A GIANT BIRTHING
At 20:30 on 27 November 2018, lucky divers and snorkellers in the northern part of the Great Barrier Reef would have borne witness to what is possibly the greatest sex show on Earth.
This spectacular phenomenon, when corals reproduce en masse over just a few days each year, is essentially synchronised sex on a grand scale as millions of corals on more than 1,000km of reefs along the Great Barrier Reef area simultaneously release their eggs and sperm into the water in the hope they’ll combine and create new coral colonies. It’s an underwater snowstorm, a sexual soup, where clouds of pink- or red-hued egg-sperm bundles whirl and swirl in all directions, carried by the waves and currents.
Coral spawning is one of the highlights on the Great Barrier Reef calendar. Yet it was only discovered as recently as the 1980s by Professor Peter Harrison, who is now director of the Marine Ecology Research Centre at Southern Cross University, New South Wales, and his colleagues.
Back then, Harrison was a 23-year-old PhD student up in Townsville at James Cook University who was fascinated by the sexual reproduction of corals and knew that what he was seeing on the reef did not match what he was reading in the textbooks. Over a two-year period of studying the corals around Magnetic Island in North Queensland, he and his colleagues realised that they reproduced by spawning instead of brooding their larvae internally, as was originally thought, and that many different coral species were in fact spawning together at the same time, triggered by the lunar cycle and a rise in sea temperature.
“It’s one of the world’s great reproductive events. I was literally surrounded by billions of eggs and sperm, and I knew that most of those were going to develop into larva,” Harrison said.
It was a spectacular discovery, and one that amazed the world when Harrison and colleagues published a paper on the event in Science magazine in March 1984.
On the Great Barrier Reef, where Harrison was, the mass spawning typically takes place four to six nights after the first full moon in November when sea temperatures get warmer, while the timing is different for the northern hemisphere reefs.