The Great Barrier Reef off the north-east coast of Australia is the largest living organism visible from space. It stretches for 1,430 miles and is made up of over 3,000 reefs of coral colonies, many of which have synchronised for their best shot at reproduction.
Spawning occurs when corals release sperm and eggs into the water column. Eggs float upwards to the surface of the water and sperm swim to find and fertilise them. Once fertilised the eggs develop into larvae before settling down to the seabed to start a colony of their own.
The phenomenon happens in springtime on the Great Barrier Reef, when water temperatures have risen enough to stimulate the sex cells to mature. In Western Australia the coral spawn in autumn, with some northern reefs reproducing in both seasons.
Spawning only takes place after the full moon, when tidal conditions are at their calmest allowing the eggs and sperm to float freely rather than just wash up on shore. The cover of night also helps reduce the chances of eggs being eaten by daytime predators but ultimately the mass spawning is an example of safety in numbers: by releasing all of their eggs at the same time, coral overwhelm their predators who can only eat a limited amount.
This synchronised effort is particularly impressive for so-called primitive organisms without eyes. Investigating Australian corals, Dr Oren Levy, Dr Bill Leggat and Prof Ove Hoegh-Guldberg discovered they have a sensitivity to blue light which explains how they respond to the lunar cycle.
Analysis of the mass spawning is a fairly recent field because, despite it occurring for thousands of years, it was only recorded for the first time in 1981. Now divers eagerly attend the annual event, describing it as an ‘underwater snowstorm’ where clouds of eggs and sperm drift through the water. The sex cells come from the corals’ guts, they effectively spit out a plume of thousands of pellets mostly in hues of red, yellow and orange.
It all culminates on the surface of the ocean in pink-tinged slicks, which can measure several miles long and are even visible in satellite images. No wonder they call it the greatest sex show on Earth!