Annual coral spawn enthralls divers

It’s not quite a once-in-a-blue-moon event, but the annual coral spawning remains one of the ocean’s most elusive sights. 

For much of the year, the coral in Cayman’s waters is, to the untrained eye, barely distinguishable from rock.  

But for 15 minutes in September, just after the full moon, it “bursts into effervescent life.” 

The timing of the synchronized spawning is connected to water temperature and to the lunar cycle, though the exact triggers are unknown.  

Scuba divers at Ocean Frontiers in East End, with the help of British marine biologist and photographer Alex Mustard, have honed to a fine art the science of predicting when the “show” will take place. 

At around 9 p.m. on Sept. 24, divers from the East End club will descend to the ocean floor, kneel in front of the reef, and wait. 

They will be waiting for the moment the corals will erupt, simultaneously spraying their bundles of spawn into the ocean, leaving the water cloudy and the surface slick with thick pools of pinkish gunk. 

“It is like diving in a blizzard, with the tiny bundles drifting slowly to the surface like snowflakes in reverse,” says Mr. Mustard, who has photographed the phenomenon on several occasions. 

This is the process by which corals, which despite their rock-like appearance are living animals, sexually reproduce – sperm and egg mixing in the water column and then scattered by the current.  

Some eggs will be eaten by fish, some will wash ashore and die, but a small few will be successfully fertilized, develop into coral larvae and settle on the right surface, at the right depth to begin the slow process of growing into coral. 

It’s a “scattergun” strategy with a “one in a million” success rate, according to Steve Broadbelt, owner of Ocean Frontiers. 

“A certain amount will land in environments that it can bind to, either rock, existing reef structure or metal, for example a shipwreck. If the conditions are right, it then starts to grow,” he said. 

Growth rates vary depending on the species, but within a few years, a microscopic egg can grow into a golf-ball sized piece of coral.  

The large heads of brain coral and star coral that form some of the most spectacular scenery on a reef can take hundreds of years to grow to the sizes seen today.  

For divers, the mass spawning is a spectacle in itself. Different species of coral release their spawn in slightly different ways at slightly different times. 

“The mountainous star corals are the real crowd pleasers,” Mr. Broadbelt said. “That is where you get the biggest visual impact, so there is a real wow factor there.” 

The star corals release all their bundles within a few seconds, like a geyser erupting. 

“It is quite spectacular to watch, and for anyone interested in marine science or in photography, it is particularly interesting,” said Mr. Broadbelt. 

This year, he said, there may be a split-spawn, with some species having already spawned following the August full moon. A much larger spawning event is expected this month. 

Estimates of when the event will take place are not precise, but divers plan to be in the water to witness the spectacle on Sept. 24, 25 and 26. It is hoped a video camera will enable a live web broadcast of the spawning. 

The spawning happens across all three of the Cayman Islands.  

Tim Austin, deputy director at the Cayman Islands Department of Environment, said Ocean Frontiers has done the most research on the phenomenon in Grand Cayman’s waters. 

For much of the year, he said, it was easy to forget that corals are animals. 

“Corals are basically colonial animals joined together by one big skin. They are living animals and they have sexual and asexual reproduction. 

“Because they can’t move to find a mate, they have to have this synchronized release of vast amounts of spawn. When they release so much at once, the predators, such as fish that gorge themselves on the released eggs, are overwhelmed, and some have the chance to survive and become new coral reefs.” 

In the Florida Keys and elsewhere, research is ongoing to test the success rate of harvesting the coral spawn and using it to grow new reefs to replace or regenerate corals damaged by disease. 

Coral cover on Cayman’s reefs has reduced from 80 percent in the 1970s to only 12 percent in the early 2000s. 

So far, no one has attempted to harvest coral spawn in the Cayman Islands, though the Department of Environment and Central Caribbean Marine Institute are involved with an ongoing project in Little Cayman to reproduce staghorn coral using a different technique, Mr. Austin said. Researchers grow coral in a nursery environment and then break off pieces to place at points on the reef where existing coral is depleted. Over time, the coral pieces replicate and grow. 

Mr. Austin added, “It is quite a recent technique, it is never going to reverse or fix the problems with the reef, but it can make a difference.” 


Boulder star coral spawning at night at a dive site in the East End, Grand Cayman. This coral has almost completed spawning and the bundles are floating away in the water column. – PHOTO: ALEX MUSTARD