My Tropic Isle, by E. J. Banfield

Chapter XIV

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-mer are also to be separately distinguished by their fumes. Anemones, great and small, seem to disperse a recognisable scent as from a mild and watery solution of fish and phosphorus. But of all the occupants of the reef none are so powerful or so characteristic in this respect as sponges. Puissant and aggressive, these exhalations are at times so strong as to almost make the eyes water, while exciting vivid reminiscences of old-fashioned matches and chemical experiments. Substantial, wholesome, and clean — though generated by a wet, helpless creature having no personal charms, and which, having passed the phase of life in which it enjoyed the gift of locomotion, has become a plant-like fixture to one spot — the gas mingles with other diffusions of the reef, recalling villanous salt-petre and sheepdips and brimstone and treacle to the stimulation of the mental faculties generally.

Invariably an afternoon’s exploration of the coral reef is followed by a drowsy evening and a night of exceptionally sweet repose. No ill dreams molest the soothing hours during which the nervous system is burnished and lubricated, and you wake refreshed and invigorated beyond measure. I have endeavoured to account for the undoubted physical replenishment and mental exhilaration largely from the breathing of air saturated with emanations from the coral and sea things generally.

In the course of three hours’ parade and splashing in the tepid water, ever so many varieties of gas more or less pungent and vitalising — gas which seems to search and strengthen the mechanism of the lungs with chemically enriched air, to tonic the whole system, and to brighten the perceptive faculties, have been imbibed. Exercise and the eagerness with which wonders are sought out and admired may account in part for present elation and balmy succeeding sleep, but the vital functions seem, if my own sensations are typical, to receive also a general toning up. Twice a month at least a man should spend an afternoon on a coral reef for the betterment of body and brain. On the face of it this is counsel of perfection. Only to the happy few is such agreeable and blest physic proffered gratis. Yet the whole world might be brighter and better if coral reefs were more generously distributed. Breathing such subtle and sturdy air, men would live longer; while the extravagant life of the reef, appealing to him in fine colours and strange shapes, would avert his thoughts from paltry and mean amusements and over-exciting pleasures. The pomp of the world he would find personated by coral polyps; its vanities by coy and painted fish; its artfulness represented by crabs that think and plan; its scavenging performed by aureoled worms.

Although students of conchology are familiar with several species of LIMA, I am eager to include it in these haphazard references, because my first acquaintance with a living specimen afforded yet another experience of the versatility of the designs of Nature. It is truly one of the “strange fellows” which Nature in her time has framed. Living obscurely in cavities, under stones, inoffensive and humble, the Lima enjoys the distinction of being, the permanent exemplification of the misfit, its body being several sizes too large as well as too robust for its fragile, shelly covering. The valves are obtusely oblong, while the animal is almost a flattened oval, the mantle being fringed with numerous bright pink tentacles, almost electrical in their sensitiveness.

Though anything but rotund, so full in habit (comparatively speaking) is the body of the lima that the valves cannot compress it. Except at the hinges they are for ever divorced, an unfair proportion of the bulging body being exposed naked to the inclemency and hostility of the world. “All too full in the bud” for those frail unpuritanical stays, the animal seems to be at a palpable disadvantage in the battle of life, yet the lima is equipped with special apparatus for the maintenance of its right to live. By the expansion and partial closing of the valves it swims or is propelled with a curiously energetic, fussy, mechanical action, while the ever-active pink rays — a living, nimbus — beat rhythmically, imperiously waving intruders off the track.

The appearance and activities of the creature are such as to establish the delusion that it is not altogether amicable in its attitude towards even such a bumptious and authoritative product of Nature as man. Its agitated demonstrations — whatever their vital purpose may be — to the superficial observer are danger signals, a means of self-preservation, as a substitute for the hard calcareous armour bestowed upon other molluscs. The fussy red rays may impose upon enemies a sense of discretion which constrains them to avoid the lima, which, though hostile in appearance, is one of the mildest of creatures. The tentacles, too, have a certain sort of independence, for they occasionally separate themselves from the animal upon the touch of man, adhering to the fingers, while maintaining harmonic action, just as the tip of a lizard’s tail wriggles and squirms after severance.

Most of the blocks of submerged, denuded coral are the homes of certain species of burrowing molluscs, the most notable of which are the “date mussels” (LITHOPHAGA). The adult of that designated L. TERES is over two inches long and half an inch in diameter; glossy black, with the surface delicately sculptured in wavy lines; the interior nacreous, with a bluish tinge. This excavates a perfectly cylindrical tunnel, upon the sides of which are exposed the stellar structure of the coral. A closely related species (STRAMINEA), slightly longer, and generally of smooth exterior, partially coated with plaster, muddy grey in colour, adds to the comfort and security of existence by lining its tunnel with a smooth material, a distinction which cannot fail to impress the observer. In each case the mollusc is a loose fit in its burrow, having ample room for rotation, but the aperture of the latter is what is known as a cassinian oval, and generally projects slightly above the surface of the coral.

The animal is a voluntary life prisoner, for the aperture has the least dimension of the tunnel. The genus is known to be self luminous — a decided advantage in so dark and narrow an habitation. It seems to me to be worthy of special note that an animal enclosed by Nature in tightly fitting valves should also be endowed with the power of mixing plaster or secreting the enamel with which its tunnel is lined and of depositing it with like regularity and, smoothness to that exhibited in its more personal covering which grows with its growth. The mollusc in its burrow in the depths of a block of coral, white as marble, with its own light and its self-constructed independent wall, appeals to my mind as evidence of the care of Nature for the preservation of types, while from such retiring yet virile creatures man learns earth-shifting lessons. A quotation from Lyell’s “Principles of Geology” says that the perforations of Lithophagi in limestone cliffs and in the three upright columns of the Temple of Jupiter Serapis at Puzzuoli afford conclusive evidence of changes in the level of sea-coasts in modern times — the borings of the mollusc prove that the pillars of the temple must have been depressed to a corresponding depth in the sea, and to have been raised up again without losing their perpendicularity.

The date-mussels play an important part in the conversion of sea-contained minerals into dry land. Massive blocks of lime secreted by coral polyps being weakened by the tunnels of the mussels are the more easily broken by wave force; and being reduced finally to mud, the lime, in association with sand and other constituents, forms solid rock.

A feature of another of the coral rock disintegrating agents is its extreme weakness. It is a rotund mollusc with frail white valves, closely fitting the cavity in which it lives. As it cannot revolve, the excavation of the cavity is, possibly, effected by persistent but necessarily extremely slight “play” of the valves; but the animal appears to be quite content in its cramped cell with a tiny circular aperture (generally so obscured as to be invisible), through which it accepts the doles of the teeming, incessant sea.

Last updated Wednesday, March 12, 2014 at 13:32