Ontogenetic Trophic Differentiation in the Amur Softshell Turtle (Pelodiscus maackii) From a Temperate Large-River System.

The Amur softshell turtle (Pelodiscus maackii) has undergone severe population declines across northeastern China, yet its trophic ecology remains poorly understood. Because ontogenetic dietary shifts can generate size-structured functional roles within food webs, clarifying diet across life stages is critical for both ecological inference and conservation. We examined diet composition and ontogenetic trophic differentiation in a remnant population of P. maackii from the midstream Ussuri River (China side) using stomach content analysis. Samples were obtained opportunistically from incidental fishery bycatch and market-derived remains collected between July 2022 and October 2025. Diet was quantified using frequency of occurrence and volumetric contribution of five functional prey categories and analyzed with multivariate ordination and permutation-based tests, and niche overlap indices. Across 41 individuals with nonempty stomachs, diet composition differed strongly among ontogenetic stages, whereas no sex-specific differences were detected after accounting for body mass. Juveniles exhibited diverse, invertebrate-dominated diets, particularly freshwater snails, while fish increased sharply with body size and dominated the diets of subadult and adult turtles. Dietary richness and within-group variability declined with ontogeny, and niche overlap was lowest between juveniles and adults but high among older stages. Together, these results demonstrate pronounced size-structured trophic differentiation in P. maackii, indicating that population declines or size-selective losses may have disproportionate consequences for riverine food-web structure and function.