The Life History of the Fresh Water Mussel

by W. P. Pycraft

Most people must have seen the fresh-water mussel in its native element. Let those who have not, search in the shallow water of the nearest river or brook till they are successful. When the stream is clear you may often see them lying on the bottom; in deeper water, you may catch them if you go out armed with a big, long-handled rake; plunge this into the water, drag it along the bottom, and carefully haul up the entangled mud and weed. Sooner or later your search should be rewarded. I have caught hundreds this way. Some of them were not more than an inch and a half long, and when placed in a glass jar were so transparent, that I could watch the beating of the heart through the shell. Indeed, I have two such little beauties before me, on my study table, as I write. One has partly buried himself in the mud, the other is lying on the surface. But, when full-grown, this transparency passes away, and they attain a perfectly huge size—six inches long at any rate!

Once upon a time, no doubt, the ancestors of these creatures lived in the sea; then they migrated to the rivers, creeping farther and farther up into fresh water, till at last their descendants have got so used to this element that they can live only in fresh water. Now, when animals gradually change their mode of life in this way, they at the same time undergo a great many structural and constitutional changes—some slight, some profound—and among these the most important are changes in the provision for the young. There is, as you know, a constant migration going on among the more active animals between the sea and the river, which is entirely on account of the needs of the young. Thus, salmon leave the sea yearly and undertake perilous journeys up the rivers, solely that they may lay their eggs there: while eels, on the other hand, as we have seen, are impelled by instinct to pursue exactly the opposite course, and to brave all dangers, that they may provide a nursery for their young in the deepest depths of the ocean.

Let us apply this to the fresh-water mussels. The ancestors of these very helpless creatures lived, I have remarked, in the sea; and we may be pretty certain that their eggs are hatched out into what we call larvae, or imperfectly developed animals, precisely similar to the young, or larvae, of the marine mussel of our seas. Now, this larva has the form of a tiny little creature covered with 'swimming' hairs. By the constant waving motions of these hairs, the little body is driven through the water, till at last, reaching a favourable spot, or tired out, they settle down at the bottom of the sea and turn into mussels, This free-and-easy life is all very well for the salt-water mussels, with the great wide sea to roam in; but such freedom in rivers would by no means be safe, because, though mussels swim, they are, by reason of their small size, quite unable to force their way against strong currents. Thus, on the outgoing tide, they would be swept off to sea, and would die even before this was reached—as soon, indeed, as the water became really salt. So, to prevent such a disaster, the fresh-water mussel carefully nurses her young between her gills, till they are old enough to help themselves. You will be surprised when I tell you the strange device they have come to adopt, so soon as they are cast adrift, whereby they may complete their days of infancy. Shielded throughout the winter months, they are turned adrift on the first warm day of Spring, a troop of very lively youngsters indeed. Each is encased in a very wonderful shell (S in the figure in the top left-hand corner of the illustration), quite unlike that of their parents, being triangular in shape, and armed with a pair of pointed teeth (T). By means of powerful muscles this shell is made to open and shut with great rapidity, and thus the body of the little creature is quickly driven through the water in a series of spasmodic jumps. Then comes a period of rest, obtained by using the long thread or 'byssus' (B) as a float, this thread being thrown out along the surface of the water. Then the hunt for a host begins again. On and on they go, till one after another—'curiouser and curiouser!'—seizes hold of a fish by means of its hooks. Having caught hold tight, each clings like grim death, and as a result of the irritation set up in the poor fish's skin, swelling follows and soon grows up all round the young mussel, and makes him a prisoner. But this is just what he wants. Snugly tucked away in his living cradle he slowly assumes his adult shape, and at last bursts his prison and falls to the bottom!

Fresh-water Mussels. Fresh-water Mussels.

There is yet another reason for this very strange and somewhat cruel procedure. The love of self, among the lower animals, is so strong that parents always drive away their young so soon as they become capable of feeding, and fending for themselves; because, if they did not adopt stern measures of this sort, famine and disease would be the result, owing to overcrowding. On the whole, this banishment is not so hard as it looks, the young having no sentiment for the place of their birth, and being probably more capable of migrating than the parents. But the method adopted by the fresh-water mussel is wasteful and dangerous; wasteful, because thousands and thousands of young ones necessarily die every year, through failing to catch their fish; and dangerous, because those who succeed are liable to contract the habit of being a parasite, and this, as always, leads to degradation and ruin. Finally, whenever young animals have to depend on other creatures to provide them with a lodging during some part of their growth, many more thousands have to be hatched than is the case where the young are dependent on themselves entirely, for it must always happen that the necessary hosts are hard to catch, and the young die in countless thousands, being unable to succeed in their search.