Diet of herbivorous juveniles modulates growth, survival, and the timing of the ontogenetic diet shift to corallivory in crown-of-thorns sea stars

Abstract Outbreaks of western Pacific crown-of-thorns sea stars (CoTS, Acanthaster cf. solaris ) remain one of the foremost contributors to coral loss on Australia’s Great Barrier Reef. For CoTS to reach maturity, juveniles feeding on crustose coralline algae (CCA) must complete an ontogenetic diet shift to reef-building corals. Here, we tested whether four different diets of CCA fed to herbivorous juveniles over 12 weeks influence the growth and survival of early-stage juveniles, and the timing of the ontogenetic diet shift to corallivory by introducing the juveniles to coral after 113 days of being offered CCA solely. Our results demonstrate considerable diet-mediated variation in growth rates (increase in number of arms, diameter, and surface area), with juveniles consuming the alga Melyvonnea sp. (mean diameter of 5.9 mm ± 1.16 SE after 12 weeks) growing significantly faster and larger, particularly compared to those consuming Sporolithon sp. (3.91 mm ± 0.96). Moreover, after 7 weeks of being offered coral, 100% of surviving juveniles fed Melyvonnea sp. transitioned to feeding on Acropora kenti contrasting with markedly lower transition rates for juveniles consuming Lithophyllum sp. (41%), Lithothamnion sp. (27%), and Sporolithon sp. (11%). Our results suggest that specific CCA (e.g., Melyvonnea sp.) promote elevated juvenile growth and earlier transition to corallivory, releasing CCA-feeding juveniles from stunted growth and high mortality, and facilitating earlier exponential growth and maturity as corallivores. The prevalence of these CCA on reefs may thus enhance individual fitness and recruitment success of CoTS, contributing to increased risk of population outbreaks and corresponding coral loss.