Morphological feminization in hermit crabs (family Paguridae) induced by parasitic peltogastrid barnacles (Crustacea: Cirripedia: Rhizocephala)

Rhizocephalan barnacles are highly specialized parasitic crustaceans that profoundly alter the morphology, physiology, and reproduction of their decapod hosts. In hermit crabs (Paguridae), parasitism by peltogastrid rhizocephalans has been reported to induce feminization of male secondary sexual traits, such as the development of female-specific pleopods and the reduction of cheliped size; however, quantitative and comparative analyses across host–parasite combinations remain limited. In this study, we examined morphological feminization in four hermit crab species (Pagurus lanuginosus, Pa. filholi, Pa. nigrivittatus, and Pa. japonicus) parasitized by three peltogastrid rhizocephalans (Peltogaster sp., Pe. postica, and Pe. aff. ovalis). The occurrence frequency of a second pleopod, a female-specific structure for egg brooding, was significantly higher in parasitized males of Pa. lanuginosus and Pa. nigrivittatus, whereas no significant difference was detected in Pa. filholi. Allometric analyses revealed pronounced reductions in cheliped length in parasitized males of Pa. lanuginosus and Pa. japonicus, with regression slopes approaching those of normal females. Effect size analyses further demonstrated that cheliped lengths of parasitized Pa. lanuginosus and Pa. japonicus were similar to those of normal females, whereas those in parasitized Pa. filholi and Pa. nigrivittatus were not, indicating the magnitude of feminization differed markedly among host–parasite combinations. These results indicate that peltogastrid-induced morphological feminization in hermit crabs is highly species-specific and depends on both host and parasite identity. Our findings highlight the importance of integrating molecular identification of parasites with quantitative morphological analyses to elucidate the diversity of host manipulation strategies in rhizocephalan barnacles.