Significant drop of fertilization of Acropora corals in 1999: An after‐effect of heavy coral bleaching?

In June 1999, after devastating coral bleaching in 1998, laboratory fertilization of Acropora nasuta , one of the most abundant reef‐building corals in Okinawa, Japan, decreased significantly from usual rates (>94%) to an average of 42% at a sperm concentration of 10 7 ml −1 . Similar decreases were observed in four other mass‐spawning acroporid corals. We also found a decrease in sperm motility in the laboratory. A series of experiments to determine the effects of sperm concentration on fertilization rates revealed that sperm of 10 7 ml −1 was needed to obtain a rate >80%. Sperm concentration in surface seawater during mass spawning was highest within 1.0 h of spawning but decreased sharply thereafter. These results suggest that gamete dilution plays an important role in limiting the fertilization of coral eggs in the sea. As successful fertilization appears to have been much lower in 1999, we suspect that production of new coral recruits was also reduced greatly. Current and future sea‐temperature increases thus pose a severe potential threat to coral reefs by increasing the frequency of coral bleaching and consequently leading to further declines of coral recruitment and hence, reef corals.