Competitive Exclusion, Harem Behaviour and Host Specificity of the Water MiteUnionicola ypsilophora(Hydrachnellae, Acari) InhabitingAnodonta cygnea(Unionidae)

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Abstract
The co‐occurring freshwater musselsAnodonta cygneaandA. anatinaserve as hosts for the water mitesUnionicola ypsilophoraandU. intermedia, respectively. MaleU. ypsilophoradisplay 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. intermediashows no antagonistic behaviour between males.A. cygneacan serve as a host forU. intermedia, but this mite species apparently is excluded from the mussel byU. ypsilophora.In this way,U. intermediais restricted by competitive exclusion to the musselA. anatina.