Genetic Variation 
121 
METHODS AND MATERIALS 
In this study, populations of the troglobitic amphipod crustacean, C. 
antennatus , were examined electrophoretically. This species is one of the 
most common and widespread aquatic troglobites in the eastern United 
States. It occurs in the Appalachian region of Virginia, Tennessee, 
Georgia and Alabama (Holsinger 1969, 1972). Crangonyx antennatus oc- 
cupies two distinctly different habitats within its range: gravel-bottom 
streams and mud-bottom pools. Significant differences in morphology 
(Dickson 1977a), behavior (Dickson 1977b) and population structure 
(Dickson and Holsinger, in press) were observed between populations liv- 
ing in these habitats. 
Populations of C. antennatus were sampled from six caves in Lee County, 
Virginia (Fig. 1). Although the populations are relatively large in caves of 
this area, small collections were taken from each cave to minimize pop- 
ulation disruption (Table 1). Three of the populations sampled inhabited 
gravel-bottom streams (Spangler, Cope and Gallohan No. 2 caves) and 
three inhabited mud-bottom pools (Roadside No. 1, Molly Wagle and 
Sweet Potato caves). Mud-bottom pool habitats generally contained 
greater quantities of available food, fewer predators and smaller numbers 
of potential competitors than stream habitats. Additional ecological data 
on these habitats were given elsewhere (Dickson and Kirk 1976; Dickson 
1977a); geological data are found in Holsinger (1975). 
After collection, amphipods were transported live to the laboratory and 
stored at — 70°C until analyzed. Amphipods were electrophoresed on 
horizontal starch gels using conditions and staining procedures similar to 
those of Selander et al. (1971) and Ayala et al. (1972). Of 21 enzyme 
systems screened on six gel types, eight allowed consistent scoring, with 
two of those exhibiting polymorphic allozyme patterns interpreted as 
segregating electromorphs. As previously noted by Gooch and Hetrick (in 
press), amphipods are not well suited for electrophoretic study, probably 
because of the release of inhibitory enzymes from the hepatopancreas 
during homogenization. 
The monomorphic systems included glucose-6-phosphate 
dehydrogenase (G-6-P), indophenol oxidase (IPO), aldehyde oxidase 
(AO), malate dehydrogenase (MDH), and general proteins I-II (GP-1, 
GP-2). The polymorphic systems scored were phosphoglucose isomerase 
(PGI) and phosphoglucomutase (PGM). The PGI locus exhibited three- 
banded heterozygote patterns, suggesting that this enzyme is a dimer. 
Two-banded heterozygote phenotypes were observed at the PGM locus, 
as expected for a monomeric enzyme. These enzyme structures are con- 
sistent with those recorded in other crustaceans, including lobsters 
(Tracey et al. 1975) and crayfish (K. Stueck, in prep.). To determine 
