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External chemical signals provide a mechanism for broadcast-spawning scleractinian corals to recognise suitable substrata for larval settlement and metamorphosis. These morphogens can be extracted from crustose coralline algae (CCA) and the skeletons of some coral species, however the precise origin of the chemical inducers has not yet been conclusively demonstrated. Microorganisms have been reported to induce metamorphosis in various species of echinoderms, molluscs, polychaetes and cnidarians. We report that Strain A3, a species of Pseudoalteromonas isolated from the CCA Hydrolithon onkodes (Heydrich), was able to induce significant levels (up to 51.5%±5.8 SE) of metamorphosis of Acropora willisae, Veron & Wallace 1984 and A. millepora (Ehrenberg, 1834) larvae in laboratory assays. This experiment was repeated daily over 4 d, and the spat developed normally into juvenile polyps in flow …