A new Synthesium species (Digenea: Brachycladiidae) from the bottlenose dolphin Tursiops truncatus (Cetacea: Delphinidae) in Southwestern Atlantic waters

One-sided infections by intestinal parasites in two sympatric porpoises bycaught from the Nemuro Strait of Hokkaido, Japan

Due to the difficulties and limitations associated with field sampling of marine mammals, quantitative surveys on their parasites are still scarce in Japan. We here report the infection statuses of intestinal parasites in harbor porpoises (n = 34) and Dall's porpoises (n = 10), bycaught between 2014 and 2018 by a bottom set-net fishery in the northern part of the Nemuro Strait, Japan. Necropsies of the harbor porpoises recovered one digenean, Synthesium nipponicum, and two acanthocephalans, Corynosoma sp. and Bolbosoma sp.; no infection was observed in the Dall's porpoises. The dominant parasites were Corynosoma sp. and S. nipponicum, with a prevalence of 73.53 and 64.71% and a mean abundance of 43.88 and 7.38 individuals, respectively. Although the two porpoise-species have a sympatric distribution in this narrow strait, the findings support the existence of trophic-link dependences, which can differentially affect vulnerability to these parasites.

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.