Parasite aggregations: Females attract conspecific males to the hosts in the ectoparasitic isopod Onisocryptus ovalis

For symbionts of marine invertebrates, migration between hosts for mate-searching entails various ecological difficulties. Aggregation in hosts is a behavioral feature sometimes employed by symbionts to overcome such challenges. This can increase mating opportunities, depending on the mating and sexual systems. In this study, we investigated seasonal bias in the ectoparasitic isopod Onisocryptus ovalis, a protandrous hermaphrodite, toward the host ostracod Vargula hilgendorfii using a two-year field survey. We additionally demonstrated the host preference of male isopods through laboratory experiments. Parasitic isopods aggregated on larger and female ostracod hosts in the field during spring, early summer, and sometimes autumn. In laboratory experiments, male parasites, which can move between hosts, consistently showed a preference for host ostracods parasitized by conspecific females compared to hosts without conspecifics. Male parasitic isopods likely select larger female hosts, which can mainly serve more eggs as reproductive resources for female parasitic isopods. Male parasites, in addition, prefer hosts parasitized by female conspecifics. Such preferences and the sexual system of protandrous hermaphrodites may increase the reproductive success of parasitic isopods under limited opportunities to encounter mates. Therefore, these traits of parasitic isopods would produce seasonal aggregation while being influenced by the life history of the hosts associated with overwintering, recruiting, and reproduction.