Unionicola formosa in West Virginia Mussels 41 
inferences about numbers of mites present. That argument is strength- 
ened by our data, which suggest that host density should also be 
considered an important factor in determining the number of mites 
present per host mussel (Fig. 1A). Dimock (1985) noted that age of host 
may be correlated with number of mites present because increase in age 
would allow for increased exposure time to invasive stages of the mite. 
That is a reasonable conclusion even though no one has convincingly 
demonstrated how to age members of this mussel species — at least 
beyond the third growing season — with any degree of confidence. 
Availability of oviposition sites, as suggested by Mitchell (1965), might 
be a good estimator of number of mites present relative to host size. 
Certainly some inventive measure of weight, or of gill area, could be 
devised to test Mitchell’s hypothesis. One feels tempted to assess the 
influence of mantle cavity volume, and perhaps host tissue response to 
mite infections as well, although the latter measure may prove ex- 
ceedingly difficult to describe. 
Over the past 3 years we have never been able to collect mussels 
from McClintic ponds in the winter months (December through 
February). Our lack of data for March and April is an unfortunate 
omission. That oversight, coupled with an unexplained population crash 
of A. imbecillis in Pond 27 that began in mid- August of 1986, further 
restricted our ability to draw definitive conclusions regarding seasonal 
influences on adult mite infections. Nevertheless, a couple of compari- 
sons can be made. Gordon et al. (1979), whose study period covered the 
same months as ours, reported no seasonal differences in either 
prevalence or intensity of U. formosa infestations in A. cataracta. Our 
findings were basically similar, i.e. the prevalence was identical for every 
month sampled (100%) and differences in mean intensities were 
statistically insignificant for male mites in both ponds and for female 
mites in Pond 27 (Fig. 1A-C). On the other hand, Dimock (1985) noted 
seasonal variation for U. formosa in A. imbecillis'. Adult females were 
most numerous in the winter and least so in late spring and summer. 
Although we detected significant differences between monthly means for 
female mites in Pond 14 (F = 2.504, 83 df; P - 0.0282), there was no 
seasonal trend. Means for August and September, for example, were 
widely separated (Fig. 1A). Thus, our knowledge of seasonal influence 
on populations of U. formosa cannot be presented as a simple 
generalization. 
ACKNOWLEDGMENTS. — We thank Dr. Malcolm F. Vidrine, 
Louisiana State University at Eunice, for his helpful suggestions during 
the course of our study and for his subsequent review of the manuscript. 
