Morphological and molecular characterization of a new freshwater Ceratomyxa species (Cnidaria: Myxozoa) from the yellow catfish, Trachysurus fulvidraco in China.

Ceratomyxa Thélohan, 1892 is one of the largest genera of the sub-phylum Myxozoa, and has a worldwide geographical distribution. Ceratomyxa species mainly infect the gallbladder of marine fish and rarely infect histozoically or coelozoically freshwater fish. In the present study, yellow catfish, Trachysurus fulvidraco (Siluriformes, Actinopteri) collected from the low reach of Yellow River was first found to be infected with an unknown Ceratomyxa species in their gallbladder which was identified to be new to science and nominated as Ceratomyxa huangheensis n. sp. by an integrative taxonomic approach for myxosporeans. Spores are typical of the genus Ceratomyxa, and matures spores are arcuate, with posterior angle of 139.2 ± 11.6 (137.0-156.0)° and rounded ends, and measures 4.7 ± 0.6 (3.3-5.5) μm in length, and 10.7 ± 1.3 (8.5-13.3) μm in thickness. Two spore valves are slightly unequal and present elongated ovoid in the lateral view. Two equal spherical polar capsules, measuring 2.2 ± 0.4 (1.4-2.8) μm × 2.0 ± 0.4 (1.0-2.5) μm in size locates adjacent to the straight suture line. The obtained partial small subunit ribosomal DNA (SSU rDNA) sequence was unique among all myxozoans in GenBank, and the highest similarity is 85.3% with Ceratomyxa sparusaurati, a marine fish-infecting congener. Phylogenetic analysis further indicated that this novel species did not cluster with other freshwater fish-infecting congeners from South America to form an independent clade, but was phylogenetically positioned within the marine fish-infecting clade. The possible presence of different radiation trajectories between Ceratomyxa huangheensis n. sp. and Amazonian freshwater Ceratomyxa lineage was discussed. This is the first report of Ceratomyxa species in the Yellow River basin and the second freshwater Ceraotomyxa species in China, even in Asia.