IN striking contrast to the marine mussels, which are plentiful, freshwater mussels are found sparsely scattered over the beds of rivers and it may be that this is a matter of necessity, owing to the comparative scarcity of food in the rivers as compared with the abundant supplies of all kinds in the seas. At all events, freshwater mussels are at some pains to ensure that their young shall be dispersed as much as possible before settling down. In this connection there is a pretty case of mutual assistance between freshwater mussels and the bitter-ling, a fish inhabiting the rivers of the continent but not found in this country.
The fish in question is quite an ordinary-looking fish, only a few inches long. In the breeding season the female bitter-ling grows a long ovipositor (or egg-laying tube) and, when ready to lay her eggs, she hovers over the gaping valves of a freshwater mussel and deposits the eggs in its gill-chambers. This appears neither to inconvenience nor to injure the mollusc since it makes no attempt to close the shell when the ovipositor is inserted nor to get rid of the eggs. Any other foreign matter introduced into the gill-chambers would be promptly surrounded by pearly matter, but this does not happen to the bitterling’s eggs.
It is almost as though there were a mutual understanding between the bitterling and the mollusc, for while the first is laying her eggs the mussel discharges its own brood, larva; in this case, and these immediately seek refuge in the gills or cling to the skin of the fish, to be carried about until the time comes for them to grow a pair of valves and live at the bottom
of the stream like their parents. Then they release their hold on the tissues of the bitterling and drop off, leaving their unwitting benefactor none the worse for the service rendered.
How such an association, or mutual exchange, came into being and what are the influences that bring about the exact timing necessary for its success are matters for speculation, but the upshot is, almost certainly so far as the mussel’s side of the bargain is concerned, that its young are scattered more effectively over the bottom of the river, and have, therefore, a greater chance of survival, than would otherwise have been the case. It is significant that the British pearl industry should have flourished around the swiftly-moving streams, and one can only suppose that the rapid movement of the water carrying the larvas about and scattering them broadcast, results in the same end as the passenger transport service provided by the bitterling in continental rivers.
Associations of this sort, between animal and animal, between animal and plant, and between plant and plant, are very numerous; not only are they of absorbing interest, but they serve to show, what is only too often overlooked, that Nature is not entirely ‘red in tooth and claw.’ On the contrary, progress in the evolution of both animals and plants has been more often due to co-operation than otherwise. Some of the associations are quite commonplace, and call for little comment. Others are of so strange an order as to place them in the realm of tall stories had we not fairly convincing proof to the contrary.