Optimising conditions for <i>in vitro</i> fertilization success of <i>Acropora tenuis</i>, <i>A. millepora</i> and <i>Favites colemani</i> corals in northwestern Philippines

Abstract

Fertilization is a critical life history event and important to population dynamics. Successful fertilization relies on encounters of compatible and viable sperm and eggs. Scleractinian corals that spawn their gametes directly into the water column may experience limitations from sperm dilution and delays in initial sperm-egg encounters that can impact successful fertilization. Notably, the same issues can be experienced when conducting ex situ larval culture. Therefore, experiments were completed using spawned eggs and sperm of the reef corals Acropora tenuis, A. millepora and Favites colemani to determine the optimal ranges of sperm concentrations (<103 to 107 mL−1) and temporal response patterns when crossing of eggs and sperm was delayed (30 min to 10 h). The results showed that sperm concentrations below 103 mL−1 yielded low rates of fertilization for all three species (<30%), but that sperm concentrations at 104 mL−1 resulted in higher rates of fertilization for the Acropora spp. (>65%) and > 90% fertilization rates for F. colemani. Sperm concentrations of 105–107 mL−1 resulted in the highest observed fertilization rates for all three species (>90%). High fertilization rates (>80%) occurred when eggs and sperm were combined after 30 min and up to 2 h after spawning for F. colemani and up to 4 h for A. tenuis and A. millepora gametes. Fertilization rates were significantly reduced (<30%) after a delay of 4 h for F. colemani and after 6 h for the two Acropora species. No fertilization occurred after a 10-h delay in combining gametes for all the tested coral species. Therefore, optimal fertilization success for these coral species rely on sperm densities of ≥104–106 mL−1 and gametes should be combined within two hours after spawning for optimising larval culture.