Comparison of the ecology of pelagic copepods as basibionts and suctorian ciliates as epibionts in the subarctic Pacific

Suctorian ciliates are widely distributed in both freshwater and marine environments. In the open ocean, they attach to crustacean zooplankton, specifically copepods, as epibiont hosts. However, criteria for host preference in oceanic areas remain unclear. In this study, we conducted a molecular analysis, examined copepod community relationships and performed laboratory-rearing experiments on suctorian ciliates in the subarctic Pacific. Suctorians belonging to five genera and species were observed on the exoskeletons of 20 copepod species across 10 genera. Additionally, species-specific host preferences were observed. Molecular analyses revealed intraspecific variations and haplotypes in suctorian ciliates, with no correlation between the haplotypes and host copepods or geographical location. Suctorians did not prefer numerically dominant or large-sized copepod species but favored deep-sea or diel vertical migration (DVM) species, specifically carnivorous copepods. A high suctorian prevalence was observed during the late copepodite stages, specifically among adult females and in regions with cold waters. These characteristics indicate that suctorians rely on prey or host molting events. Laboratory experiments confirmed novel suctorian attachments and growth from swarmer to trophont within 24 h. This study indicates that suctorian epibionts prefer late copepodite stages in deep-sea habitats, exhibiting DVM, prolonged stage duration and low predation risk.