There's a marine parasite that 'reprograms' male hermit crabs to grow female body parts just so they can baby-sit its offspring.
March 17, 2026
Original Paper
Seasonal dynamics of sex ratio, reproduction, and parasite-specific feminization in the hermit crab Pagurus filholi at a fixed coastal site in Chiba, Japan
EcoEvoRxiv · 10.32942/X2308K
The Takeaway
The study documents how rhizocephalan parasites castrate their male hosts and force them to develop a second pleopod—an appendage normally found only on females—to help carry the parasite's eggs. This reveals a bizarre level of morphological hijacking where a parasite can rewrite the sexual anatomy of its host.
From the abstract
Reproductive output in intertidal crustaceans is reshaped by seasonal changes in host demography and its interaction with parasitic castration, yet parasite–species–resolved time series remain scarce for hermit crabs. We conducted year‐round monitoring of the hermit crab Pagurus filholi at a fixed intertidal site in Chiba Prefecture, Japan, with monthly sampling from January to December 2025. Hosts were sexed and female reproductive status (ovigerous vs. non-ovigerous) was recorded, and rhizocep