Rapid in situ settlement following spawning by Acropora corals at Ishigaki, southern Japan

The principal role of larval recruitment in reef corals is still unknown: it could be for self-recruitment or long-range dispersal. In this study, peak settlement period, which is ecologically (i.e. demographically) important, was estimated in the field by counting the number of settled corals every 3 d after a mass-spawning of Acropora corals. We found that ~90% of the total recruits settled within 8 d after spawning. The number of swimming larvae also peaked from the fifth to the seventh day after spawning and decreased rapidly on the ninth day. Although we could not determine whether the coral larvae remained around the natal reef or if larval supply from upstream regions concentrated in this study area, no multiple peaks were found in the settlement pattern of Acropora corals. This finding supports the hypothesis that even broadcast spawners adopt the strategy of self-recruitment, at least in a single generation (i.e. settlement pattern was influenced by intrinsic controls such as larval behavior). Otherwise, this short, intensive settlement pattern could be a region-specific event because the result was obtained from only a single reef during a single study year (i.e. extrinsic controls such as hydrodynamics); in this case, these areas should be preferentially designated as 'protected' because such intensive settlement would be important to the resilience of Acropora corals.