WAYS OF THE WILD.

Auckland Star, Volume LXVIII, Issue 150, 26 June 1937, Page 25 (Supplement)

WAYS OF THE WILD.

TEETH OF OUR SHELLFISH.

SOME INTERESTING FEATUB4R3

(By A. W. B. POWELL.)

It will surprise most people to learn that most shellfish, that is, practicality all but the bivalves, are furnished with elaborate series of teeth. These teeth are not arranged in an upper and lower jaw as in the mammals, but appear a« numerous symmetrically placed projections on a flexible tongue-like ribbon. This dental structure, which is absolutely peculiar to the shellfish, is termed the radula. Both the ribbon and th* teeth of the radula are composed of chitin, that is, the same horny material as the hard parte of all insects. The radula lies enveloped in a kind ol membrane, in the floor of the mouth and throat, being quite flat in the forward part, but usually curving up so aft to line the sides of the throat further back, and in some cases eventually forming almost a tube. The upper surface over which the food passes i« covered with teeth of the most varied shape, size, number and disposition, which are almost invariably arranged in symmetrical rows. These 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 or card 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 part of the radula are often much worn and probably fall away by degrees, their place being taken by others successively pushed up from 4 behind.

Variety in Radula Formation. The length and breadth of the radula varies greatly in different groups of shellfish. In the periwinkle it is very narrow and several times the length of the whole animal. It is kept coiled away like a watch spring at the back of the throat, only a small 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 nwnr ber, 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 chiselshaped. Some shellfish have one or two teeth only, while others have such vast numbers that 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 they ever suffer from toothache!

Radula as Effective Drill. Certain of the whelks and allied shellfish have developed the radula as a most effective drill. One species in particular, the oyster borer, Lepsiella scobina, occasions the Marine Department much concern owing to its depredations among the rock 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 the 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.

Two other shellfish related to the Lepsiella, but much larger, are also destructive to the beds. They are Xeothais scalaris and Lepsia haustrum. Owing to its size and strength, the former does not take the trouble to bore the oyster's shell, but uses bruto strength by attaching its sucker-like foot to the upper valve of the oyster and thus gains leverage on the rock base with the edge of its shell. When the oyster has been forced to open iU shell sufficiently the Xeothais wedges the edge of its own shell between the two valves of the oyster, who is then at the mercy of the rasping teeth of the enemy. Venomous Cone Shells of Tropics.

The radula has proved a valuable guide to the classification of manv groups of the shellfish. Its design and the shape of thi teeth are verv 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 actuallv unrelated groups, and here, again, studv of their dentition emphasises this dissimilaritv.

A Press report earlier in the rear from Brisbane described how a voung man. on a pleasure ciuise to Havman 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 coinplained that his eyp.-ight was failing and that he had a burning sensation round the mouth. He soon became unconscious and died while being conveved to the hospital.

Certain of the tropic-:;] cone shells have long been known to possess venomous qualities, and this ha- been traced definitely to their liighly specialised radulae. The teeth are few. arranged in pairs, and each is fitted with a sharp barb and a definite prison duet. Visitors to the tropics should l*«aro of these cone shells, particularly as their beautiful colouring cannot fail to

attract the collector's eye. However, living ones are not likely to be found 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 -.wually hidden away in crevices. Fortunately "we have none of these shellfish in New Zealand waters.