The influence of seawater temperature on the timing of coral spawning

We report the influence of seawater temperature on the timing of coral spawning for three time-scales; months, days, and hours for four scleractinian coral species (Acropora hyacinthus, Acropora gemmifera, Dipsastraea speciosa, and Favites pentagona). We monitored the timing of in situ coral spawning in Lyudao (Green Island), Taiwan from 2010 to 2020, at two sites (Gonggan and Dabaisha) that had an average seawater temperature difference of 0.1-2.1 degrees C due to upwelling. We also conducted a reciprocal transplantation experiment between the two sites using A. hyacinthus in March 2015 (1-2 months before the expected spawning). Results indicated consistent patterns among the four coral species. Annual spawning always occurred between April and June, when seawater temperature was rapidly rising and reached the monthly average between similar to 26-29 degrees C. Spawning days (days after full moon) were negatively correlated with average seawater temperatures over the 1 week prior to spawning, as were spawning hours with average seawater temperatures between sunset and spawning time. Spawning tended to occur on earlier days and hours at the warmer site (Dabaisha). Transplants of A. hyacinthus to the warmer site spawned in May, compared to those at the cooler site that spawned in June. Our results demonstrated the pervasive influence of temperature on the timing of coral spawning at three relevant time-scales. This can probably be attributed to temperature influencing the time course of various reproductive processes that operate on different time-scales; e.g., gametogenesis (months), the final gamete maturation (days), and the preparation of spawning (hours).