The crown-of-thorns seastar species complex: knowledge on the biology and ecology of five corallivorous Acanthaster species

Coral-eating crown-of-thorns seastars (CoTS, Acanthaster spp.) are major contributors to the coral reef crises across the Indo-Pacific region. Until recently, CoTS throughout the Indo-Pacific were regarded to be a single species, Acanthaster planci. However, genetic and morphological analyses demonstrated that there are at least four distinct species: Acanthaster benziei in the Red Sea, Acanthaster mauritiensis and A. planci in the Indian Ocean, and Acanthaster cf. solaris in the western Pacific. Acanthaster cf. ellisii in the eastern Pacific needs more taxonomic attention. Here, we review the biological knowledge for each species adapting a pragmatic geographical species definition and using a systematic literature review complemented with more focused searches for individual species. The vast majority of CoTS research (88%) was conducted on A. cf. solaris, with much of this research undertaken on the Great Barrier Reef or in Japan. Many studies of A. cf. solaris are focused on monitoring or documenting incidences of outbreaks, though there is a solid base of knowledge on larval, juvenile and adult ecology derived from field and laboratory experiments. By contrast, most of the published studies on the four remaining species simply document cases of population outbreaks. The major taxonomic bias in CoTS research constitutes a significant limitation for understanding and managing these species for two reasons. First, even for A. cf. solaris, which is the most studied species, limited fundamental knowledge of their biology and ecology constrains understanding of the drivers of outbreaks and hinders corresponding management actions for prevention and control of these events. Second, understanding and management of other species are predicated on the assumption that all CoTS species have similar biology and behaviour, an unsatisfying assumption for ecosystem management.