Sex on the beach: The annual hard coral spawning event

Every year a wonderful event occurs during the summer on CNMI’s nearshore coral reefs….the corals spawn, releasing tiny gametes into the water column that will some day become the next coral community we swim with. There are two key factors that determine if and when corals will spawn: sunlight and moon phase. Many corals begin their reproductive cycle during the spring months when the sun makes its closest approach to the CNMI, providing ample energy (solar radiation) for the creation and storage of coral eggs and sperm (known as gametes). If things go well and the corals are not stressed from pollution, a passing typhoon, or other disturbances during the spring and early summer, these gametes will fully develop. Now the tricky part…because the corals can’t get up off the reef and find an appropriate partner to mate with they had to develop alternative forms of communication. Through time corals have evolved to communicate with their neighbors by releasing chemical signals when the time is right. These signals are not sound waves containing Marvin Gaye’s inspirational notes, rather, they are termed pheromones declaring that unfertilized gametes are in the water column and need to be fertilized! All of this is triggered by the July full moon in the CNMI. Quite predictable, scientists have documented this coral spawn to regularly occur 2 – 7 days following the July full moon, shortly after sunset but prior to moon rise. Why this time and place? While researchers have hypothesized many reasons for this exact time frame, I’ll share the explanations I like best. First, the full moon is associated with the highest tides and thus, the greatest water movement around our islands. More water movement increases the chances of a encountering a friendly neighbor. Second, why after sunset but before moonrise? This exact time frame is when the tide is highest yet the light (both sun and moon) is lowest, reducing the chance of predators feeding upon the floating coral larvae. Once fertilized, coral larvae typically develop and settle upon the reefs within three weeks, and begin to grow in to the future generation.

Hopefully by now your interest has been sparked your ready to witness this event for yourself. The July full moon is on the 18 and so your best chances of observing the main event will be between 2 to 7 days following the full moon, or the 20th to the 25th. Not all coral colonies spawn on the same night so any of these evenings should provide some excitement, however each year there is usually one or two nights when pink eggs fill the water column completely, and my best guess would be the 22nd and 23rd. Just find a calm spot to go over the reef and stay focused upon the 10 ft. contour with your flashlight in hand. Be patient because last year the action didn’t start until 9 PM…and enjoy! For further information you can contact the Division of Environmental Quality at 664-8500.