Seasonal dynamics of sex ratio, reproduction, and parasite-specific feminization in the hermit crab Pagurus filholi at a fixed coastal site in Chiba, Japan

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 rhizocephalan infections were diagnosed by externa morphology and subsequently assigned to Peltogasterella gracilis or Peltogaster postica using DNA barcoding. Across 12 sampling occasions, 3,134 crabs were recorded (1,586 males; 1,548 females). Sex ratio varied significantly among months, shifting from female-biased in winter–early spring to strongly male-biased in late summer–early autumn. The proportion of ovigerous females also showed strong seasonality, peaking in mid-winter, declining through spring, reaching zero in August–September, and increasing again toward winter. Both parasites exhibited sharply seasonal prevalence with a pronounced June peak: for P. gracilis, 29/166 in June vs. 8/2,968 in other months, and for P. postica, 19/166 vs. 36/2,968. Male secondary sexual morphology responded in a parasite-specific manner: infected males were more likely to bear a second pleopod, a female-specific egg-brooding structure, than uninfected males, driven primarily by P. gracilis (26/28) rather than P. postica (3/35). Together, these results provide a high-resolution baseline linking reproductive phenology and parasite-specific feminization in a single P. filholi population.