Members of the bopyrid subfamily Pseudioninae are obligate parasites of crustaceans, with the majority of species infesting the branchial chambers of their hosts. These parasites infest a broad range of decapod crustaceans, including members of the Anomura (hermit crabs and their relatives). The purpose of the current research was to expand knowledge of these parasites from the Indo-West Pacific and especially from the Philippines, a region that is an epicenter of marine biodiversity. In total,

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... 2,175 hermit crabs from the Philippines were examined; 112 (5.1%) were infested with pseudionines belonging to four genera, including three species that are new to science. In addition, three species are reported from the Philippines for the first time and include new host records. Asymmetrione Codreanu, Codreanu & Pike, 1965 , is represented by two species: A. asymmetrica , found infesting the hermit crabs Calcinus gaimardii, C. minutus, and Dardanus lagopodes, and A. harmoniae, new species found infesting the hermit crab D. lagopodes from the Philippines and Indonesia. We redescribe Bopyrissa magellanica Nierstrasz & Brender à Brandis, 1931, the type species of Bopyrissa Nierstrasz & Brender à Brandis, 1931, and designate a lectotype. Bopyrissa xiphidiostega, new species was found infesting C. gaimardii, C. minutus, and D. lagopodes; females possess a unique lateral extension of the first oostegite that overlaps the basis of pereopod 1. Bopyrissa marami, new species is described based on specimens infesting C. gaimardii, C. latens and C. minutus whereas Pseudione novaeguineensis P. kensleyi Williams &, are both transferred to Bopyrissa, with B. kensleyi recorded for the second time from Singapore. We erect Eremitione, new genus to accommodate nine species previously placed in Pseudione Kossmann, 1881, that infest hosts in Lithodidae and Paguroidea. Eremitione clibanaricola , new combination was found infesting C. gaimardii and C. minutus. The holotype of Pseudione calcinii (now Eremitione calcinii (Shiino, 1933, new combination) was examined and illustrated. Finally, Pagurion tuberculata Shiino, 1933, was found infesting D. lagopodes in the Philippines and Indonesia; study of these specimens supports the synonymy of Pagurion arrosor An, Li & Markham, 2013 with P. tuberculata. These findings show that the diversity of bopyrid fauna in the Indo-West Pacific has been underestimated. A key to all genera of pseudionines that infest hermit crabs is provided, as well as keys to all species of Asymmetrione, Bopyrissa, and Eremitione, new genus. Total Parasitised (%) Parasite speciesnumber (%) Calcinus gaimardii (H. Milne Edwards, 1848)* 992 86 (8.7) Bopyrissa marami new species -72 (7.3) Bopyrissa xiphidiostega new species -8 (0.8) Eremitione clibanaricola new combination -5 (0.5) Unidentified branchial bopyrid -1 (0.1) Calcinus latens (Randall, 1840) 505 1 (0.2) Bopyrissa marami new species -1 (0.2) Calcinus minutus Buitendijk, 1937 208 10 (4.8) Asymmetrione asymmetrica -1 (0.5) Bopyrissa marami new species -5 (2.5) Bopyrissa xiphidiostega new species -3 (1.4) Eremitione clibanaricola new combination -1 (0.5) Dardanus lagopodes (Forskål, 1775) 123 11 (8.9) Asymmetrione asymmetrica -1 (0.8) Asymmetrione harmoniae new species -2 (1.6) Bopyrissa xiphidiostega new species -7 (5.7) Pagurion tuberculata -1 (0.8) *C. gaimardii is very similar to C. morgani Rahayu & Forest, 1999 (see Malay et al., 2018 and the species are sympatric. Our specimens appear to most closely match C. gaimardii based on morphology and color pattern.