Rescuing The Reef

Justin Gilligan joins scientists on an expedition to the far northern Great Barrier Reef to witness the annual mass coral spawning spectacle and to look for ways to help this ecosystem under pressure.

- Justin Gilligan

EACH YEAR, AFTER a late spring or early summer full moon, a spectacular synchronised coral spawning occurs on the Great Barrier Reef (hereafter the GBR or the Reef). This nocturnal natural phenomenon is like an aquatic fireworks display, as red, yellow and orange bundles of sperm and eggs are released en masse into the ocean.

Most coral species are hermaphrodites – simultaneously both male and female – and typically release tightly packed bundles of sex cells, which open at the ocean’s surface allowing eggs to meet compatible sperm. During peak spawning, more than 100 species release their sex cells – their gametes – during just a few nights. This phenomenon was first discovered off Townsville in 1984 by a group of young scientists. It was a turning point in their careers and earned them a prestigious Australian Museum Eureka Award for Environmental Research in 1992. They initially documented that 32 coral species had spawned shortly after late spring full moons in 1981 and 1982 at three different locations.

Mass coral spawning is now known to be the most common reproductive mode for the Reef’s corals, and is the culmination of months of development for their sex cells. Timing of the phenomenon is linked to seawater temperature, lunar phases and factors such as the daily cycle of light and dark. In some years, when the full moon falls early in the October–November spawning season, some colonies hold off for a later lunar cycle, resulting in multiple spawning events.

By expelling eggs and sperm at the same time, coral increases opportunities for fertilisation. This mass approach also overwhelms predators: many eggs will be eaten, but at least some will make it through to be fertilised. These develop into mobile larvae that at first move in ocean currents but search out and finally settle on hard substrates in suitable locations to begin growing into new coral colonies.