Parasite species identity reshapes geographic patterns of morphology in the hermit crab Pagurus filholi across Japan

Asami Kajimoto , Haruki Shinoda, Noriko Kaataoka, Keito Tsunoda, Daisuke Uyeno, Michitaka Shimomura3, Tsuyoshi Ohira, Kenji Toyota

Geographic variation in morphological traits of marine invertebrates is commonly attributed to local adaptation or phenotypic plasticity, yet such interpretations can be confounded when parasite assemblages — and their morphological effects on hosts — differ systematically among localities. Here, we tested whether the identity of five parasite species—rhizocephalans Peltogaster postica and Peltogasterella gracilis, and bopyrids Parathelges enoshimensis, Athelges takanoshimensis, and Eremitione sp.—alters the interpretation of geographic morphological variation in the hermit crab Pagurus filholi across six Japanese localities (31–42°N), where rhizocephalan barnacles tend to be more common at northern sites and bopyrid isopods at southern ones. We measured six morphological traits in 643 individuals spanning unparasitized P. filholi and five parasite-status categories, and applied principal component analysis, allometric regression, ANCOVA, and within-locality effect size analyses (Hedges’ g) to partition the contributions of body size, locality, sex, and parasite species. Unparasitized individuals showed significant multivariate geographic differentiation (PC1: 86.3% of variance), with allometric slopes heterogeneous among localities. The rhizocephalan Peltogasterella gracilis, sampled across multiple localities, reduced right chela length—the primary sexually dimorphic trait in Paguridae—in parasitized males (g = −0.47) but increased it in females (+0.72), consistent with parasitic feminization; the bopyrid Parathelges enoshimensis reduced multiple size-corrected traits in females (right chela length g = −1.41) without affecting body size, consistent with energetic costs. These contrasting, sex-specific effects demonstrate that parasite species identity is an independent and biologically meaningful driver of morphological variation, and that geographic comparisons of hermit crab morphology require explicit identification of parasite species present.