Once a year, during a period of about 15 minutes, many hard coral colonies of the same species will spawn synchronously in the same area. Corals cannot move, that’s why reproductive success depends on the accuracy in spawning time of all the colonies.
Triggered by the moon cycle and the sunset light, broadcast spawning corals release millions of eggs and sperm at the same time, which are externally fertilized at the ocean´s surface where there is a greater chance of compatible gametes meeting. The fertilized eggs will develop into embryos and then become motile larvae with the ability to swim and find a suitable home. After traveling for a few days, a very small number of them will make it to the reef and metamorphose into a sessile coral polyp that will begin reproducing asexually to become a coral colony. After a few years, this colony will grow to a mature size and be able to spawn, coming back full-circle.
Not much is known about how corals synchronize themselves to simultaneously release the gametes, but it is widely thought that the moon cycle has a very important role on the event. Corals are able to detect the moonlight, and even sense which phase it is in.
Every year, a few days after the full moon, many colonies of scleractinian corals spawn on Koh Tao. For us to be able to predict when this magic event will occur, we just need to count a few days after the night of the full moon, usually five. This year though, the complete full moon phase occurred the morning of the 22nd of April, meaning that the night of the 21st, the moon was almost full, and the following one, it was starting to decrease. Since broadcaster corals always spawn after sunset, it was hard for us to determine which night would the corals interpret as full moon. Many bets were on amongst the team members. Will they spawn the 26th or the 27th?
The answer to our doubts came as a big surprise. The night of the 26th, four and a half days after the full moon, half of the colonies of many genera spawned, like Symphylia, Goniastrea and Diploastrea. But we had a bit of a surprise, as the following night the other half of the colonies spawned, together with other genera like Galaxea, Fungia or Platygyra.
Now, more doubts come into our mind. Is nature not as precise as we think, giving always some margin of error? Were the corals as ‘confused’ as us, and half of them bet for one night and the other half for the next one? Or are corals “smarter” than that, and decided ‘not to put all their eggs in one basket’?
Unfortunately, this means that the occurrence of natural fertilization must have been lower than in previous years, as the abundance of gametes and the number of genetic individuals mixing were reduced. That’s why we are so glad that our team managed to collect plenty of egg and sperm bundles from Goniastrea colonies on the first spawning night to use in our larval culturing program, in this way helping the precious coral reef we all love.