SHELLFISH CAN KILL

Grey River Argus, 12 February 1938, Page 5

SHELLFISH CAN KILL

WITH THEIR TEETH. The microscope opens up a field of ever-inereasing wonder, hidden worlds of knowledge that man never dreamed of, prior to perfection of this fascinating and extremely useful instrument. For instance, it will surprise most people to learn that most shellfish—that is, practically all but bivalves — are furnished with elaborate series of teeth. They are not arranged in an upper and lower jaw, as in mammals, but appear as numerous symmetrically placed projections on a flexible tongue-like ribbon. This dental structure, absolutely peculiar to the shellfish, is termed the radula. Both the ribbon and the teeth of the radula are composed of chitin—that is, the same horny material as In the hard parts of all insects. The radula lies enveloped in a kind of membrane, in the floor of the mouth and throat, being quite flat in the forward part but usually curving up so as to line the sides of the throat further back, in so.me cases eventually forming almost a tube. The upper surface over which the food passes is covered with teeth of the most varied shape, size, number and disposition, almost invariably arranged in symmetrical rows. The teeth are attached to the cartilage on which they work, by muscles which serve to erect or depress them; probably also the radula as a whole can be given a forward or a backward motion, so as to rasp the food which passes over it. The action has been likened, not to dragging a harrow over a field, but to the reversal of that. process. The teeth on the front j part of the radula are often much worn and probably fall away by de- | grees, their place being taken by others successively pushed up from behind.

The length and breadth of the radula varies greatly in different groups of shellfish. In the periwinkle it is very narrow and several the length of the whole animal. It is kept coiled away like a watch spring at'" the back of the throat, only a gmall proportion of the whole being in use. In some of the larger shellfish the radula teeth may be felt distinctly if it is drawn across the hand. Limpets have the radula extremely large in proportion to the size of the shell, and that is the reason why gastronomic discomfort often follows if these shellfish are eaten, although to the initiated this is avoided by the removal of the dental ribbon before the shellfish is cooked. The number of teeth in the radula varies greatly. When the teeth are very large they are usually few in number; when small they are numerous. In the carnivorous feeding kinds as a rule the teeth are comparatively few and powerful, while in the herbivorous feeders they are many and small. Large hooked and sickle-shaped teeth, sometimes furnished with barbs like an arrow head, are characteristic of kinds which feed on flesh; vegetable feeders, on the contrary, have the teeth blunt and chisel shaped.

Some shellfish have one or two teeth only, while others have such vast numbers they almost baffle calculation. The introduced garden snail, for instance, has over 15,000, equal to 400,000 to the square inch, and in the little-known umbrella shell over 750,000 have been estimated. I wonder if thev ever suffer from toothache! Certain of the whelks and allied shellfish have developed the radula as a most effective drill. One species in particular, the ovster borer, Lepsiella scobina, occasions the Manne Department much concern owing to its depredations among rack oysters. The borer drills a neat circular hole less than an eighth of an inch in diameter in a surprisingly short time, the animal within the oyster then being extracted piecemeal through the tiny hole. I once timed one of these borers, and the drilling of quite a thick oyster shell took no longer than three-quarters of an hour. This rapid penetration is made possible by secretion of sulphuric acid by the borer, which converts the carbonate of lime of the oyster’s shell into sulphate of lime, thereby' greatly reducing the resistance. The radula has proved n valuable guide to classification of many groups of shellfish. Its design and the shape of the teeth are conservative in allied species, and in cases where external divergence in the shells has made their relationships obscure, study of the teeth will show their basic origin. Similarly, shells that have a certain shape, brought about by their conditions of living, frequently have a deceptive resemblance to other actually unrelated groups, and here again, study of their dentition emphasises this dissimilarWhen the scientific importance of the study of shellfish dentition was first realised, a German specialist became most enthusiastic and sought to get the living animal of many rare shells in order Io establish their true systematic relationships. He tried in particular to get the animals of two apparently closely related West African species, and finally, in desperation, offered a considerable reward to a native, who promptly provided specimens containing animals. The results startled the specialist, for he found that although the shells seemed identical, the teeth were entirely different in the two specimens. He published his remarkable results, and it was not until many years later that he realised that he had been hoaxed, for the native had extracted the animals from a common and totally unrelated species and rammed them in the shells of the species sought by the specialist. A press report last year from Brisbane described how a young man, on a pleasure cruise to Hayman Island, died as the result of a sting received from a shellfish. While he was examining a beautifully marked cone shell a spike came out and pierced his hand. Although he felt no pain at the time, a short while afterwards he complained that his eyesight was failing, and that he had a burning sensation round the mouth. He soon became unconscious, and died while being conveyed to the hospital. ‘ " ' ■ Certain of the tropical cone shells have long been known to possess venomous qualities, and this has been traced definitely to their highly specialised radulae. Teeth are few—arranged in pairs. Each is fitted with a sharp barb and a poison duct. Visitors to the tropics should beware of these cone shells, particularly as their beautiful colourings cannot fail to attract the collector’s eye. However, living ones are not likely to befound exposed on a beach, it being necessary to search for them by looking under ledges, and also by

turning over blocks of coral where they are usually hidden away in crevices. Fortunately, we have none of these shellfish in New Zealand waters.