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 (<5 mm test diameter, TD) were reported within established C. rodgersii habitats along the coast of New South Wales. We investigated populations of T. australiae at 9 sites in this region in 2025 and 2026. To investigate species distributions relative to foliose macroalgal cover surveys were conducted to 20 m depth. We also assessed T. australiae size, recording the test diameter of 443 individuals across sites. Generalised Linear Mixed Models (GLMMs) showed that T. australiae abundance was positively correlated with macroalgal cover while C. rodgersii was negatively correlated. In one location T. australiae were predicted to be more abundant at depth, while C. rodgersii were always abundant in the shallows. Most T. australiae were large (>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.