Diel timing of planulation and larval settlement in the coral Isopora cuneata (Scleractinia: Acroporidae)

The prevailing environmental conditions when reproductive products are released in the water column can have a profound influence on the biology and ecology of marine organisms. In reef-building corals, brooding species that release azooxanthellate larvae are expected to release their larvae before sunrise, similar to species releasing zooxanthellate larvae as established in previous studies. This study investigated the diel timing of release of asymbiotic larvae by Isopora cuneata in northwestern Philippines during 2 years through ex situ observations on gravid colonies collected from the reef. Planulation mainly occurred after sunset until midnight, with grand mean hour of release at 19:35 h (95% confidence limit:18:44-21:02 h) in April 2010 and 19:41 h (18:10-20:59 h) in March 2011. This pattern of release timing is distinct from what has been previously reported for brooding scleractinian coral species with zooxanthellate larvae (i.e., release close to sunrise). The release coincides with the low tidal stand, which may provide opportunity for the prompt settlement of newly released, buoyant larvae to shallow reef areas where adult colonies are generally distributed. The larvae were able to settle with 20 +/- 25% success within a day of release, though a distinct settlement peak at 3 days post-release (70 +/- 25%). This study provides new information on the early life strategies of the reef-building coral Isopora cuneata. The unique pattern of larval release time and the settlement behavior may influence the population dynamics and success of the species through space and time.