Morphology and phylogeny of Ameson portunus n. sp. (Microsporidia) infecting the swimming crab Portunus trituberculatus from China.

Ameson portunus n. sp. is a new microsporidian species that infects the skeletal muscle of Portunus trituberculatus, a pond-reared swimming crab from China. This parasite was characterized using morphological and molecular phylogenetic data. Light and transmission electron microscopy revealed that this microsporidian experienced disporogonic and polysporogonic (chain-like) life cycles. Mature uninucleate spores appeared ovoid, measured 1.4±0.06×1.0±0.07μm on ultrathin sections, and exhibited no dimorphism. The isofilar polar filament was coiled in 8-9 turns. Of these coils, 5-9 were arranged in large regular outer layers; the remaining coils (0-3 coils) were situated internally. According to phylogenetic analyses based on the small subunit (SSU) rDNA gene, A. portunus n. sp. belonged to the group comprising Ameson spp. and Nadelspora canceri. The result of comprehensive analysis of ultrastructural features, molecular phylogenetic data, host and geographical differences among known species supports the establishment of a new Ameson species for this parasite. Ameson portunus n. sp. is the first Ameson species described from the coasts of East Asia.