Molecular detection of novel epibiont ciliates associated with the gill of Manila clam Ruditapes philippinarum: a potential first case of mussel protozoan X (MPX)-like ciliates in Japan

The Manila clam is well known to be parasitized by the protist Perkinsus. However, other parasitic organisms affecting this clam remain poorly understood. In our previous study, 18S rRNA gene sequences (uncultured clones named WJR_C1 and WJR_C2) derived from two unknown ciliates were detected in the gills of Manila clams. Based on phylogenetic analyses, these sequences have been suggested to belong to the ciliate subclass Suctoria; however, their ecology remains unclear. In this study, along with re-evaluating the taxonomy of WJR_C1 and WJR_C2, we investigated the prevalence of both ciliates using a specific polymerase chain reaction (PCR)-based detection method. We also examined the localization of the ciliate represented by WJR_C1 in the host gill and its effect on the host using in situ hybridization and hematoxylin–eosin staining. Consequently, WJR_C1 and WJR_C2 were shown to belong not to Suctoria but to the subclass Rhynchodia, and to be particularly closely related to a parasite known as “mussel protozoan X” (MPX). The prevalence of these ciliates varied across regions and seasons, and the WJR_C1 ciliate was localized on the surface of host gills rather than inside the gills. The WJR_C1 ciliate did not appear to have any negative histological effects on host gills.