SEX RATIO IN GAMBUSIA. 1 



SAMUEL F. HILDEBRAND, 

 DIRECTOR, U. S. FISHERIES BIOLOGICAL STATION, BEAUFORT, N. C. 



INTRODUCTION. 



The unequal division of the sexes in adult Gambusia and other 

 viviparous minnows is a subject discussed by many writers. The 

 present author has had an opportunity, in connection with the 

 study of the value of Gambusia as an agent for the control of 

 mosquitoes, to make many observations and to sex numerous fish 

 consisting of both adults and juveniles and the results from this 

 work form the basis for the present paper. 2 The material upon 

 which the present study was based was obtained chiefly in three 

 localities, namely, Augusta, Ga., Beaufort, N. C., and Greenwood, 

 Miss. Therefore, the data, according to the most recent classifi- 

 cation (Geiser, 1923, and Hubbs, 1926), are based on two species 

 of Gambusia, namely holbrookl from Augusta and Beaufort, and 

 affinis, according to Geiser, or patruelis according to Hubbs, from 

 Greenwood. The present writer is unable to discuss the merits 

 of the further division between affinis and patreulis, recently 

 recognized by Hubbs. However, the differences in the structure 

 of the distal part of the anal fin of adult males from Augusta and 

 Beaufort (Atlantic drainage) and those from Greenwood (Mis- 

 sisssippi River drainage) are unmistakable and they are correctly 

 described and figured by Geiser (1923). Furthermore, the pres- 

 ent writer finds that when the rays in the dorsal fin are accurately 

 and uniformly enumerated in both species, that is, when the last 

 two rays that are more or less united, are counted either as one, 

 or as two rays, a constant difference of one ray becomes evident. 



1 Published by permission of the U. S. Commissioner of Fisheries. 



2 The writer wishes to acknowledge the very valuable assistance rendered 

 by Irving L. Towers, recently junior aquatic biologist, U. S. Bureau of 

 Fisheries, who was associated with the author for several years in the 

 Gambusia-mosquito studies and who personally sexed many adult fish and 

 made numerous observations herein recorded. 



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