Cardimyxobolus iriomotensis n. sp. (Cnidaria: Myxosporea: Ortholineidae) parasitic in gill filaments of Rhinogobius species (Gobiiformes: Gobiidae) in Japan.

Cardimyxobolus iriomotensis n. sp. (Cnidaria: Myxosporea: Ortholineidae) is described from the gill filaments of Rhinogobius brunneus (type-host) and R. aonumai aonumai (Gobiiformes: Gobiidae) collected from two rivers on Iriomote-jima Island, located in southern Ryukyus, Okinawa Prefecture, Japan. This new species represents the second record of the genus Cardimyxobolus in Japan and the fourth species of the genus globally. The plasmodium of the new species is spherical, and its spores are ovate, appearing wider than long in valvular view. The shell valves are smooth, and the pair of pyriform polar capsules are subequal in length, occupying the anterior half of the spore body and containing 3-5 coils of the polar tubule. The foramina of each polar capsule are positioned laterally at approximately one-third of the distance from the anterior end of the spore. Cardimyxobolus iriomotensis n. sp. can be distinguished from its congeners by the following key features: 1) a smooth shell valve, in contrast to the 1-6 V-shaped folds found in C. leshanensis; 2) significantly thinner spores (4.4-6.1 μm) compared to C. rhodeus (9.0-10.0 μm); 3) pyriform polar capsules, differing from the spherical capsules in C. leshanensis and C. rhodeus; and 4) narrower spore width (7.5-10.0 μm) and smaller polar capsules (2.6-4.0 μm × 1.4-2.7 μm) than those of C. japonensis (11.3-12.5 μm and 4.2-5.0 μm × 2.9-3.8 μm, respectively). Molecular analysis of 18S rDNA revealed the affinity of the new species for C. japonensis and Triangula percae (Ortholineidae). Cardimyxobolus iriomotensis n. sp. appears to be endemic to the southern Ryukyu region, particularly Iriomote-jima Island, with a geographically restricted distribution, at least in Japan. In addition, this study includes taxonomic notes on the genus Triangula.