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John R. Holsinger and David C. Culver 
troglobites in North America, including C. antennatus from the study 
area, are represented by some populations with vestigial eyes, but the 
presence or absence of eyes may vary both within and between 
populations. 
Recent studies by Dickson (1976, 1977a, 1977b, 1979) and Dickson 
and Holsinger (1981) on the ecology of C. antennatus in Lee County, 
where the species is very common in caves, have revealed what are 
apparently two microgeographic races corresponding to habitat types. 
One race is found in mud-bottom drip pools, and the other in small 
gravel-bottom streams. Dickson has found small differences in behavior, 
ecology, and morphology between the races. In addition to ecological 
studies, six populations from the same area were genetically analyzed by 
electrophoresis (Dickson et al. 1979), but allozyme allele frequencies 
were determined at only two polymorphic loci. This study revealed a 
high degree of allele frequency heterogeneity among the populations 
and indicated a tendency for stream and pool amphipods to cluster in 
populations distinct from one another (see Table 1 in Dickson et al. 
1979). 
Despite some apparent isolation between the two habitat types and 
the microgeographic heterogeneity among populations, additional 
observations on C. antennatus indicate that at least a limited amount of 
dispersal can occur between cave populations. One of us (Holsinger 
1969a, 1978) has shown that this species may also inhabit perched 
groundwater above cave passages and occasionally enter caves from this 
habitat in dripping vadose water. The ecological flexibility of C. 
antennatus that allows it to inhabit simultaneously several different 
types of subterranean groundwater habitats undoubtedly accounts in 
part for its large range, which extends far southwest of the study area. 
The fact that C. antennatus does not appear to be far removed 
taxonomically from surface congeners and is not a highly specialized 
troglobite indicates to us that it has evolved directly from an epigean 
ancestor in fairly recent times. 
In contrast to Crangonyx, Bactrurus and Stygobromus are ex- 
clusively subterranean groups in which all known species are eyeless and 
unpigmented, and apparently highly specialized stygobionts. Species of 
these genera inhabit a wide variety of groundwater biotopes: cave 
streams, pools and phreatic lakes; interstitial media; small springs and 
seeps; wells; drain tiles; and, rarely, even Pleistocene relict lakes. Some 
species are restricted primarily to caves per se, where they are often 
associated with small streams; others simultaneously inhabit caves 
(usually drip or seep pools) and groundwater habitats outside caves; and 
even others inhabit groundwater habitats outside karst areas and are 
never found in caves (Holsinger 1967a, 1969b, 1972, 1977, 1978; Culver 
1982). Although Bactrurus and Stygobromus are apparently closely 
