108 
David C. Culver and Timothy J. Ehlinger 
P <C 0.01). In summary, there is no clear evidence for divergence in size 
where the species are syntopic. 
Table 4. Length measurements of Caecidotea holsingeri and Caecidotea cannulus, 
in mm. 
C. holsingeri C. cannulus 
Cave X S.E. N x S.E. N 
Linwood 
5.7 
0.2 
24 
— 
— — 
Harman 
2.8 
0.2 
21 
— 
— — 
Bowden 
2.5 
0.1 
23 
8.2 
0.4 19 
Alpena 
3.6 
0.3 
8 
8.8 
0.4 3 
Glady 
3.6 
0.1 
12 
7.0 
0.4 17 
Harper 
4.0 
0.2 
4 
7.9 
0.6 5 
Cave Hollow 
— 
— 
— 
7.05 
0.3 4 
For the four caves where more than 20 individuals were collected, 
the size distributions of isopods together with the size distributions of the 
stream gravels are shown in Figure 1. The data given in Table 2 strongly 
suggest the two distributions should be correlated, although the actual 
gravel sizes that an isopod of a given size uses are not known. However, 
there are some striking similarities in the qualitative aspects of the gravel 
and isopod size distributions for each cave, which are summarized in 
Table 5. In Bowden Cave, both distributions are strongly bimodal. In 
Glady Cave, the gravel size distribution is weakly bimodal, while isopod 
size distribution is apparently unimodal with a broad size range. By con- 
trast, in Harman Cave both gravel and isopod size distributions are 
strongly unimodal and skewed to the left. In Linwood Cave, gravel sizes 
are uniformly distributed for small gravels, and skewed to the right 
overall. Isopod sizes are strongly unimodal, with a narrow size range. 
There are two discrepancies between isopod and gravel size distributions. 
First, in Glady Cave, gravel size distributions are weakly bimodal and 
isopod sizes are not, although they do have the expected broad size range. 
Second, in Linwood Cave there is a broad size range of gravels and a 
narrow size range of isopods, and there is little concordance of the two 
distributions. With these two exceptions, there was a good fit between the 
two distributions. 
DISCUSSION 
There are two interrelated hypotheses about body size of Caecidotea 
cannulus and C. holsingeri. The first is that small isopods suffer less 
washout (and less mortality) in small gravels and large isopods suffer less 
