By: Dr Sean Porter
On a hot and humid moonlit February evening, during the month of love, corals were caught in the act – scientists were able to document corals spawning in situ at Sodwana Bay for the very first time! Millions of sex cells, comprising eggs and sperm, were simultaneously released into the water by corals hoping to engender the next generation. This event occurs a few nights a year when conditions are just right, and our scientists were able to catch a rare and unique underwater glimpse of this phenomenon.
As the full moon began to rise and its reflection sparkled over the ocean, the intrepid scientists geared up and rolled overboard into the darkness below. Many years of research soon paid off, as it was not long before the first signs of coral spawn were spotted: egg-and-sperm bundles floating in the water column at first, and then bundles being expelled from the polyps of different coral colonies. On this particular night, as the stars seemed to align in the milky way above, it was ironic that the hard coral Galaxea was recorded spawning, it’s thousands of egg bundles reflecting in the torch light, mirroring the stars above. Other corals such as Acropora, were also broody and in the process of releasing eggs and sperm. Witnessing and recording corals spawning underwater for the first time was an exciting way of celebrating the Coral Biodiversity and Ecology team’s 30th anniversary.
The coral reefs of Sodwana Bay are unique in being the southern-most coral reefs on the African continent. The southward flow of tropical water bathes these reefs in warm clear waters, allowing corals to thrive at this high latitude. These factors have created excellent conditions for luxuriant growth of over 100 different species of corals on the ancient submarine fossilized dunes. When these reefs were discovered in the 1970s, it was thought that they were too far south and marginal to allow for sexual reproduction of corals and it was postulated that the coral communities were dependant on larvae floating southward from Mozambique. Thirty years of research on these reefs by ORI has, however, shown that corals at Sodwana Bay do indeed spawn and exist as self-sustaining populations that are supplemented by larvae from further north. In more tropical regions, such as on the Great Barrier Reef, coral spawning events can be so massive that the slicks of eggs and sperm on the ocean surface can be seen from space!