Selective pairing in gammarids

Abstract

Studies on the sexual coupling of organisms have shown it necessary to recognize that association of mates may be selective rather than random. It is evident that such selective coupling may have important evolutionary consequences. 1 The problem of selective coupling on the basis of somatic characteristics, however, is an entirely different problem from that of selective union of germ cells. This point must be clearly in mind; the distinction has occasionally lapsed in discussions of the topic. 2

It has been shown by Pearl and by Jennings that paramœcia assort with respect to size; and the nature and effects of this assorting have been pointed out. More recently it has been shown that the nudibranch Chromodoris zebra, which practises internal fecundation, shows a rather high degree of assortative mating according to size. 1 Crozier pointed out some of the likely consequences of large individuals mating with large, and small with small, and he showed that an adaptive result is probably thus attained in a purely mechanical, automatic manner.

It is possible that this phenomenon, in varying degrees, is of wide occurrence among animals. As further evidence there is here presented a study of naturally occurring pairs of Gammarus locusta (Linn.), a small salt-water crustacean. It is possible to study size relations of the members of breeding pairs, because the male carries the female about with him for a considerable time. The formation of such pairs is by the usual sex-recognition methods of the crustacea. It is agreed by those who have studied sex-recognition in typical crustaceans that it is a purely mechanical affair, 2 and this was observed to be true in Gammarus. The male shows certain strong clasping reflexes, and itself resists being clasped. The female, however, is usually passive when clasped.