A new species of Athelges Gerstaecker, 1862 (Isopoda: Epicaridea: Bopyridae) parasitizing hermit crabs in Japan, with molecular phylogenetic analyses

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Abstract Epicaridean isopods are obligate parasites of crustaceans, and members of Athelginae are abdominal bopyrid parasites of hermit crabs. Athelges Gerstaecker, 1862 is the most species-rich genus in the subfamily, but its recognized species diversity in Asia remains low despite many records from the region. Because species identification of taxa in Athelges is highly challenging, molecular phylogenetic analyses are needed to clarify species boundaries in the genus. We describe a new species of Athelges collected from the dorsal surface of the pleon of Pagurus decimbranchiae Komai & Osawa, 2001 from Tateyama, Chiba Prefecture, Japan. The new species is distinguished from all known congeners mainly by the hook-shaped pleotelson in the female, which features a distal end divided into three pointed processes. Partial sequences of the 16S rRNA and 18S rRNA genes were obtained, and phylogenetic analyses showed that the new species is genetically distinct from the two congeners previously known from Asia, A. takanoshimensis Ishii, 1914 and A. pileus Shinoda & Shimomura in Shinoda et al., 2026. The present study represents the first record of any bopyrid from P. decimbranchiae and provides new information on the host associations of Athelges species in Asia. This discovery suggests that the species diversity of Athelges in Asia remains underestimated and supports the usefulness of combining morphological and molecular data for species delimitation in this taxonomically difficult genus.

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