Crown-of-thorns starfish larvae are vulnerable to predation even in the presence of alternative prey

Many predators reported to feed on crown-of-thorns starfish (CoTS, Acanthaster spp.) are generalist and opportunistic feeders. However, research into predation on CoTS tends to examine these predator-prey interactions in isolation, and it remains unknown whether many potential predators will prey on CoTS when other, potentially more palatable, food sources are available. Assessing predatory responses to changes in prey availability is critical for gauging the capacity of predators to regulate prey populations. Here, we explored prey preferences and tested for prey switching across nine species of planktivorous damselfish offered varying densities of Pacific CoTS (Acanthaster cf. solaris) larvae versus larvae of a common and co-occurring starfish, Linckia laevigata. Results show that planktivorous damselfishes will consume crown-of-thorns starfish larvae, even in the presence of alternative prey. Feeding responses varied among the nine planktivorous predators with five damselfishes (Acanthochromis polyacanthus, Amblyglyphidodon curacao, Dascyllus reticulatus, Pomacentrus amboinensis and Pomacentrus moluccensis) exhibiting increased consumption of Acanthaster larvae with increasing density, despite the presence of alternative prey. Moreover, Abudefduf sexfasciatus and P. amboinensis exhibited preference for larvae of A. cf. solaris over larvae of L. laevigata. Despite these predation patterns, prey switching between starfish larvae was not observed. These results add to a growing body of evidence which suggests that predators of the early life stages of A. cf. solaris could be important in regulating settlement and recruitment patterns of this starfish, especially at low, non-outbreak, densities.