Heart of the Nation: coral spawning on the Great Barrier Reef
A few nights after full moon in late spring and early summer, there’s a frisson among the corals of the Great Barrier Reef.
A few nights after full moon in late spring and early summer, there’s a frisson among the corals of the Great Barrier Reef. Taking their cue from the lunar light, great swathes of them – hundreds of different colonies and species – get it on en masse, synchronously releasing little bundles of sperm and eggs into the water. “Trillions of them, like an underwater snowstorm,” says Professor Peter Harrison, who in 1981 was part of the team that documented the phenomenon on reefs off Magnetic Island – the first scientists in the world to do so. Now, his clever idea for “coral IVF” using collected spawn is one of the shining hopes for our beleaguered Great Barrier Reef.
Queensland Museum photographer Gary Cranitch captured this image after midnight on Vlassoff Reef, off Cairns; just under the surface, he fired off a powerful flash to illuminate the egg-and-sperm packets – and the bluish streaks of baitfish feeding on them – like so many stars in the night sky.
A biological mechanism prevents those packets, 2mm across, from self-fertilising as they float to the surface and form a great slick, where all the genetic mingling happens. Larvae form from fertilised eggs within 24 hours, and after a week will descend to the sea floor in search of a place to settle. If successful, each larva will develop into a young coral, a hand-span across, within two years. The natural success rate, though, is vanishingly small – and that’s where Harrison’s work, funded by the Great Barrier Reef Foundation, comes in: he’s growing millions of larvae from harvested spawn and directly “seeding” bleached reefs to kick-start their recovery.
Anyway, what was it like being engulfed in that cloud of spawn? Says Cranitch: “It was like a scene from Star Wars!”