Establishment of the barrens habitat forming urchin, Tripneustes australiae on rocky reefs will increase impacts on macroalgae in Southeast Australia

Worldwide, grazing by sea urchins can create barrens habitat devoid of foliose macroalgae. Climate change has enabled urchins to poleward range extend, colonizing rocky reefs and reducing macroalgal forests. Tripneustes australiae, typically a vagrant on rocky reefs in southeast Australia where Centrostephanus rodgersii dominates, now appears at high densities. In 2024, many small T. australiae (90 mm TD) indicating established adult populations. Information on these species where they co-occur with C. rodgersii is scant, and our results suggest synergistic grazing impacts. In contrast to C. rodgersii, grazing by T. australiae is not limited by depth or time of day. Aggregations of T. australiae at its southernmost range limit is a new phenomenon, suggesting a potential range extension front. As T. australiae prevalence increases, deep water may provide less refuge for macroalgae going forward.