Ontogenetic and sex‐based trophic niche divergence in the river shrimp <i>Macrobrachium borellii</i> : evidence from the isotopic composition from different tissues

Abstract Sex‐ and stage‐related trophic strategies can shape energy allocation to reproduction, particularly in dynamic environments. We investigated whether diet varies according to sex and ontogenetic stage in the river shrimp Macrobrachium borellii , an omnivorous decapod inhabiting floodplain lakes of the middle Paraná River. We predicted that (1) juveniles and females would consume a higher‐quality, more specialized diet than males, (2) females would show higher δ 13 C in tissues during vitellogenesis and (3) egg isotopic composition would reflect either capital (muscle) or income (hepatopancreas) sources. Using stable isotope analysis (δ 13 C, δ 15 N), mixing models and isotopic niche metrics, we examined trophic patterns across sexes and stages. The results partially supported the first prediction: females – especially those in the reproductive stage – and juveniles showed greater trophic diversification than males but consumed higher‐quality prey. In agreement with the second prediction, females exhibited higher δ 13 C values in both tissues during vitellogenesis. For the third prediction, egg isotopic values were more closely aligned with the hepatopancreas than with muscle, indicating reliance on local food income. Overall, females exhibited flexible foraging and an income‐breeding strategy, adjusting their diet to exploit locally abundant resources to meet reproductive energetic demands. This strategy is well‐suited to the fluctuating hydrological and trophic conditions of floodplain ecosystems and highlights the interplay between trophic flexibility, reproductive tactics and environmental dynamics in crustaceans.