SHARKS IN OUR WATERS

Press, Volume CIII, Issue 30454, 30 May 1964, Page 4

SHARKS IN OUR WATERS

[Reviewed by A.M.E.]

Sharks and Rays of Australian Seas. By David G. Stead. Australian Museum. Angus and Robertson. 211 pp.

This lively book provides a great deal of information on all known species of sharks and rays found in Australian waters; their description, breeding habits, attack patterns and distribution, much of the subject matter being applicable to New Zealand. The treatment is very readable but at the same time does not lack scientific definition and detail for those who require more precise information.

It opens with a general description of sharks and rays. The skeleton is wholly cartilaginous, the gills are uncovered and the creatures are found in all seas, and many fresh water places, from the Arctic to the Antarctic in depths ranging from a few feet to some thousands of feet in the great oceans. In the majority of species the sharks and rays are viviparous, that is to say, the young are born alive but in some species the egg is hatched within the female and the young extruded alive. In all species, fertilisation takes place within the female as with the mammals, and not externally as with most bony fish where the eggs are fertilised after being deposited by the female. There is no external female organ but the male is equipped with two organs in the form of claspers which, when erected, come together and form a channel along which the fertilising fluid is passed to the female. Although the general appearance of a shark resembles that of a fish, there are many important differences such as the construction of the fins. In the fishes these are soft and flexible but in the sharks they are hard and erect. In the evolution of the elasmobranch family as the sharks and rays are correctly called, there is an almost perfect graduation from sharks proper to the rays, the latter being adapted to bottom dwelling. In the elasmobranch structure there is no swimbladder which is so common in bony fishes. The elasmobranch have been about for a very long time and many different species appear in the fossil record. They form the most primitive group of backboned animals although their skeletons are not calcified but are wholly cartilaginous. One can hardly think of sharks without thinking of teeth. In most sharks these are arranged in rows, those on the outer edge being erect and functional and those in the inner rows being successively less so. As the functional teeth are lost they are replaced by those from the inner rows which, with the gum in which they are imbedded, move out and allow the teeth to become erect. There is an interesting section on the development of

the senses. The most important is that of smell and it is apparently by this means that they hunt down their prey. Some very interesting experiments with food crabs are recorded. Sound, and sight, appear to be of less importance but in some species there are special feelers by means of which they feel for the movements of their prey on, or in, the sandy or muddy bottoms on which they live. However, these senses do not assure a regular supply of food and there is a very large liver to tide the animal over the long periods of near-starvation which it has to endure from time to time.

The introductory section concludes with a classification of the fishes and the origin of the name “shark.” It is interesting to note that it appears to have been derived from the Latin name given to the Great White Shark of the Mediterranean, Carcharus. Of the remainder of the book, by far the greatest part is taken up with descriptions of all sharks and rays likely to be encountered in these waters. This begins with a description of the physical characteristics, distribution and habits of the more primitive seven-gill and six-gill sharks. The descriptions are a comfortable amalgam of scientific detail and interesting narrative of incidents involving sharks, from which it would appear that the diet is extremely varied. “In its stomach was a man’s straw hat, on the band of which was the name of a well-known Sydney hatter, there was also the cover of a book on philosophy, a piece of cloth and a spinal vertebra—not necessarily human.” Scattered throughout the text are accounts of attacks by sharks on humans, and as they have been seen as far as 140 miles up river from the sea, this is something which is likely to happen at any time and at any place. The author devotes several pages of one section to this matter and in doing so produces some extremely interesting reading and also some good advice to those who appear to enjoy presenting themselves as potential shark bait. One of his conclusions has at least one redeeming feature. “One of the most remarkable facts about attacks by this shark on swimmers or surfers is that, without exception in my experience, once the shark has snapped at the victim and perhaps torn an arm or a leg off or taken part of the victim’s body, it does not come back to the attack.” To remain whole, it might be better to accept the precautions which he suggests. There is no evidence to support the notion that a shark will lie in wait for humans. Indeed it appears to be a rather conservative creature and the initial snap is likely to contain an element of surprise in so far as the shark is concerned, after which its natural caution is likely to cause it to hold off whilst' it considers this new

experience so unlike the normal catching of freeswimming fish. The author quotes 12 case histories of attack and it is apparent that an observable behaviour pattern does exist. The electric rays are a surprising adaptation of nature. “All species possess two electric organs, one on each side of the head. These are the batteries; collections of hexagonal cells filled with a jelly-like matter. Some of the larger species are able to throw out a surprisingly powerful shock ... it will surprise many when I say that quite a lot of these electric rays are eaten by sharks.” Trawlermen fishing off the New Zealand coast and near the Chatham Islands meet with one of these species and have learnt to respect it because of the powerful shocks which it can administer.

There is a section on Stingrays, (so called because of the poisonous barbs on the back of the tail), Devil Rays and Great Sea Bats and the book concludes with a brief chapter on Pilot-fish and Suckers. This a very readable combination of popular information and text-book data. It would appear to anyone associated in any way with the sea and to the biology student who requires easily assimilated information. For the latter there is a scientific appendix which covers all elasmobranch species, a bibliography of further reading and a good index. There are 64 illustrations which would be of considerable assistance to those interested in identification.