Parasitic rhizocephalans exploit male hermit crabs for reproduction: Feminization and decreased cheliped size

Rhizocephalans, known as parasitic castrators that rob their hosts of reproductive capabilities upon infection to exploit them for their own reproduction, infect Pagurus hermit crabs and alter males to display female-like traits. A joint research group including Researcher Asami Kajimoto of Hiroshima University and Kanagawa University, Associate Professor Kenji Toyota of the Graduate School of Integrated Sciences for Life at Hiroshima University, and Professor Tsuyoshi Ohira of Kanagawa University focused on the phenomenon where "the male hermit crab's body changes like a female (morphological feminization)" caused by peltogastrid rhizocephalans. The group made it clear that the degree of feminization is not a single reaction but differs depending on the species of the parasitic rhizocephalan and the species of the host Pagurus hermit crabs. These findings were published in the online edition of Marine Biology.

Rhizocephalans are parasitic castrators that infect crustaceans such as hermit crabs and other types of crabs, take away the host's reproductive capacity by spreading root-like tissues inside the host's body, and use it for their own reproduction. From previous studies on hermit crabs in the family Paguridae, it has been reported that morphological feminization occurs due to the infection of rhizocephalans, such as the characteristic large cheliped (pincer leg) of male hosts becoming smaller.

However, because it is extremely difficult to distinguish rhizocephalan species based solely on external morphology, quantitative evaluations paired with precise species identification have been limited. Consequently, how the degree of feminization varies across different host-parasite combinations had not been fully understood.

The research group investigated four species of Pagurus hermit crabs collected from the coasts of various parts of Japan (Asari-cho in Otaru City, Hokkaido Prefecture; Noto Town, Ishikawa Prefecture; Sado Island, Niigata Prefecture; and Chikura-cho in Minamiboso City, Chiba Prefecture): Pagurus lanuginosus, Pa. japonicus, Pa. filholi, and Pa. nigrivittatus. By molecular identification using the mitochondrial COI gene, they identified three species of rhizocephalans parasitizing the hosts (Peltogaster sp., Pe. postica, and Pe. aff. ovalis).

For these samples, they compared the "cheliped size" between uninfected individuals and infected individuals by allometric analysis and calculation of standardized effect size. As a result, regarding the cheliped size, they confirmed that infected males of Pagurus lanuginosus and Pa. japonicus significantly shortened to a size similar to that of uninfected females. In contrast, infected males of Pa. filholi and Pa. nigrivittatus showed no significant size difference from uninfected males, and shortening to a size similar to females was not confirmed.

From these results, it became clear that even though they belong to the same group called Peltogastridae, the appearance of morphological feminization is not uniform and depends strongly on the species-specific interaction between the host hermit crab and the parasitic rhizocephalan.

In this research time, by integrating the molecular identification of rhizocephalans and quantitative morphological analysis, it became clear that the "strength of host modification" differs depending on the combination of rhizocephalan and hermit crab. Moving forward, combining these findings with molecular biology techniques is expected to lead to the elucidation of the mechanisms by which parasitic organisms manipulate the sex and morphology of their hosts.

Journal Information
Publication: Marine Biology
Title: Parasite-induced feminization across host-parasite combinations in pagurid hermit crabs infected by peltogastrid rhizocephalans (Crustacea: Cirripedia: Rhizocephala)
DOI: 10.1007/s00227-026-04860-3