Sympatric two-species infestation by rhizocephalan parasites in the spider crab Pugettia aff. ferox Ohtsuchi & Kawamura, 2019 (Brachyura: Epialtidae) was investigated in the Vostok Bay (Peter the Great Bay, northwestern Sea of Japan). Morphological and molecular analyses showed that this crab was infested simultaneously by Sacculina pugettiae Shiino, 1943 and Parasacculina pilosella (Van Kampen et Boschma, 1925) (Cirripedia: Rhizocephala). Sacculina pugettiae was found in the northwestern Sea of Japan for the first time. The two rhizocephalan species are clearly distinguishable by the morphology of their external cuticles, the shape and position of their receptacles, and the structure of their colleteric glands. Retinacula are present in the mature externae of both species. Molecular analysis showed that these rhizocephalans are unrelated, although both species parasitize Pugettia aff. ferox and are sympatric. Sacculina pilosella should be placed in the genus Parasacculina Høeg & Glenner, 2019, belonging to the family Polyascidae Høeg & Glenner, 2019. The intensity of infestation reached two externae in P. pilosella and three externa in S. pugettiae per host. A simultaneous settlement of two rhizocephalans on the same crab specimen was shown for the first time. The intensity of the two-species multiple infestations reached four externae per host. Externae with developing embryos occurred from June to September in P. pilosella and July to September in S. pugettiae, at water temperatures of 15-24°C, indicating that the reproductive periods of these species are confined to the summer months in the investigated locality.
Keywords: Morphology; Multiple infestation; Parasacculina pilosella; Rhizocephala; Sacculina pugettiae.