The Mystery of Star Coral Spawning

Article written by Steve Rosenberg and Greg Bassett

Photography by Steve Rosenberg

Video by Greg Bassett

We were sitting on a dive boat off the south shore of Grand Cayman, in the middle of the night, already geared up and ready to witness — and hopefully capture images of — a spectacular display of reef regeneration. Each year, usually in August or September, many of the corals in Cayman’s reefs spawn simultaneously over a three-night period, releasing bundles of eggs and sperm at the same time so that they can cross-fertilize and scatter in the currents. If this year was to be the same as in years past, for approximately 45 minutes to an hour and a half on three consecutive nights, many corals, including several species of star corals and one species of brain coral, would release tiny packets containing millions of sperm and eggs. The predicted dates are calculated roughly five to seven days after the eighth full moon of the year, usually in August, and a couple of hours after sunset. Generally this event will take place between August 30 and September 26. This year the full moon was on August 29, making September 3, 4 and 5 the nights when everything was going to happen.

Earlier in the evening, we had been treated to a terrific presentation by Steve Broadbelt, one of the owners of Compass Point and Ocean Frontiers, a full-service dive resort on the East End of Grand Cayman. He pointed out that mass spawning is necessary for coral fertilization to occur. The release of millions of small, peach-colored sacs, which contain either eggs or sperm, along with a critical synchronization of the timing allowing coral species to spawn, is nature’s design to sustain coral colonies. The factors required for coral to release millions of sacs to achieve fertilization include the full moon, water temperature, sunset and a chemical that acts as a trigger for each coral colony to spawn individually within 15 minutes of another colony’s release to achieve reproductive success.

Steve explained that the mountainous star corals and the lobed star corals put on the most spectacular show. “These are the most visible, and it happens all at once,” he said. Our goal for the evening was relatively simple. We just had to be in the water at the right time, fortuitously select the right corals, and hope that our skills would allow us to record and document the event. Steve selected a relatively shallow dive on the south side of the island with a high density of these corals, called Playing Field Reef.

On this particular evening, we anticipated focusing on these two species of star corals, which are both male and female, releasing some bundles containing eggs and some containing sperm. Some individual star corals, such as the giant star corals, are either male or female. The surface of these corals is covered with little round dots or circles, which are polyps about a quarter of an inch or smaller across. Each one of the polyps releases a ‘bundle,’ and each of these is full of eggs or sperm, which suddenly appear in the middle of the polyps to signal that the process is beginning. The bundles then hover on the surface of the coral for a few moments while they soak up salt water, making them buoyant, and then they slowly float up to the surface in unison. We would observe that when the bundles drift to the surface, it’s like a reverse snowstorm. When they get to the surface, the bundles break apart, allowing the eggs and sperm to mix.

The whole reef doesn’t spawn at the exact same moment. Different sections of a single coral head may spawn at different times. These corals usually spawn on two consecutive nights, giving them a greater chance of surviving. On a single coral head, you may have a 15-minute window of action after the bundles appear in the center of the polyps or it may be imminent. You may even see one section spawning on one night, with other sections of the same coral head releasing bundles on the next night.

The presence of brittle stars out on the surface of the coral is usually one of the best predictors that a coral is getting ready to release its bundles. As Steve pointed out, the spawning may not take place for 20 minutes, but it will happen on that coral. Fish eat a lot of the spawn after the bundles are released; only a very small percentage of the corals will actually make it back to the bottom of the ocean, graft to the sea floor and become a new coral.

Steve told us that the September 3 would probably feature the soft corals, which do not photograph very well, so we should concentrate on the 4th and 5th for the star corals, although predicting the due date of the spawning isn’t an exact science. The timing of the synchronized spawning is connected to water temperature and the lunar cycles, but the exact triggers are unknown. Our plan was to buddy up with him and take his direction as to which coral he thought might present an opportunity to photograph and get video of the event. We were a bit anxious about being in the right place at the wrong time and vice versa. I must also admit that I had trouble wrapping my mind around the fact that the exact time when this event was to take place could be so accurately predicted. We had enough air for approximately one hour underwater, and a lot of things had to magically coincide for our dive to be successful. After making a final check of our camera and camcorder, we took a giant stride into the inky water and slowly descended toward our objective.

During the dive, Steve excitedly gestured toward several star corals. Amazingly, the predicted day and time for the spawning was literally within 10 minutes of the actual time. After shaking off our initial amazement, Greg and I were able to find and document several corals that demonstrated this marvel of nature. We found it absolutely uncanny that Steve was not only able to predict the nights, but also the timing of the spawning so that it took place over the course of a 25-minute window during our dive. While coral spawning dates and the volume can vary, anyone who has witnessed this coral reef regeneration phenomenon will retain a memory of the event forever. Observing up-close the releases of bundles of eggs and sperm by the corals and the slow rise in unison of these tiny sacs slowly drifting upward through the water column should be on every diver’s bucket list. For us, it was added to the bucket list of things that we had to experience again.

To see more from Steve Rosenberg, check out his website.

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