Population Dynamics of Adult Unionicola formosa 
(Acari: Hydracarina), a Parasite of Anodonta imbecillis 
(Mollusca: Bivalvia), in West Virginia 
James E. Joy and Jeffrey W. Hively 
Department of Biological Sciences 
Marshall University 
Huntington, West Virginia 25701 
ABSTRACT . — Population dynamics of a parasitic aquatic mite, 
Unionicola formosa , were studied at the McClintic Wildlife Station 
(West Virginia) in two ponds that supported different densities of the 
host mussel, Anodonta imbecillis. Pond 27 with 26.0 host indivi- 
duals/m 2 was categorized as a high-density pond, whereas Pond 14 
with 8.6 host individuals /m 2 was considered a moderate-density pond. 
Collections were made monthly from May through November 1986. 
All hosts in both ponds were infested by female mites, but only 57 of 
90 hosts from Pond 14 and 60 of 79 from Pond 27 were infested by 
males. Intensity of infestation, as mean adult mites/ host, was lowest in 
May (5.4 for Pond 14; 19.7 for Pond 27) and highest in August (12.9 
and 31.3) for those ponds. Although the number of mites per host was 
positively correlated with host shell length for mussels in Pond 27, 
there was little or no correlation in Pond 14. Mite sex ratios were 
heavily female-biased at 10.7:1 in Pond 14, and 18.5:1 in Pond 27. 
May (1983) demonstrated, by an illustration based upon math- 
matical modeling, that a relationship existed between the number of 
animal parasites per host and the host population density. In theory, at 
relatively high parasite burdens and correspondingly high levels of host 
mortality, a host population could be regulated by parasitic infestation 
(Anderson and May 1978). Lanciani (1975) showed that increased 
numbers of an ectoparasitic water mite, Hydryphantes tenuabilis 
Marshall, reduced the rate of population increase of its aquatic insect 
host, Hydrometra myrae Bueno, in a laboratory setting. 
It is often difficult to assess the extent to which a parasite regulates 
host population growth in nature. Therefore, the primary goal of this 
study was to evaluate intensity of infestations of a parasitic aquatic mite 
in two freshwater mussel populations at different densities. This study 
was carried out during the 7-month seasonal period when mussels are 
most active, and a secondary objective was to examine changes in 
seasonal intensity levels of mites in the two host populations. We also 
attempted to correlate intensity of infestation with host size, a rela- 
tionship investigated in several previous mite/ mussel studies. This paper 
Brimleyana 16:33-42, July 1990 
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