Identification of Thelohanellus pseudonikolskii n. sp. and Myxobolus koi Kudo, 1920 from Carassius auratus

During a parasitological survey of goldfish Carassius auratus (Linnaeus, 1758) in China, two myxosporeans were collected in the present study including a Thelohanellus species and a Myxobolus species. To identified the two myxosporeans, a morphological, histological and molecular study was performed. The Thelohanellus species was characterized by round, white plasmodia in fins of goldfish. Compared with all described Thelohanellus species, spore morphology of the Thelohanellus species in the present study was distinct from them. BLAST search showed that SSU rDNA sequence of this Thelohanellus species did not match any sequence in GenBank database. Phylogenetically, the Thelohanellus species obtained in the present study was sister to Thelohanellus nikolskii Achmerov, 1955. So, the present Thelohanellus species was identified as a novel species and named Thelohanellus pseudonikolskii n. sp. The Myxobolus species was characterized by a number of round or irregularly shaped, white plasmodia in gills of goldfish. When compared with all reported Myxobolus species, spore morphology of the present Myxobolus species was most similar to Myxobolus koi Kudo, 1920. Histological analysis showed that plasmodia of this Myxobolus species simultaneously developed in stratified epithelium of filament and in lamellae, which was same to M. koi. Moreover, BLAST search showed that SSU rDNA sequence of the present Myxobolus species was identical to M. koi. Thus, the morphological, histological and molecular analysis demonstrated that the present Myxobolus species was conspecific with M. koi. Due to M. koi consistently described from gills of common carp Cyprinus carpio Linnaeus, 1758 and koi carp C. carpio in the previous researches, M. koi was first found to infect gills of goldfish in the present study. Considering koi carp and goldfish both as ornamental fish frequently cultured together in China, the possible host shift of M. koi from koi carp to goldfish is of interest for further investigation.