The dominant urchin in southeastern Australia is an omnivore, not a herbivore

Abstract Urchin diets are a topic of global interest due to their impacts on subtidal reefs. Urchins can consume large amounts of macroalgae, leading to the loss of macroalgal forests and creation of “barrens”. However, there is scant dietary information in the southern hemisphere for the temperate sea urchin, Centrostephanus rodgersii , and what they consume within barrens, which lack macroalgae. Here, we sampled C. rodgersii to quantify its nutritional sources in macroalgal forests and barrens along ~ 630 km of the SE Australian coastline. Urchins were collected from macroalgal forests and barrens and the stable isotope values (δ 13 C, δ 15 N, δ 34 S) of urchin muscle tissues were analysed. Using Bayesian stable isotope mixing models, we found macroalgae contributed to nutrition in both habitats (~ 30%). However, grouped sources containing invertebrates contributed more to urchin diets in both barrens (> 50%) and macroalgal forests (> 60%). Composition changed with habitats – crustaceans or similar invertebrates were the dominant source in barrens while a more diverse invertebrate array was relied upon in macroalgal forests. However, proportional contributions differed along the coast more than between habitats within locations. These data provide new context for C. rodgersii , as these urchins are not primarily herbivorous. Furthermore, this study indicates that conceptual models for barrens in SE Australia are more complicated than previously considered, as C. rodgersii were omnivorous irrespective of habitat. This could suggest the core C. rodgersii diet is invertebrates rather than macroalgae, raising questions about their perceived ecological role.