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. I n 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. I n this way, U. intermedia is restricted by competitive exclusion t o the mussel A. anatina.
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