Some Curious Bivalves

Though certain species of molluscs have their respective habitats, and that which is considered rare in one part may be common in another, there are few which have not a general interest for the scientific conchologist. Collectors prize shells on account of their rarity and beauty; the man of science because of the assistance they afford in the working out of the universal problems of nature. Neither a collector nor a scientific student, my attitude towards marine objects is that of a mere observer — an interested and often wonder-struck observer — so that when I classify one species of mollusc as common and another as rare I am judging them in accordance with my own environment and information, not from a general knowledge of one of the most entertaining branches of natural history. From this standpoint I may refer to four or five species which stand out from the rest in interest and comparative rarity.
An oyster (OSTREA DENDOSTREA FOLIUM), too mean of proportions, too dull and commonplace of colour to be termed pretty, worth nothing, and justifying, in appearances its worthlessness, is remarkable for the exercise of a certain sort of deliberate wit in accordance with special conditions. Nature provides various species of the great oyster family with respective methods of holding their own in the sea, and in the case under review she permits the individual to exercise a choice of two different methods of fixture as chance and the drift of circumstances decide its location. From the bases of the valves spring three or more pairs of hook-like processes which, if Fate decides upon a certain coral host, encircle a slim “twig,” creating for the mollusc a curious resemblance to a short-limbed sloth hugging tightly the branch of a tree. When the spat happens to settle in places where coral is not available the hooks or arms are but crudely developed. It becomes a club-footed cripple, its feet adherent by agglutination or fusion to a rock or other and larger mollusc, dead or alive. In fact, the shrewd little oyster responds to its environment, clasping a twig with claws or cementing itself to an unembraceable host in accordance as contingencies insist.
Another mollusc (AVICULA LATA), sometimes found in company with the clinging oyster, resembles, when the fragile valves are expanded, a decapitated butterfly, brick-red in colour, with an overshirt of fine and elaborate network, orange tinted. The interior is scarcely less attractive, the nacre having a pink and bluish lustre, while the “lip” is dark red. This is found (in my experience) only in association with a certain species of coral (GORGONIA), which flourishes in strong currents on a stony bottom three or four fathoms deep. Apart from the unusual shape and pleasing colours of the shell, it is remarkable because it seems to be actually incorporated with its host. The foot of the mollusc is extended into a peduncle, consisting of fibres and tendons, by which the animal is a fixture to a spur of coral. At the point of union (to facilitate which there is a hiatus in the margins of the peduncle) the sarcode or “flesh” of the coral is denuded, its place being occupied by ligaments, which by minute ramifications adhere so intimately to the coral stock or stem that severance therefrom cannot be effected without loss of life to the mollusc.
On a single spray of ruddy Gorgonia several of these commensal molluscs may occur in various stages of development — the smaller no bigger than the wing of a fly and almost as frail, the larger three and four inches long, and each whatsoever its proportions securely budded on and growing from a spur, while frequently the valves of the large are bossed with limpets and other encumbrances. In appearance the shell represents a deformity in usurpation of a thin pencilate “growth” of coral a foot long, for the exterior colouration is that of the coral. Quite independent of their host for existence, these molluscs are not to be stigmatised as parasites, though the individual spur to which each is attached is invariably destroyed by the union, merely sufficient remaining for the support of the intruder. Natural science provides many illustrations of symbiosis, or the intimate association of two distinct organisms. This example may be out of the common, and therefore worthy of inclusion in a general reference to the life of the coral reef.
A third species, rare in a certain sense only, is of a most retiring, not to say secretive, disposition. For several years I sought in vain a living specimen of a flattened elongated bivalve (VALSELLA), buff-coloured externally, very lustrous within, with a hinge the centre of which resembles a split pearl. The blacks could offer no information beyond that which was delightfully indefinite. “That fella plenty alonga reef. You look out. B’mbi might you catch ’em!” “Tom,” who never wilfully parades his ignorance, boldly asserted that they favoured rocks, but he had no name for them, and no living specimen was ever forthcoming to substantiate confident opinions.
An exceptionally low tide revealed several hitherto cautiously preserved secrets of the reef, among them the location of a species of sponge, dark brown, some semi-spherical, some turreted in fantastic fashion. Embedded upright in the sponges, like almonds in plum-puddings, so that merely the extremities of the valves were visible as narrow slits, were the long-sought-for molluscs. Judging by the extreme care of the species for its own protection — for it is ill-fitted in model and texture for a rough-and-tumble struggle for existence — one is inclined to the opinion that it must have many enemies. The valves are frail and brittle, and only when they gape are they revealed, and the gape is self consciously polite. The sponge embraces the slender mollusc so maternally that rude yawning is forbidden. It may lisp only and in smooth phrases, such as “prunes” and “prisms”; and, moreover, the host further insures it against molestation by the diffusion of an exceptionally powerful odour, which, though to my sense of smell resembles phosphorus, is, I am informed on indubitable authority, derivable from the active form of oxygen known as ozone. Experimentally I have placed these molluscs in fresh water, to find it quickly dyed to a rich amber colour while acquiring quite remarkable pungency. Even after the third change the water was impregnated.
Interest in the mollusc became secondary upon the discovery of the host and in consideration of the part it plays in the production of one of the special effects of coral reefs; but the mollusc serves another and timely purpose — purely personal and yet not to be disregarded. It indicates a dilemma with which the wilful amateur in the first-hand study of conchology is confronted. Although, as I have said, no local knowledge of identity was available, reference to a well-disposed expert secured the information that its title in science is VULSELLA LINGULATA; that some twenty species are known; that they all associate with sponges, and that possibly different species inhabit different kinds of sponges. It may seem unpardonably gratuitous on the part of one professedly ignorant to offer general observations upon natural phenomena; but as I find myself among the great majority who do not know and who may be more or less interested and anxious to learn, I claim justification in describing that which to me is novel and rare. In this splendid isolation I cannot hope to illuminate primary investigations with the searching light in which science basks unblinkingly, for the nearest library of text-books is close on a thousand miles away. Nor can I keep all my observations to myself. There are some which, like murder, “will out,” conscious though I am of meriting the censure of the learned.
With this off my mind, let me return to the tenement sponges, which may be likened to so many independent and flourishing manufactories of ozone. Apart from the odour of brine common to every ocean and the scents of the algae and some of the flowering plants of the sea, which are similar all over the world, a coral reef has a strong and specific effluence. The skeletonless coral (ALCYONARIA) has a sulphurous savour of its own, and the echini and bêche-de-............