Assessing feeding electivity in Acanthaster planci: A null model analysis

Feeding electivity was investigated in a non-outbreaking population of the crown-of-thorns starfish Acanthaster planci (L.) from North Sulawesi, Indonesia. A null model-based approach was used to assess the feeding pattern of Acanthaster in relation to the availability of coral prey in the field. Of a total of 70 species of corals recorded as prey, massive species, particularly of Faviidae, tended to be more frequently consumed than would be expected under the assumption of random feeding by A. planci. Branched and encrusting/laminar forms of corals that occurred in relatively exposed sites were apparently not preferred, pointing to the importance of non-acroporan massive species of corals in cryptic habitats as prey for A. planci. The null model-based electivity index Z introduced here directly measured the deviation from random feeding, while two common indices (Ivlev's and Vanderploeg and Scavia's) only partially reflected such deviations (hence, prey selection cannot be accurately demonstrated by these). Electivity values (Z) for poritid species and Acropora palifera, the most common Acropora species in the study site, were significantly negative, indicating apparent avoidance of them by Acanthaster. Our results indicate that accessibility to different coral species and the choice/avoidance of certain species are the important elements of feeding in non-outbreaking populations of Acanthaster inhabiting spatially variable reef environments. A similar consideration may apply to the feeding patterns of other corallivores that possess superior/inferior mobility to Acanthaster. The present study emphasizes the merit of testing the observed patterns, using null models for a rigorous assessment of feeding preferences.