Settlement of crown-of-thorns starfish: role of bacteria on surfaces of coralline algae and a hypothesis for deepwater recruitment

Abstract

Settlement trials conducted with larvae of Acanthaster planci revealed a moderate degree of substratum specificity. Highest rates of settlement and metamorphosis occurred on coral rubble and the crustose coralline alga Lithothamnium pseudosorum, but rates were more variable on the coralline. Treatment of highly inductive shards of L. pseudosorum with antibiotics reduced their inductive activity to low levels, suggesting that induction of settlement and metamorphosis of A. planci by L. pseudosorum may be mediated by epiphytic bacteria. Larvae always settled on sections of thallus having high densities of bacteria, but never on adjacent areas where epiphytic bacteria were sparse. The inductive stimulus is likely to be chemical. The spatial distribution of coral rubble and L. pseudosorum on and around Great Barrier Reef midshelf reefs, the location of hydrodynamic retention cells around reefs, and the pattern of outbreaks on the GBR, suggest that mass settlements of A. planci are more likely to occur in deep than in shallow water. This would explain the paradox that outbreaks of A. planci on the GBR are not heralded by increases in abundances of juveniles in shallow water, but are first observed as adult starfish ascending from deepwater. -from Authors