Molecular and ultrastructural characterization of a novel cryptic species of the Mesomycetozoea clade isolated from Manila clam, Ruditapes philippinarum, on the west coast of Korea

In the present study, a cryptic species (IchX) was isolated from the hemolymph of the Manila clam, Ruditapes philippinarum, collected from the west coast region of South Korea. Following comprehensive molecular analysis, a partial sequence resembling the small subunit of the ribosomal RNA (SSU rRNA) gene was obtained, indicating that this species belonged to the class Mesomycetozoea, also known as Ichthyosporea. Detailed phylogenetic analyses based on SSU rRNA sequences placed IchX in a distinct clade within the order Dermocystida, class Mesomycetozoea, and showed that IchX is closely related to Ichthyosporea sp. Microscopic examination of in vitro cultured IchX cells revealed life-cycle stages of different sizes, from the endospore to sporangium through vegetative stages. An ameboid-like structure was observed in the early endospore stages as the characteristic feature of zoospores. Ultrastructural analyses using scanning electron microscopy revealed that all endospores and vegetative cell stages are spherical. Transmission electron microscopy revealed characteristic features, including a spindle pole body and membrane-decorated hyaline vesicles, consistent with those previously described in Mesomycetozoea. In addition, a prominent fibrillar structure was observed. Notably, the cell wall of mature IchX sporangia was digested with 2 M NaOH, while that of the endospores was resistant. This is the first report of a novel Mesomycetozoean from the Manila clams. Further taxonomic study of this organism and elucidation of its pathological characteristics are necessary.