Temporal partitioning: dynamics of alternating occupancy of a host microhabitat by two different crustacean parasites

Abstract

Double infection by 2 crustacean parasites was found on the mysid Siriella okadai collected from the Seto Inland Sea, Japan. An epicaridean isopod, Prodajus curviabdominalis, and a siphonostom- atoid copepod, Neomysidion rahotsu, both occupied the lumen of the host marsupium, and fed vora- ciously upon host eggs. Interestingly, adult females of these 2 parasites occurred alternately on the host, with almost no overlap: the copepod occurred from mid-winter to summer when water temperatures were <20°C (with mean prevalence of 7.2%), whereas the isopod occurred exclusively from mid- summer to late autumn when water temperatures exceeded 20°C (with mean prevalence of 9.1%). Pos- sible factors responsible for generating and maintaining this alternation of microhabitat occupancy on the host are discussed on the basis of the life cycles and host-specificities of the 2 parasites. In addition, we explored the possible means by which these obligate parasites survive during the periods when they are not on the mysid host. In the case of the isopod, we hypothesise that survival away from the mysid host involves the utilisation of an intermediate host by an as yet undiscovered microniscus larva, as in other epicarideans. For the copepod, we considered the available evidence in support of a number of hy- potheses. A new pattern of behaviour, unique within the copepod family Nicothoidae, was discovered in N. rahotsu. After the infective female copepodid stage attached to the host, it moulted into an imma- ture female and penetrated the host tissue. It then migrated internally, typically through the dorsal trunk musculature of the host, finally emerging into the host marsupium. This behaviour was observed exclu- sively during the season when copepods occurred within the marsupium. We inferred that this internal migration behaviour in some ways facilitates the synchronisation of the parasite life cycle with the oviposition by the host into the marsupium. It remains uncertain how and where N. rahotsu passes the exclusive occurence of P. curviabdominalis.