A new myxosporean species, Kudoa septempunctata n. sp. (Myxosporea: Multivalvulida), is described from the trunk muscles of an aquacultured olive flounder (Paralichthys olivaceus) imported from Korea. This species formed pseudocysts in the myofiber without inflammatory reactions, and the infection was not evident macroscopically. Spores of the new species were irregularly stellate in apical view, with the majority having seven unequal valves, each with a polar capsule of variable size (the remaining spores had six valves and polar capsules). The spores had dimensions of: width 11.8 (11.1-13.1); thickness 9.4 (8.9-10.0); length 8.5 (7.9-8.9); polar capsule length 4.6 (3.7-5.3); and polar capsule width 2.4 (2.2-2.8; mean with range in parentheses; n = 10; all measurements in micrometers). Scanning electron microscopy of the spores revealed unequal positioning of the seven valves without a definite center, rounded posterior ends of valves, and tiny projections at the apex of each valve. The small subunit ribosomal RNA gene (SSU rDNA) sequence of the new species was closely related to Kudoa spp. with five or more valves, particularly Kudoa thalassomi (97.6% identity) recorded from the trunk muscles of a moon wrasse (Thalassoma lunare) around the Australian continent. However, the latter species has six valves with a pointed edge and six polar capsules of a uniform size. The new species was also distinct from all presently known Kudoa spp. with seven valves and polar capsules, i.e. Kudoa yasunagai and Kudoa lethrini, regarding tissue tropism (trunk muscles versus brain), spore shape or external appendages, and SSU rDNA sequence.

Infection of marine fish by certain myxosporean species of the genus Kudoa results in unsightly cyst formation in the trunk muscle or post-mortem myoliquefaction, causing a great economic loss to aquaculture industries, capture fisheries, and fish dealers. In addition, consumers encountering unsightly Kudoa cysts in fish fillets believe them to be unknown foreign materials acquired during processing. To identify prevalent Kudoa spp. encountered in daily life by the Japanese population, fresh fish slices (sashimi) or fish fillets with whitish spots were collected during a 7-month period (May to December 2008) at local markets in the city of Yamaguchi, western Japan. Kudoa cysts were found in three Japanese seaperches (Lateolabrax japonicus), two black sea bream (Acanthopagrus schlegelii), two Japanese jack mackerel (Trachurus japonicus), and one albacore (Thunnus alalunga). Kudoa iwatai was identified in all the examined Japanese seaperch and black sea bream from Japan's Inland Sea, as assessed by morphology and genetic analysis of the 18S and 28S ribosomal RNA gene (rDNA). Kudoa trachuri n. sp. from two Japanese jack mackerel fished in the Japanese Sea off Nagasaki and Kudoa thunni n. sp. from one albacore fished in the Pacific Ocean had a spore, which was semiquadrate in shape in apical views and ovoid in lateral views, with four equal shell valves and drop-like polar capsules. Scanning electron microscopy revealed that these three Kudoa species had different types of small projections at the apex of each valve. The 18S and 28S rDNA sequences of K. trachuri n. sp. and K. thunni n. sp. were found to be closely related to those of Kudoa crumena; however, these sequences were distinct in each of the species, which additionally exhibited different morphological features.