Genetic Variation 
125 
expectations. Heterozygosity values displayed no correlation with dis- 
tance from the Powell River. 
Sample sizes and numbers of loci scored were small for reasons noted 
earlier, so the results must be interpreted with extreme caution. Estimates 
of genetic variability fall near the middle of the range of values previously 
reported in crustaceans from other aquatic habitats (H ranges from 0.038 
in the lobster, Homarus amencanus , to 0.211 in the krill, Euphausia 
distinguenda; see reviews in Nevo 1978 and Powell 1975). Heterozygosity 
values in C. antennatus are considerably above mean values reported for 
eight other cave species (H = 0.056, Table 3). 
A striking aspect of the data is the high degree of allele frequency 
heterogeneity among C. antennatus populations, particularly at the PGM 
locus where sample sizes were fairly large (Table 1, Figure 1). 
Heterogeneity among localities can be expressed by the standardized 
genetic variance (F ST ), which provides a common scale for comparing the 
relative levels of interlocality allele frequency heterogeneity at different 
loci, or in different sets of populations (Wright 1978). For five elec- 
tromorphs at the PGM locus, weighted F ST = 0.300 among populations 
of C. antennatus inhabiting the approximately 60 km 2 area of the study. 
This variance in allele frequency is greater than mean values among snail, 
Helix aspersa , populations in different California cities or house mice, Mus 
musculus, populations on different farms in Texas (Selander and Kaufman 
1975). It is comparable to observed values between bluegill sunfish, 
Lepomis macrochirus, populations inhabiting distinct reservoirs within large 
southeastern drainages (Avise and Felley, in press). The microgeographic 
heterogeneity in C. antennatus is consistent with the contention that caves 
may be highly partitioned habitats, analogous to islands or archipelagos 
(Culver 1971). 
Genetic similarity values were calculated between pairs of cave popula- 
tions using Nei’s (1972) identity statistic (T). This statistic can assume 
values from 0 (no electromorphs shared) to 1 (identical electromorph fre- 
quencies). For the populations of C. antennatus studied, I values fell be- 
tween 0.858 and 0.968, within the range typical of conspecific populations 
in other invertebrates. The identity matrix was employed to generate a 
biochemical dendrogram (Fig. 2) according to the unweighted pair-group 
method with arithmetic means (Sneath and Sokal 1973). There is no clear 
tendency for stream and pool populations to cluster distinct from one 
another. Based on the relatively distinct pattern of allele frequencies in 
the population from Roadside No. 1 Cave (Figs. 1, 2; Table 1), it appears 
that the Powell River has acted as a partial barrier to gene flow. 
