On the nocturnal emergence of reef-dwelling urchins: continental-scale patterns and correlates

Sea urchins are renowned for their capacity to shape shallow reef community structure and function, especially when at high densities. However, most quantitative assessments of urchin densities are based on diurnal surveys, despite the nocturnal emergence of urchins being a well-recognised phenomenon. The value of such diurnal density estimates is, therefore, inherently dependent on the extent of nocturnal urchin emergence and the consistency of this diel behaviour under different contexts. Yet, our understanding of nocturnal urchin emergence under different local conditions at broad scales including between tropical and temperate reefs, remains largely unresolved. Here, we evaluate the extent of nocturnal urchin emergence, and assess how this varied with potential predictors, using paired day and night surveys across a total of 42 reef sites spanning tropical (n = 23) and temperate (n = 19) Australia. Among kelp-dominated temperate rocky reefs, urchin densities and biomass were ∼7-8-fold greater than observed on tropical coral reefs. Regardless of realm, urchin densities and biomass were consistently higher at night, with tropical reefs showing the greatest diel differences (3.5-fold higher density and 3.0-fold higher biomass) compared to temperate reefs (1.4-fold and 1.7-fold, respectively). Gradient Boosted Regression Trees revealed that mean sea surface temperature (SST) was the strongest predictor of the relative extent of nocturnal urchin emergence, with higher relative emergence on reefs with warmer SST, corresponding with strong patterns in nocturnal emergence peaking toward equatorial latitudes. Our findings show that daytime surveys likely greatly underestimate urchin activity, population size, and ecological impacts, particularly on tropical reefs, with important consequences for reef monitoring and management.