Molecular Characterization and Morphological Observation of “Zeylanema” Nematodes in the Digestive Tract of Fire Eels (Mastacembelus erythrotaenia) Raised in an Aquarium in Japan

The family Camallanidae Railliet and Henry, 1915 comprises parasitic nematodes found in the digestive tracts of fishes, amphibians, and reptiles. In this family, the taxonomic status of several genera is still under debate. Zeylanema Yeh, 1960, is a genus of uncertain status that was segregated from CamallanusRailliet and Henry, 1915. Some researchers have argued that Zeylanema is either a subgenus or junior synonym of Camallanus based on morphological characteristics. No genetic information for Zeylanema has been reported to date, and therefore the molecular phylogenetic relationships of the genus have not been investigated. In this study, nucleotide sequences for 18S and 28S ribosomal DNA and mitochondrial cytochrome c oxidase subunit I of Zeylanema worms were analyzed, and these data were combined with detailed morphological observations. Molecular phylogenetic trees based on 18S and 28S sequences were constructed. The sequences for the Zeylanema worms were included in a single Camallanus clade. This result supported the previous opinion that Zeylanema is a subgenus or junior synonym of Camallanus. These results should prompt further studies to evaluate the taxonomic status and relationships of other genera within the Camallanidae.