Glimpses of Ocean Life; Or, Rock-Pools and the Lessons they Teach

by John Harper

To see a Mussel 'flit,' is a sight one may often watch and wait for without success. On the other hand, when least expected, the self-willed mollusc may commence operations. When about to take up a new home, the animal shaves off its beard entirely, or in more scientific language, 'rejects its byssus' altogether. In order to excite the locomotive instincts of my specimens, I used to cut all the threads of their cable except one. The animal being suspended, of course its whole weight was then thrown upon a single fibre. Such a state of insecurity was by no means agreeable, and I generally found in the course of a few hours that fresh threads were rapidly thrown out, and an entirely new byssus formed; the old one, which was broken off at the root, being left behind as useless.

Another singular peculiarity of the Mussel which came under my observation has not been, so far as I am aware, noticed by previous naturalists. I allude to the power which the animal possesses of lengthening out the root or stem of the beard, apparently to an unlimited extent. This power appears to be seldom exercised, for although I have had hundreds of specimens of the Mytili, in only one instance have I witnessed the phenomenon in question.

A large specimen of this bivalve, procured accidentally from a fishwife in the street, was dropped into the aquarium, and placed close against the surface of the glass. The animal seemed highly delighted with its change of situation, for in a few moments the valves were opened, and a long draught of water taken in to bathe its branchiæ, and furnish a hearty meal. Having satisfied its appetite, the next process, of course, was to find out what kind of a home he had been introduced into. The foot, a noble specimen, was soon protruded, and one after the other, in rapid succession, various threads were formed. By next morning the animal, advancing by a series of easy stages, had reached the surface of the water, which was exactly five inches deep. I knew it would not remain long in this position, and was anxious to discover what plan would next be adopted. Several courses were open to him. For instance, like a marine Captain Cook, he might circumnavigate his little Globe,—or he might let go his cable and drop plump to the bottom,—or he could follow the route I had often seen taken by his relations, viz., to journey back to the place from whence he started. It pleased him, however, to strike out into a new path,—to devise a method of his own. While located near the top of the tank, he threw out exactly ninety-eight threads, not certainly for security, but merely, it would appear, for pleasure.

Then slowly but surely, day by day, he lengthened out the stem of his byssus tree, until it reached the extreme length of nearly five inches. To what further degree it would have been extended, had the mollusc not reached the base of the tank, it is impossible to conjecture.

No sooner did the shell touch terra firma, than the cable which had taken so long to spin was immediately broken off. I have succeeded in keeping the same animal by me for the last twelve months, but have seen no attempt at a renewal of the operation, in the progress of which I had taken so lively an interest. I may add that this Mussel taught me another lesson; it was this: in my early studies regarding the habits of the Mytilus, I had adopted a certain theory of the manner in which the beard was formed; and having watched so long, and witnessed the process so repeatedly, I thought myself justified in forming certain conclusions. My belief was that the creature could not form more than one thread at a time, without withdrawing its foot into the shell, as I believed, in order to procure a fresh supply of material. That this notion was erroneous, this animal proved to my entire satisfaction. Not only may one, but two, three, four, and even six threads be attached to any selected object, the point of the foot being passed from one position to another, without the organ being withdrawn into the valves until the whole of the threads are formed. How many more the Mussel is capable of producing at one 'stretch,' I have no means of knowing, but six is the largest number that any of my specimens in such case have ever fabricated.

The general idea seems to be that the Mussel works in the same manner as the spider, who emits a drop of liquid against some foreign substance, which, being allowed to harden somewhat, is then drawn out as the spider recedes. This notion, I may state, is quite erroneous. When the sucker of which we have spoken is formed, the thread is completed. It is true that the foot as it retires into the shell generally glides down the newly-constructed filament, but this is not of necessity, nor does such circumstance invariably occur. Indeed, while busily engaged in attaching a disc to the glass, the muscles of the foot will contract, and thus throw open the folds of the groove, situated in the middle of that organ; when thus exposed, the byssus thread may be seen in the furrow, stretched like the string of a harp or dulcimer.

While the end of the thread is being attached to a certain spot, a conspicuous muscular action is perceived going on in the foot, which alternately swells and contracts, as if something were being pumped up through the byssal channel, until it reached a certain point. There being dilated and spread out in successive layers, it assumes a trumpet-like disc, which is firmly fixed to the foreign object. Indeed, I am by no means certain that the thread is not, when first produced, exactly like a trumpet in shape. It also conveys the idea of being blown out in a similar manner to a piece of bottle glass. After being exposed to the air for some little time, the hollowness of the thread is not so apparent as when it is newly fabricated.

The mucous fluid, from which the fibres are formed, is secreted in a gland situated at the base of the foot, whence it is apparently expelled at the will of the animal into the furrow already referred to, and is there spun into threads. The toughness of these filaments, considering that each is finer than the thinnest strand of silk, is remarkable. Their strength, however, may be easily accounted for, when we know that each is composed in reality of innumerable delicate threads, bound together by a subtle gelatinous fluid. This phenomenon may be made out quite distinctly with a common hand lens, if the following simple experiment be adopted: Make a Mussel construct its thread in such a way that the disc of each is planted on the face of the glass. Then place the fine point of a common needle upon the outer edge of a chosen disc or sucker, and gently draw the former away to a little distance, and you will find that by so doing the stretched string becomes peeled. Continue this process carefully, and before the thread gives way you will have divided it into a dozen parts at least, all of which are visible to the naked eye, but clearer when the hand lens is used, and still more distinctly and beautifully defined, of course, if the microscope be brought into play.

The foot of the Mussel appears to be firmly strapped on, as it were, to certain transverse muscles, by a contraction of which the animal closes its shell with surprising force. This strap, composed of a powerful tendon which passes under the adductor muscles, is attached at either end to the base of the foot. Thus we account for the remarkable strength which is evidently seated in the foot, and makes it of so much importance to the animal. At first sight nothing appears more easy than to pluck out this organ by the roots, but an attempt will prove the experiment to be more difficult than many persons suppose.

The colour of the foot varies considerably in different specimens, even of the same species. Some, for instance, are of a chestnut brown; others of a kind of mauve or purple, covered with a peach-like bloom during life; others, again, are of a deep-toned umber, while not a few are pearly white, and streaked sometimes with pink like a tulip.