ASSORTIVE MATING IN CHROMODORIS ZEBRA 287 



the result of biparental inheritance. Beyond commenting on 

 the possibihty of such an effect, there is no good reason to 

 speculate upon the matter. 



The coloration of C. zebra is rather variable. Certain fairly 

 distinct types of coloration may be distinguished. There is no 

 correlation between color variation and size, and there is no de- 

 tectable tendency for specimens of the same color type to pair 

 together. 



3. Species crossing. One other species of Chromodoris, C. 

 roseapicta Verrill, is also found at Bermuda. It is connmonly 

 of about the same size as C. zebra, but it is much less numerous 

 and its habits are quite different. The two species have never 

 been found in association. So far as size merely is concerned, 

 there seems no a priori reason why members of these two species 

 should not pair. In nature it is probable that they meet rarely 

 if at all. Even when they are confined in small dishes I have 

 never succeeded in inducing C. zebra to mate with roseapicta. 

 The mutual attraction of members of each species probably de- 

 pends upon reactions to specific substances which the ripe indi- 

 viduals secrete. Assertive mating can play no part in the pre- 

 vention of interspecific crossing. 



VIII SUMMARY 



1. Conjugating pairs of the nudibranch Chromodoris zebra 

 mating under natural conditions were secured to the number of 

 148. Copulating individuals ranged from about 4 to 18 cms. in 

 total length. 



There is found a rather high degree of correlation between 

 total length (and other dimensions) of the two components of a 

 pair. For the total length, r = 0.608. 



2. Mass, experiments under laboratory conditions substantiate 

 this finding. The index of correlation for total length, in 119 

 pairs, was r = 0.721. 



3. Direct observation of mating behavior shows how this cor- 

 relation is the result of assortive mating. In most cases large in- 

 dividuals mate successfully with large, small individuals with 

 small. The homogamic correlations for different dimensions of 



