New insights into the entrance of Perkinsus olseni in the Manila clam, Ruditapes philippinarum.

In order to understand interactions between Perkinsus olseni and its host mollusk species Manila clam Ruditapes philippinarum, this study focused on invasion processes of the parasite, particularly the mechanisms of zoospore transformation to trophozoites in its portal entry into the host. We exposed Manila clam to P. olseni zoospores, then periodically quantified parasite intensity in various host organs and tissues. We detected large numbers of parasite cells within gills and labial palps of the host clam from the early to the final stages, moderately within mantle and digestive organs but low numbers within hemolymph, foot and adductor muscles. Our results suggest that P. olseni first invades the gills and labial palps of the host clam with limited translocation throughout the host body via the host's circulatory system until 12 days post exposure to zoospores. P. olseni zoospores exposed to extracts of gills and labial palps transformed into trophozoites more efficiently than they did when exposed to other tissues; this transformation was not observed when zoospores were exposed to heated organ extracts. Our results suggest the involvement of a host molecule in the transformation of P. olseni zoospores, leading to initial infection primarily within gills and labial palps of the host clam.