Revision of the taxonomic status of Synthesium elongatum (Ozaki, 1935) (Brachycladiidae), an intestinal digenean of narrow-ridged finless porpoise (Neophocaena asiaeorientalis)

Synthesium elongatum (Brachycladiidae) is an intestinal digenean described from the finless porpoise (Neophocaena asiaeorientalis) in Japan. Few records of this species exist and there is a remarkable morphological similarity between S. elongatum and S. tursionis, such that a synonymy between the species has been suggested previously. However, no morphological and/or molecular analysis has been carried out to clarify the taxonomic status of S. elongatum. In this study, we collected specimens of Synthesium sp. from N. asiaeorientalis in western Japan. The specimens possess lobed testes within the third quarter of the body, a round ovary, and vitellaria extending to level of uterine field, which are diagnostic characters for both S. elongatum and S. tursionis. They were morphologically identified to S. elongatum or S. tursionis due to the fact that the available morphometric data for both species overlap remarkably. A molecular analysis of the mitochondrial ND3 gene showed that the pairwise nucleotide distances between these specimens and S. tursionis were small, and phylogenetic analysis showed that these specimens and S. tursionis are in the same clade. Therefore, it was indicated that S. elongatum and S. tursionis are the same species and, consequently, S. elongatum is a synonym of S. tursionis.