Molecular and microscopic identification of Eomarteilia granula infection in Manila clam Ruditapes philippinarum off the south coast of Korea

A report on the new species Eomarteilia (=Marteilia) granula infecting Manila clam Ruditapes philippinarum from Japan in 2014 suggests the possibility of E. granula infecting other Manila clam populations in the Northwest Pacific region, including Korea. In this study, we report the first infections by E. granula in Manila clams off the south coast of Korea. Histology revealed Marteilia-like plasmodia in the digestive tubule epithelia. Tissue imprints demonstrated that each parasite sporangium enclosed 4 spores and transmission electron microscopy (TEM) revealed ultrastructure of primary cells enclosing secondary cells, which contained spores. Mature spores consisted of 3 sporoplasms: outermost, intermediate, and innermost. The outermost sporoplasm showed a peripheral electron-dense monolayer characteristic of E. granula. The 18S rDNA amplified from the Marteilia-like parasite yielded 1784-bp PCR amplicon sequences which were 99.8% similar to that of E. granula previously reported (as M. granula) from Japan. In the molecular phylogenetic analysis, the novel Marteilia-like organism formed a well-supported clade with E. granula. Accordingly, we concluded that the novel Marteilia-like parasite that we found infecting some Korean Manila clams is Eomarteilia granula. Field surveys revealed that the infection was limited to clams of the south coast of Korea, with the prevalence ranging from 3.3 to 5.0%.