The morphological dietary preferences of an outbreaking population of corallivorous crown-of-thorns sea stars (Acanthaster sp.) in the Gulf of Thailand

Abstract

The current study investigated the morphological dietary preferences of an outbreaking population of corallivorous crown-of-thorn sea stars (Acanthaster sp.) in Koh Tao, situated in the Gulf of Thailand. The local effects of such populations deemed to be in outbreak are currently poorly understood. Therefore, the aim of this study was to investigate the localised feeding behaviours of Acanthaster sp. within an outbreak population, so as to better understand the reefs vulnerable to future outbreaks. Data was collected over a 2-year period, using underwater visual census methods. Survey parameters included sea star abundance, as well as substrate composition and frequency of observed predation scarring. Acanthaster sp. populations were observed to consistently exceed regional outbreak definitions (34.4 individuals ha⁻¹), and demonstrated unimodal size distribution consistent with a primary outbreak. Analysis of feeding behaviour displayed some evidence to support previous claims of density-dependent predation within Acanthaster sp. populations. Van der Ploeg and Scavia tests of dietary preference appeared to suggest that Acanthaster sp. have a marked preference for coral morphologies belonging to tabular and arborescent coral types, and a strong avoidance for massive, laminar, submassive, and solitary coral morphs. Furthermore, Acanthaster sp. displayed a preference towards small coral colonies between 0 and 20 cm. This finding has potential significance for reef resilience and diversity within areas at risk of CoT outbreaks, and seemingly opposes previous literature in other regions. Overall, the current study displays a host of evidence shedding light on Acanthaster sp. ecology and dietary preferences within an outbreaking population density in the Gulf of Thailand. Moreover, there is clear evidence highlighting large spatial variation in the ecology of crown-of-thorns sea stars feeding ecology, and as such demonstrates the requirement for increased localised studies of Acanthaster sp. that accounts for environmental and ecological heterogeneity.