Competitive Exclusion, Harem Behaviour and Host Specificity of the Water Mite Unionicola ypsilophora (Hydrachnellae, Acari) Inhabiting Anodonta cygnea (Unionidae)

The co‐occurring freshwater mussels Anodonta cygnea and A. anatina serve as hosts for the water mites Unionicola ypsilophora and U. intermedia , respectively. Male U. ypsilophora display a territorial behaviour. They fight with other males, and as a result, there is usually only one male per host. As a consequence, this intrasexual aggression results in female‐defence polygyny, or a harem mating system. In contrast, U. intermedia shows no antagonistic behaviour between males. A. cygnea can serve as a host for U. intermedia , but this mite species apparently is excluded from the mussel by U. ypsilophora. In this way, U. intermedia is restricted by competitive exclusion to the mussel A. anatina .