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.