Genetic Variation in the Eastern Cottonmouth, Agkistrodon 



piscivorus piscivorus (Lacepede) (Reptilia: Crotalidae) at 



the Northern Edge of its Range 



Donald A. Merkle 



Department of Natural Sciences, Longwood College, 



Farmville, Virginia 23901 



ABSTRACT. — Genetic variation was examined by electrophoresis in 

 six populations of Agkistrodon p. piscivorus from the northern edge of 

 the species' range in southeastern Virginia. Twenty-three presumptive 

 loci were found to be monomorphic, while three loci were polymorphic 

 in some populations. Average observed heterozygosity values ranged 

 from 0.9% to 2.9%, with a mean for all populations of 1.6%. Nei's 

 index of genetic identity reveals that all Virginia populations sampled 

 have a very high degree of genetic similarity, with a minimal value of 

 .964. 



INTRODUCTION 



Advances in biochemical techniques have enabled researchers to 

 collect a great deal of information on the amount of genetic variation 

 that occurs in natural populations (Selander and Johnson 1973). Most 

 of the studies dealing with snakes have involved the family Colubridae 

 (Sattler and Guttman 1976; Gartside et al. 1977). Those studies that 

 included species of the family Crotalidae analyzed only components of 

 the venom. 



The Eastern Cottonmouth, Agkistrodon piscivorus piscivorus 

 (Lacepede), occurs primarily in the Coastal Plain of the eastern United 

 States, where it ranges from Alabama to its northern distributional lim- 

 its in the southeastern corner of Virginia. For years, the James River 

 was thought to mark the northernmost limit for this species, just as it 

 does for a number of other reptiles and amphibians (Wood 1954; 

 Conant 1975). Cottonmouths, however, occur north of the James River 

 in the vicinity of the Newport News-Hampton area (Engeling 1969; Lin- 

 zey and Clifford 1981). 



The distribution of the cottonmouth in Virginia is discontinuous. 

 There is an isolated population along Swift Creek and the Appomattox 

 River just west of Hopewell in Chesterfield County. "This population is 

 at least 60 km from the nearest known locality in the main part of the 

 range. . ." (Blem 1981:117). Populations of this species on the barrier 

 beaches along Back Bay are separated from the mainland by several 

 miles of water. Other populations, such as that at Sea Shore State Park, 



Brimleyana No. 1 1:55-61, October 1985 55 



