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 crown-of-thorns starfish Acanthaster planci revealed a moderate degree of substratum specificity. Highest rates of settlement and metamorphosis occurred on coral rubble and the crustose coralline alga (CCA) Lithothamnlum pseudosorum, but rates were more vanable on the coralline. Interpretation of settlement on the rubble is difficult because rubble always supported some CCA. Settlement was significantly lower on other CCA (Porolithon onkodes and Neogoniolithon foslei), non-calcareous crustose red algae (Peyssonellia sp.), and fouled ceramic tdes. These results were consistent irrespective of whether larvae were offered a choice of substrata or not. When larvae were separated from L. pseudosorum by mesh, settlement was highly variable but sometimes occurred at high rates, suggesting that contact with the algae is not obligatory for induction. Larvae were not induced to settle by GABA (y-amino butyric acid), elevated K+ concentrations, or coral blocks fouled for 9 d, and settlement rates were virtually zero in controls without a known added inducer. Treatment of highly inductive shards of L. pseudosorum with antibiotics reduced their Inductive activlty to low levels, suggeslng that induction of settlement and metamorphosis of A. planci by L. pseudosorum may be mediated by epiphytic bacteria. Other results were consistent with the notion of bacteria-mediated induction. The inductive ability of different regions on individual L. pseudosorum plants varied greatly, as did densities of bacteria on the plant surface. 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 since it was inactivated by boiling or autoclaving, and may be a relatively large molecule since it was not detected in water, ethanol or chloroform extracts of lnductlve algae or coral rubble, and was retained by dialysis tubing of pore size 10000 Daltons. The spatlal distribution of coral rubble and L. pseudosorum on and around GBR 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 Prehminary deep water videotransects off Davies Reef showed that rubble and CCA were abundant m deep water (30 to 65 m) adjacent to the area where aggregations of adult starfish were first seen moving up from deep water, but the substratum in deep water off other sections of the reef was sand.