Identification of the Larval Cestode Plerocercoid Found in the Body Cavities of the Japanese Smelt, Hypomesus transpacificus nipponensis, and Review of the Literature

Abstract

Plerocercoid, generally known as one of the larval forms of the Pseudophyllidea cestode, was found in the body cavities of the Japanese smelt, Hypomesus transpacificus nipponensis. The larva was analyzed genetically to identify the species. Homology searching of the partial ribosomal RNA (rRNA, 1,238 bp) and mitochondrial cytochrome oxidase subunit 1 (cox1, 428 bp) gene sequences, obtained using the FASTA program, showed that the sequences of the larva had 98.3% and 98.5% identity, respectively, with those of Diphyllobothrium ditremum. The larva was also grouped with D. ditremum according to molecular phylogenetic analysis of the cox1 sequence (393 bp), so that the larva from the Japanese smelt appeared genetically to be D. ditremum. However, Diphyllobothrium plerocercoids have already been detected in Japanese surf smelt and olive rainbow smelt and assigned the new species name D. hottai based on the morphological and biological differences between the isolates from the two species of smelt and D. ditremum. Since the sequence data of D. hottai remains unavailable, it was not possible to make a genetic comparison of the plerocercoid found in the Japanese smelt and D. hottai. The present report highlights problems in differentiating morphologically or biologically similar Diphyllobothrium species.