Morphological feminization in the hermit crabs (family Paguridae) induced by the rhizocephalan barnacles

Rhizocephalans (Thecostraca: Cirripedia) are parasitic crustaceans that infect a wide range of decapod hosts, including hermit crabs, crabs, and shrimps. These parasites exert profound effects on their hosts, inducing parasitic castration, suppressing the development of secondary sexual characteristics, feminizing male crabs, and altering male behavior that resembles that of females. In this study, we examined the secondary sexual characteristics of two hermit crabs– Pagurus lanuginosus from Asari Town (Hokkaido, Japan) on the Sea of Japan coast and Pagurus filholi from Chikura Town (Chiba, Japan) on the Pacific coast–both parasitized by Peltogasterella gracilis and Peltogaster sp., respectively. Specifically, we assessed the presence of secondary pleopods and the length of the right large cheliped. Our findings demonstrate that male P . lanuginosus and P . filholi parasitized by P . gracilis and Peltogaster sp. exhibit morphological changes and characteristics of females, confirming morphological feminization. Moreover, the magnitude of parasitic effect on morphological feminization varies between the two host species depending on the rhizocephalan genus. It indicates that the extent of feminization varied depending on the parasite genus. Notably, different parasite genera induced varying degrees of host modification, even within the same host species. Similarly, the level of feminization caused by a single parasite genus differed between host species. Our study highlights the importance to understand the characteristics of both the hermit crab host and rhizocephalan parasite to offer a crucial insights into the morphological feminization of the parasite within its host.