Settlement cue selectivity by larvae of the destructive crown-of-thorns starfish

Abstract

Population irruptions of crown-of-thorns starfish (COTS) cause extensive degradation of coral reefs, threatening the structure and function of these important ecosystems. For population irruptions to initiate and spread, large numbers of planktonic larvae have to successfully transition into their benthic life-history stage (i.e. settlement), whereby larval behaviour and the presence of settlement cues may shape spatial patterns of recruitment and adult densities. Our results demonstrate that a wide range of coralline algae species induce COTS larvae to settle; however, the capacity to promote settlement success varied manyfold among algal species, ranging from greater than 90% in Melyvonnea cf. madagascariensis to less than 2% in Lithophyllum cf. kotschyanum and two Porolithon species at 24 h. Because many coralline algae species that promote high settlement success are prevalent in shallow reef habitats, our findings challenge the hypothesis that COTS larvae predominantly settle in deep water. Considering both larval behaviour and algal ecology, this study highlights the ecological significance of coralline algae communities in driving recruitment patterns of COTS. More specifically, the local abundance of highly inductive coralline algae (especially, Melyvonnea cf. madagascariensis) may explain some of the marked spatial heterogeneity of COTS populations and the incidence of population irruptions.

Keywords: Acanthaster; coralline algae; larval settlement; metamorphosis; population outbreaks; recruitment.

Figure 1.

Larval development through metamorphosis in crown-of-thorns starfish: (a) brachiolaria larva; (b) metamorphosing larva absorbing the larval body; (c) post-metamorphic juvenile (photographs by Ciemon Caballes and Peter Doll).

Figure 2.

(a) Settlement cue responses of crown-of-thorns starfish larvae to 15 coralline algae species at 24 and 48 h (means ± s.e., n = 12). Letters denote statistical differences among treatments, with treatment means not sharing any letter being significantly different (Benjamini–Yekutieli-adjusted p < 0.05). (b) Differences between the mean settlement rates at 24 and 48 h after experiment commencement. FSW = filtered seawater.