Resilience of juvenile crown of thorns starfish and the implications for population outbreaks

Abstract

Population outbreaks of the corallivorous crown of thorns starfish (COTS, Acanthaster sp.) are one of the leading causes of coral loss throughout the Indo-Pacific. Decades of research and significant investment have been directed towards understanding and managing the potential causes of outbreaks with a focus on the larval and adult stages. However, the role of the juvenile stage in population dynamics of COTS is poorly understood. This thesis provides new insights on the biology and ecology of juvenile COTS addressing critical knowledge gaps in their growth, diet, behaviour and physiology.

Juvenile COTS are initially obligate herbivores with a known diet of crustose coralline algae (CCA) before they transition to a coral diet. I found that juveniles are also capable of eating a geniculate coralline algae, Amphiroa sp., and can survive on biofilm with diet-dependent growth rates. An important finding is that the juveniles exhibited extreme growth plasticity and paused their growth for more than six years on an algal diet in the absence of coral and continue growing when offered coral. During the transition to coral, juveniles were injured and killed by coral and, if they survived, they regenerated the damaged tissue. Juveniles also exhibited defensive behaviours that were affected by the presence of their natural enemy, coral guard crabs. I also determined the metabolic rate of both herbivorous and corallivorous juveniles.

To reduce the loss of critical coral reef habitat and manage COTS outbreaks, it is crucial to understand and address the inherent biology of COTS that underlie their success. The growth plasticity and diet flexibility of juvenile COTS is a novel finding of this thesis that is likely to drive their resilience in unfavourable conditions, their success when conditions benefit their development, and contribute to the boom-and-bust dynamics of COTS populations.