feet deep. Most sharks are not markedly gregarious, though a few species such as the Basking Shark seem to form into definite schools' but with rather limited numbers, while others, including the common Spiny Dogfishes which live in depths down to about 100 fathoms or more, may be caught in such large numbers as to suggest schools of a very considerable size.
Fig. 1, Whaler Shark (Galeoidea). Fig. 2, Bullhead Shark. Fig. 3, Frill-Gilled Shark. Fig. 4, Saw Shark. Fig. 5, Angel Shark. Fig. 6, Thresher Shark. Fig. 7, Hammerhead Shark. Fig. 8, Mako Shark. Fig. 9, Tiger Shark. Fig. 10, Oxynotus sp. (Figures redrawn from illustrations in Bigelow and Schroeder, Goodrich, Waite and Whitley.)
All sharks are carnivorous, with the possible exception of the Whale Shark which may include seaweed as part of its regular diet. The larger predaceous sharks feed chiefly on fish, squid, and other animals not always of small size, as for example seals, sea-birds and turtles. A Greenland Shark was even found to have swallowed a reindeer. A lack of discrimination is shown in the diet of some sharks, for carrion of any sort, and rubbish of all kinds including such unlikely articles as a sack of coal, a kerosene tin, and bottles both empty and full have at times been found to be acceptable to large hungry sharks. The largest sharks, however, the Whale Shark and the Basking Shark, feed only on small planktonic organisms such as shrimps and small schooling fishes, in the same manner as whalebone whales. Like the whales, they have a sieving apparatus for straining this minute food from the water, and consisting of bristle-like gill-rakers in the Basking Shark, and a sponge-like tissue in the Whale Shark. In both cases the teeth are minute. The teeth of most sharks are directed slightly backward, and hence are suited mainly for seizing and retaining food rather than for cutting or crushing it. Even in those sharks with very broad, triangular, blade-like teeth, such as the White Pointer, the gaps between adjacent teeth are so considerable as to prevent a complete cutting action. The upper and lower teeth may also be totally different in appearance, especially in some of the deep-water Spiny Dogfishes, in which the lower teeth are reclined blades while the uppers are awl-shaped or even needle-like. The Smooth Dogfishes and the Bullhead Sharks have flat, pavement-like teeth, which are used to crush the shell-fish, crabs and prawns which form a major part of their diet. In contrast to these hard-shelled feeders it can be cited that some omnivorous sharks will even utilise gelatinous medusae or jellyfish as food.
The above few examples illustrate a little of the variety seen in the food-habits of sharks. It is to be hoped that they will indicate, together with the other features of form, habitat and development mentioned before, something of the diversity and hence the interest of the group as a whole. Such interest is worthy of greater appreciation than the attention which is commonly focussed on sharks due to the sinister reputations of only a few of the species.