392 SAMUEL F. HILDEBRAND. 



SEX RATIO IN ADULT Gambusia. 



The literature dealing with the sex ratio of adult Poeciliids, as 

 previously indicated, is extensive and scattered and it will not be 

 reviewed here. The reader, however, is referred to Dr. Samuel 

 W. Geiser's admirable paper, " Sex-ratio and Spermatogenesis in 

 the Top Minnow, Gambusia liolbrooki Grd.," (1924) for a re- 

 view of this literature and a concise discussion on the subject. 

 It is sufficient to say here that the inequality of the sexes in adult 

 fish is an unmistakable and a positive fact. As further proof, 

 Table I., based on 103,150 fish, examined for sex, is offered. In 

 explanation, it may be stated that the fish were collected either 

 with a bobbinet seine or a dipnet made of the same material. 

 Therefore, it was impossible for the smaller males to escape 

 through the meshes of the nets. 



In general fish having a total length of about 21 mm. (using 

 "eye measure") and over were considered adult fish and are in- 

 cluded in Table I. The modified anal fin, serving as a copulatory 

 organ, of course, was used in separating the sexes. That is, when 

 a fish possessed a modified anal fin it was classed as a male, 

 otherwise as a female. Many males, as will be explained later, 

 undoubtedly were classed as females, because of the late develop- 

 ment of the copulatory organ in some individuals. Occasionally 

 this organ is fully developed when the fish is 18 mm. long and 

 generally if not fully formed it is at least sufficiently developed to 

 admit of recognition when the fish is about 21 mm. long. How- 

 ever, many exceptions are found, as is shown in the section of this 

 paper dealing with the sex ratio of immature fish. It is enough 

 to say here, that the disparity of the sexes, that is, the minority of 

 males, is not as great as shown in Table I. The number of males, 

 classed as females, nevertheless, is entirely too small wholly to 

 account for the large inequality in the sexes, and the fact that the 

 males are greatly in the minority among adult fish unquestionably 

 remains. 



It was shown by Barney and Anson (1921) that a "seasonal 

 male frequency " takes place. The data presented in Table I. 

 bear out the information offered by these writers, namely that the 

 males are comparatively much more numerous during the fall, 



