Host-epibiont relationship between calanoid copepods and suctorian ciliates in the northern North Pacific Ocean

Abstract

This study investigated the host-epibiont relationship between calanoid copepods and suctorian ciliates collected widely from the northern North Pacific Ocean. Mid- and deep-water adult female calanoids without dormant stages were found to be infested with suctorians, probably because of their larger size, longer intermolt duration and longevity in comparison to males and surface water species. A total of 259 calanoids belonging to sixteen species were infested with five suctorian species. Among them, Candacia columbiae, Gaetanus minutus, Lucicutia bicornuta, Metridia similis, Pleuromamma scutullata, and Racovitzanus antarcticus were reported to be infested with suctorians for the first time in the present study. Ephelota coronata infested only Metridia pacifica and displayed a strong host preference, whereas the other suctorians displayed a weaker host preference. Any suctorian species that infested Metridia pacifica and M. similis attached almost exclusively to their urosome. Suctorians might avoid or could not stay attached to the anterior parts of these fast-swimming copepods. The attachment sites of Actinocyathula pleuromammae and Paracineta gaetani extended to the anterior part of the copepod body as the number of attached suctorians increased, suggesting they were obliged to attach to suboptimal parts that were exposed to faster flow speed.