40 
James E. Joy and Jeffrey W. Hively 
recovered from Pond 27 hosts in May, July, and September exceeded 
the maximum numbers of female mites collected from mussels in Pond 
14. Still, these findings must be tempered somewhat because they 
describe an association between host populations and the populations of 
their acarine parasites in only two ponds. 
Although sex ratios of Unionicola species vary considerably (Humes 
and Jamnback 1950, Mitchell 1965, Gordon et al. 1979, Hevers 1980, 
Dimock 1983, 1985), a female-biased situation is almost universal. This 
condition, referred to as “harem-defense polygyny” by Dimock (1985), 
was also seen in the present study with female: male sex ratios of 10.7:1 
and 18.5:1 in Ponds 14 and 27, respectively. 
Humes and Jamnback (1950) reported an inverse relationship 
between prevalence of Najadicola ingens (Koenike, 1895) and size of 
Elliptio complanata (Lightfoot, 1786) and Anodonta cataracta Say, 
1817, whereas Mitchell (1965) found no correlation between host size 
and any parameter of the population biology of Unionicola fossulata 
(Koenike, 1895). Conversely, Gordon et al. (1979) and Dimock (1985) 
cited positive correlations between host size and the presence of U. 
formosa, with which our findings in Pond 27 concur (Fig. 2). Previous 
workers tend to group all sample data on a single scatter plot to show 
correlations between host shell length and number of mites present. 
There is a possibility, however, that this approach obscures seasonal 
correlations. For example, if a disproportionate number of small mussels 
are examined in the spring, with predominantly larger mussels sampled 
in the fall, the question then becomes: Is the correlation size-related or 
season-related? To approach that question we attempted to collect A. 
imbecillis individuals across a broad spectrum of shell lengths for every 
sample month. Scatter diagrams were then constructed for each month 
(Fig. 2 and 3). Thus, in Pond 27 (Fig. 2) it is quite apparent that the 
positive correlations are indeed related to host shell length. The lack of 
correlation between host length and number of mites present in Pond 14 
(Fig. 3) is not easily explained, but the seasonal factor has been 
removed because of a wide-ranging distribution of host lengths for each 
month (except for October and November). 
An understanding of the growth rates and anticipated life 
expectancy of A. imbecillis individuals reveals why shell length may not 
be a good indicator of mites present. Harmon (1987) has rather 
convincingly argued that 70-79 mm will likely be the dominant size class 
of mussels in a population at McClintic Wildlife Station. Small A. 
imbecillis individuals grow rapidly, approaching their maximum shell 
length of ~80 mm after 3 to 4 years. Because a 75-mm-long individual 
could be in its fourth growing season, or in its eighth or ninth, it could 
be argued that shell length alone provides insufficient information for 
