Forage preferences and size-mediated trophic segregation among green turtles Chelonia mydas in a resource-limited tropical marine ecosystem

The ontogenetic shift paradigm is critical for understanding the drivers of habitat and resource use in marine consumers. Size-based habitat and diet transitions reflect adaptive responses to ecological trade-offs and environmental conditions, including resource availability, quality, and predation risk. Although ontogenetic habitat and dietary shifts are well known in green turtles Chelonia mydas , they remain poorly documented in tropical marine ecosystems of the western Indian Ocean. Here, we collected skin tissues of 57 juvenile green turtles foraging along the western coast of La Réunion (Mascarene Islands, SW Indian Ocean) using stable carbon and nitrogen isotope analysis (δ 13 C, δ 15 N). Cluster analysis identified 2 distinct isotopic groups: one of larger juveniles foraging in the inner reef systems (mean curved carapace length [CCL] = 61.2 ± 8.9 cm; mean ± SD) and another of smaller juveniles foraging on the outer reef slope (mean CCL = 50.1 ± 10.4 cm). Furthermore, the 2 clusters exhibited contrasting relationships between isotopic values and body size, with one cluster showing an ontogenetic trend (Cluster 2) while the other remained relatively stable across ontogeny (Cluster 1). Stable isotope mixing models suggest that larger turtles (Cluster 1), associated with lagoon habitats, have a diet dominated by seagrass (46%) and red algae (36%). In contrast, smaller turtles (Cluster 2) displayed an omnivorous diet dominated by brown algae (52%) and crustaceans (36%) in forereef habitat. Multiple isotopic niche indicators revealed no significant overlap between clusters, suggesting habitat and resource partitioning. Size-mediated trophic segregation likely mediates intraspecies competition, but the local decline of seagrass meadows might promote trophic overlap or dietary shifts, particularly as green turtle populations are recovering, as observed worldwide.