Among the many living things that contribute to the never-ceasing attractions of a coral-reef are those which depend for their existence on association with others remarkably different in appearance, character and habit. Observers of skill and experience have proved that the association is for mutual benefit; but in some cases, and those of the more obvious nature, the advantages may appear one-sided.
What gain is it to certain species of fish to have a rotund crab in a cell in close proximity to the eyes — a phenomenon, it should be said, by no means confined to the warm waters of the tropics? And what advantage is it to the shark and turtle and dugong to have the company of sucker-fish? The latter, of course, is spared the exertion of self-propulsion through the water, and the host may not be seriously inconvenienced by the slight additional tax on its energy; but when, as sometimes, the blacks use the sucker-fish as a means for the capture of its erstwhile host, the end is tragedy.
It is easy to appreciate the reasons for association between the huge, bright-blue anemone, common in tropical waters, and a tiny fish of wonderfully brilliant tints. The latter hides from its enemies in the folds of its host, and seems distraught with anxiety if, as it flits above and around, it is by a sudden manoeuvre driven a few inches therefrom, to be easily captured. In addition to a tenant which is believed to be welcome, the blue anemone has been shown to be troubled by multitudes of small crustaceans, the colour of which exactly matches that of their host. Authorities assert that some such crustaceans form the food of the herring, which is popularly believed to live on water.
Just as terrestrial animals have their parasites and hyperparasites, so the denizens of the reef may support other creatures of diverse habits and mode of living. The fleas of the seas have lesser fleas; but such insignificances are beyond the notice of the individual who is looking for living jewellery in the warm waters. Here is something more attractive. Upon a dead bailer shell stands an anemone about four inches high — a neutral-coloured stem surmounted by a head of pink and purple waving filaments. From the shell is obtruded the head and legs of a hermit crab, deep red in colour and bristling with repellant points. The anemone — delicate feeder — thrives on the scraps and shreds scattered by the clumsy and greedy crab. The body of the crab is soft, and no doubt appetizing to fish that could draw it from its refuge; but the anemone is armed with irritating weapons which fish dread. Independently, neither crab nor anemone would lead a contented life. Together they are fairly safe, and the picture they present is particularly rich in tint.
In other species the anemone is attached to the back of the crab, and in the case of a nervous, soft-bodied crab lodging with a living sponge a third associate is invariably present — a worm which acts as scavenger and keeps the hermit’s quarters clean. The reason for the presence of the third and most humble partner is obvious. Ordinarily, the hermit crab moves to a larger refuge in accordance with its growth. The elasticity of the sponge renders the change unnecessary, and the sanitary worker becomes essential. The provider, the protector and the domestic thus establish an ordered if unenterprising community, each contributing to the maintenance according to its aptitudes, each obtaining the gratification of its needs.
If the brighter and prettier examples of associated lives among marine creatures are to be contemplated, one must visit the coral-reefs; even then, if he be untrained and inexpert, he will see only the more public exponents of a phase of existence of most varied nature, in some instances edifying, in others bewildering. There is a slim mollusc never found except in association with a particular species of sponge. The shells are of social habit, dozens living in a community, each buried to the extremity of its valves in the compact mass of the sponge. Some of the living sponges have the gift of being able to warn off enemies by, it would seem, the discharge of an irritating fluid. The host of the present example has no weapon of offence appreciable in its effects by human beings, whilst the shells are not nearly as frail as others that lead independent lives.
One of the marvels connected with the jewellike prawns associated with sea anemones is the power they possess of controlling their colours to match those of their hosts. Indeed, it is possible to watch certain degrees of the changes. The phenomena has been thus described by an expert, Mr. R A. Potts, who studied the life of the reefs in Torres Straits:—
“The prawn has the power of forming red, yellow and blue pigments, and by altering their relative proportions in the chromatophores it can acquire a green, brown, blue or red ground-colour, and is thus able to adapt itself to the varied colours of seaweeds and hydroids. The pigment may be laid down in longitudinal strips or horizontal bars, and in this way a colour-scheme can be formed matching whatever seaweed the prawn shelters in.”
Proof of the accuracy of this observation was given when the tides favoured an investigation of a portion of the reef. As the tide rose a small anemone emerged from the sand and began to wave its singular filaments — dull-yellow in tint and in form not unlike a cauliflower run to seed. There was nothing out of the common about the animal; but, the habit of watching such things having been established, its ordinary movements served for entertainment. Presently a shadowy form passed slowly over a portion of the moving filaments, and on close inspection a slender, almost transparent, prawn was seen. It glided hither and thither with almost imperceptible motion, now and again being, as it were, absorbed by its host. Alert, almost, as a sunbeam when aroused, the little creature would have been safe from capture but for its attachment to its home.
The general tint was that of the host — yellowish-green with white blotches, the main part of the body transparent, the pattern of the host being duplicated in miniature. The opaque white spots on each side were not of uniform size nor were they arranged according to uniform design. Each of the scales at the end of the tail was ornamented with a circular purple spot, and these spots not only varied in intensity of tint, but in size and position, showing that they were under the control of the animal.
Some species sting with more than the intensity of the nettle. This host did not in any way molest an interfering finger, and its tender associate seemed to depend for its safety on the fact of close resemblance. On restoring the prawn, it was noticed that it had become darker in colour, the transparent parts being slightly clouded and the purple spots of the tail almost black. In a few minutes the ordinary tints were resumed and the little creature moved, spectre-like, over the filaments of its host — a living jewel.
Last updated Tuesday, August 25, 2015 at 14:05