Three Unicapsula species (Myxosporea: Trilosporidae) of Asian marine fishes, including the description of Unicapsula setoensis n. sp. in the yellowfin goby (Acanthogobius flavimanus) from the Inland Sea of Japan.

The myxosporean genus Unicapsula (Multivalvulida: Trilosporidae) is defined as having a spore with three unequal shell valves and polar capsules, of which one is prominent and the two other polar capsules are rudimentary. Genetic characterization of members of the genus, currently 11 nominal species, is, at present, unsatisfactory yet when comparing to the closely related genus Kudoa (Multivalvulida: Kudoidae). In the present study, we characterized long ribosomal RNA gene (rDNA) sequences of three Unicapsula spp., namely Unicapsula pyramidata, Unicapsula seriolae, and a novel myxosporean species, Unicapsula setoensis n. sp., from Asian fishes. Elongated plasmodia of U. pyramidata were found in the trunk muscle of Japanese threadfin breams, Nemipterus japonicus, fished off northern Vietnam in the South China Sea. Semitriangular spores, 5.5-6.4 μm in length and 5.6-9.6 μm in width, consisted of three shell valves with two caudal appendages, 7.2-7.4 μm in length. One prominent polar capsule, 2.0-2.4 μm in diameter, was located in the apical shell valve and two rudimentary polar capsules, 0.4-0.5 μm in diameter, in each caudal shell valve. Elongated plasmodia of U. seriolae were found in the trunk muscle of a greater yellowtail, Seriola dumerili, aquacultured in Japan. Semispherical spores, 5.9-7.4 μm in length and 6.3-7.4 μm in width, also consisted of three shell valves and one prominent polar capsule, 3.4-3.8 μm in diameter, with two rudimentary polar capsules, 0.7-1.0 μm in diameter. Plasmodia of U. setoensis n. sp. were found in the trunk muscle of yellowfin gobies, Acanthogobius flavimanus, fished off Hofu, Yamaguchi Prefecture, in the Inland Sea of Japan. Semispherical spores, 5.6-6.9 μm in diameter, displayed three shell valves and one prominent and two rudimentary polar capsules. The former functional polar capsule was 1.9-2.5 μm in diameter and extruded a 9.4-13.8-μm-long polar filament. Nearly the whole length of the 18S rDNA and more than 2,200 bp of the 28S rDNA of the three Unicapsula spp. were sequenced along with nucleotide sequences of the 5.8S rDNA and internal transcribed spacer-1 and spacer-2 of U. pyramidata and U. setoensis n. sp. Molecular genetic analyses supported the morphological species differentiation of U. pyramidata and U. seriolae, and the distinctness of U. setoensis n. sp. from hitherto known species.