Description of Myxobolus iwagiensis n. sp. (Myxosporea: Myxobolidae), infecting medaka Oryzias latipes (Temminck & Schlegel, 1846) (Beloniformes: Adrianichthyidae) in Japan.

Wild medaka Oryzias latipes collected from a brackish water reservoir on Iwagi Island, Japan, were parasitized by a myxosporean belonging to the genus Myxobolus. Morphological and molecular analyses were carried out to identify the myxosporean parasite. The myxospores measured 12.1 ± 0.4 μm long, 9.8 ± 0.3 μm wide, and 7.9 ± 0.2 μm thick. Oblong to oval shape in valvular view and elliptical in sutural view. Two, unequally sized, pyriform polar capsules, running parallel to each other. The larger polar capsule measuring 6.4 ± 0.2 μm long and 4.0 ± 0.2 μm wide. The smaller measuring 5.8 ± 0.3 μm long and 3.5 ± 0.2 μm wide. Polar filament form 4-6 coils in the smaller polar capsule and 6-8 coils in the larger polar capsule. In histological examination, plasmodia containing mature spores were observed in peripheral nerves, including the cranial nerves, spinal cord and ganglia, muscular and skin connective tissues, and beside to the gill arch cartilage. A BLAST search based on the small subunit ribosomal DNA sequence showed <82 % similarity with species in the family Myxobolidae. On the basis of the host species, infection site, spore morphology and molecular analyses, we describe the myxosporean from O. latipes as a novel species, named Myxobolus iwagiensis n. sp. This is the first report of myxosporean infection in medaka in Japan.