Reprinted from the Proceedings of the NATIONAL ACADEMY OF SCIENCES, 

 Vol. 3, pp. 519-522, August, 1917 



EVIDENCE OF ASSORTIVE MATING IN A NUDIBRANCH 



By W. J. Crozier 



BERMUDA BIOLOGICAL STATION FOR RESEARCH, AGAR'S ISLAND. BERMUDA' 

 Communicated by E. L. Mark, June II, 1917 



In man there is found, according to Pearson and others, a slight but 

 appreciable degree of positive correlation between the members of 

 mating pairs as regards their stature and certain other characters. 

 For Paramecium a similar, but higher, correlation was proved by Pearl 

 (1907) to exist between the lengths of members of conjugating pairs. 

 Jennings (1911) substantiated Pearl's discovery that in Paramecium 

 large individuals are usually found mated with large, small individuals 

 with small, and made more certain the conclusion that this correlation 

 (homogamy) is due to real assortive mating, as Pearl had previously 

 maintained. 



This matter of assortive mating, which may have various important 

 implications for evolution, appears not to have been studied in animals 

 other than Paramecium and man. With reference to characters con- 

 cerning the size of the organism, at least, it should, of course, be pos- 

 sible for assortive mating to take place only when there is available 

 some physical basis for the required process of selection. Hence, 

 although echinoderms and some other marine animals appear to con- 

 gregate at their times of breeding, and may even be conspicuously dis- 

 posed in pairs (Orton) , it is not to be expected that invertebrates prac- 

 ticing external fertilization would, in general, yield any evidence of 

 assortive mating. Among gastropods, however, the case is different, 

 and notably so with nudibranchs. In the latter animals, which are 

 hermaphroditic, a true copulation of two individuals seems a prerequi- 

 site for fertilization of the eggs. In some nudibranchs the male and 

 female genital openings, two or three in number, situated on the right 

 side of the body, are separated by a considerable distance, and the 

 behavior of the animals in qopulation shows that it is necessary for the 

 'male' and 'female' openings of one individual to be brought simul- 

 taneously into close relation with the appropriate openings of another 

 (e.g., in Cenia, as described by Pelseneer, 1899). 



Other nudibranchs, such as chromodorids, have the reproductive 

 openings concentrated upon a single small papilla; but, in some cases, 

 at least, their behavior during the maneuvers preliminary to actual 

 copulation strongly suggests that here also there is a rather well-defined, 

 though not absolute, mechanical necessity for equality in the nzes of 



