Metamorphosis of broadcast spawning corals in response to bacteria isolated from crustose algae

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, how- ever the precise origin of the chemical inducers has not yet been conclusively demonstrated. Micro- organisms 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-through aquaria. Approximately the same number of larvae underwent partial metamorphosis, forming flattened discs that were not attached to the substrata. Larvae underwent full settlement, attachment and metamorphosis only in the presence of Pseudo- alteromonas A3 plus inert chips of the coral skeleton Porites sp., indicating that the calcareous matrix may play a role in the synthesis of inducers from Pseudoalteromonas Strain A3. This discovery pro- vides evidence for a widening range of morphogenic sources and demonstrates the role that micro- organisms may play in fine-scale coral recruitment. In addition, the synthesis of chemical inducers by Pseudoalteromonas Strain A3 may have biotechnological applications for reef re-seeding.

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