Annual coral spawning imminent

If the predictions are accurate, divers taking the plunge after dark between 17-19 September will be treated to a rare sight.  

These are the dates when it is predicted the corals around Grand Cayman will spawn.  

Although coral reefs often look like verdant underwater gardens, the minute polyps that band together in colonies to form corals are in fact small invertebrates. Being animals, the majority of corals reproduce sexually, but as they anchored to the seabed corals cannot easily go and find themselves a mate, said Steve Broadbelt of Ocean Frontiers.  

They have therefore developed other means of reproducing: Individual coral polyps release sperm and eggs – or in some cases an egg and sperm bundled together – into the sea. With luck these will fertilise and, given the right conditions, begin new colonies. 

Until recently, scientists thought corals spawned at intervals throughout the year. However, it is now known that corals of all species synchronise their spawning, so that it all takes place once a year. The simultaneous release of sperm and eggs by all colonies of the same species is thought to maximise the chances of cross-fertilisation and may also be designed to overwhelm the appetite of predators. 

Coral spawning in the Cayman Islands has in the past taken place during the course of three consecutive nights. Each species, however, usually spawns on only one of these nights, and individual coral heads may release their eggs for only very short periods of time that night. 

“On the first night, the soft corals and gorgonians spawn. This is a more subtle event. On the second night 60 to 70 per cent of mountainous star coral will spawn and on the third night, the brain corals and the balance of the star corals spawn,” Mr. Broadbelt said. Once released, the bundles float up toward the light at the surface. If the weather is calm, one may be able to see these slicks drifting on the surface for several days before they sink back down to the sea floor where they settle and establish new colonies. 

While vast tracts of reef synchronise spawning over a few short nights, predicting which nights this will be is no mean feat.  

“We know it happens when the sea is at its warmest, but the dates vary with latitude. Other reefs on the same latitude as Cayman may be spawning on the same days, but further north it will happen a month earlier; further south it will be a month later,” Mr. Broadbelt said.  

Given the limited time frame in which coral spawns and the difficulty in predicting when this will be, it is a wonder that scuba divers – whose time underwater is often limited to an hour or less – ever get to witness this event.  

Mr. Broadbelt and marine biologist and photographer Alexander Mustard have done years of painstaking research that means interested parties in Grand Cayman have a chance to see the event.  

“Year after year we tried different dates and different nights,” Mr. Broadbelt said. “It was trial and error. Eventually we narrowed it down to a two week period. Then we just went diving every night.”  

They are able to be more precise in their predictions now: For several years they have observed corals spawning on the fifth, sixth and seventh night after the ninth full moon of that calendar year, starting about two hours after sunset. That makes it the 17-19 September, from 9pm onward this year. 

There are no iron clad guarantees, however.  

“I have a 100 percent track record [in accurately predicting the dates] over the last eight years,” said Mr. Broadbelt. “Will it happen on the same dates this year? I don’t know. I wouldn’t bet my life on it.”