Larval starvation to satiation: influence of nutrient regime on the success of Acanthaster planci

High density populations of the crown-of-thorns seastar, Acanthaster planci, are a major contributor to the decline of coral reefs, however the causes behind periodic outbreaks of this species are not understood. The enhanced nutrients hypothesis posits that pulses of enhanced larval food in eutrophic waters facilitate metamorphic success with a flow-on effect for population growth. The larval resilience hypothesis suggests that A. planci larvae naturally thrive in tropical oligotrophic waters. Both hypotheses remain to be tested empirically. We raised A. planci larvae in a range of food regimes from starvation (no food) to satiation (excess food). Algal cell concentration and chlorophyll levels were used to reflect phytoplankton conditions in nature for oligotrophic waters (0-100 cells ml^-1; 0-0.01 μg chl a L^-1), natural background levels of nutrients on the Great Barrier Reef (GBR) (1,000-10,000 cells ml^-1; 0.1-1.0 μg chl a L^-1), and enhanced eutrophic conditions following runoff events (100,000 cells ml^-1; 10 μg chl a L^-1). We determine how these food levels affected larval growth and survival, and the metamorphic link between larval experience and juvenile quality (size) in experiments where food ration per larvae was carefully controlled. Phytoplankton levels of 1 μg chl a L^-1, close to background levels for some reefs on the GBR and following flood events, were optimal for larval success. Development was less successful above and below this food treatment. Enhanced larval performance at 1 μg chl a L^-1 provides empirical support for the enhanced nutrients hypothesis, but up to a limit, and emphasizes the need for appropriate mitigation strategies to reduce eutrophication and the consequent risk of A. planci outbreaks.