SEX RATIO IN GAMBUSIA. 



When counting the last, partly united, rays as two in 81 specimens 

 examined from the Atlantic drainage (Augusta, Ga., Beaufort, 

 N. C., Orangeburg, S. C, and Key West, Fla.) the dorsal fin 

 constantly had 8 rays. Similarly, in 78 specimens examined from 

 the Mississippi River drainage (Greenwood, Miss., Memphis, 

 Tenn., Mound, La., and Little Rock, Ark.) 7 dorsal rays invari- 

 ably were present. The variation of two and three rays reported 

 in published works, therefore, probably is due chiefly to a differ- 

 ence in enumeration. The difference in the number of dorsal 

 rays, then, is helpful in separating the otherwise bothersome fe- 

 males, as well as all juveniles which have not yet developed the 

 external sexual characters. 1 



Although the data, forming the basis for the present paper, are 

 founded upon two species of Gambusia, they will not be delt with 

 separately, as no differences between the species with respect to 

 sex ratio, neither seasonal, for adults, nor for young fish were 

 noticed. The specimens, collected from 1921 to 1926, were taken 

 in a large variety of w r aters, that is, ponds of various sizes (with 

 and without vegetation) borrow pits, swamps (both fresh and 

 brackish), ditches and sluggish creeks. The specimens, in fact, 

 were taken in nearly every environment in which Gambusia lives. 

 The data, therefore, are representative of animals living under a 

 large variety of conditions, and not of any particular environment. 



Although many authors, as already stated, have discussed the 

 sex ratio in adult Pceciliids, little work has been done on the sex 

 ratio of immature fish. The only account of the sex ratio of 

 young Pceciliids, of any importance, that has come to the writer's 

 notice is the one by Geiser (1924). Even Doctor Geiser's data 

 are rather limited, yet they show quite accurately the sex ratio of 

 young Gambusia, as will be demonstrated by the extensive data 

 that are offered in a subsequent section of this paper. 



1 It appears to be of interest to note here that a small collection (26 

 specimens) from Camilla, Ga., is at hand in which occur five females with 

 only 7 rays in the dorsal fin, whereas all the others, including two males, 

 have 8 rays. Geiser (1923) studied specimens from the same collection and, 

 basing his identification on the minute structure of the intromittent organ of 

 adult males, found them to represent the eastern form, holbrooki, although 

 Camilla is in the Gulf drainage. It would now appear as if both species 

 occur in this locality. Unfortunately no males with 7 dorsal rays are at 

 hand with which to verify this identification. 

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