A physiological crisis drives the coral recruitment bottleneck.

Recruitment failure is an important factor contributing to population declines of tropical corals. Because the causes of death for juvenile corals are unclear, it is challenging to predict how recruitment bottlenecks will change in the future. We tested the hypothesis that depletion of metabolic reserves increases mortality of juvenile corals under thermal stress. Metabolic reserves of juvenile colonies (<30 mm diameter) of broadcast spawning from Moorea, French Polynesia, were manipulated using elevated temperature to increase respiration, and reduced day length to decrease photosynthesis, and estimated as biomass. Corals with high or low biomass were incubated at 28°C and 31°C for 15 days. Juvenile with high biomass were six times more likely to die at 31°C versus 28°C, but corals with low biomass were 48 times more likely to die at 31°C versus 28°C. When juvenile were grown in seawater augmented with bicarbonate to reduce the cost of skeletogenesis in support of growth, growth was not affected, but energy expenditure was reduced by 20% to reduce reliance on metabolic reserves. Resource limitation of juvenile corals can affect their response to elevated temperatures, supporting the hypothesis that a physiological crisis initiated by resource limitation mediates the stringency of recruitment bottlenecks.