EDWARDS, D.D.*; VIDRINE, M.F.; University of Evansville; Louisiana State University at Eunice: Patterns of host specificity among unionicolid water mites of North America

North American water mites of the genus Unionicola (Acari: Unionicolidae) are a diverse group of more than 70 species, parasitizing freshwater sponges and molluscs. Most species parasitize freshwater mussels, living on the gills or mantle and foot of their hosts and using these tissues as sites of oviposition. Although host records for North American species have been well documented, there is little information available regarding patterns of host specificity. Host records can be a misleading representation of host specificity because they fail to consider how closely related host species are to each other. The present study addresses host specificity of North American Unionicola using a specificity index that considers the average taxonomic distance between reported hosts. Of 59 species of mites examined, 29 had at least two host species and were assigned specificity indices. Eleven species parasitized two host species and had relatively low indices, indicating that these mites occurred in association with hosts that were taxonomically similar. For 18 species with three or more species of hosts, specificity indices correlated positively with the number of reported hosts, indicating that mites with many hosts are more likely to include species from broader taxonomic origins. However, the variance of these specificity indices did not significantly covary with the number of reported hosts. Thus, mites with few a hosts are just as likely to parasitize a broad taxonomic assemblage of species as are mites with many reported hosts. Specificity indices for gill mites were significantly different from those of mantle mites, with the former parasitizing a taxonomically narrower range of hosts. These results may reflect the longer evolutionary history gill mites have had with host mussels or increased competition among gill mites for oviposition sites, leading to increased host specialization.