In a new study, researchers have discovered that corals need to be within a few metres apart to successfully reproduce. 

“In what came as a surprise, we saw that corals needed to be within 10 metres of one another, and preferably closer than that for fertilisation to take place,” Professor Peter Mumby, from The University of Queensland, said.

This proximity requirement is much closer than previously thought.

The study

In the study, containers were placed above 26 coral colonies on a reef in Palau, Micronesia during the spawning. After some of the coral eggs, the containers rose to the surface, then drifted with the tide. Sperm were able to enter the containers to fertilise the eggs.

“After an hour of drifting, the proportion of fertilised eggs was noted for each type of coral along with the distance to similar established corals,” said Mumby. 

It was found that fertilisation averaged 30 percent when corals were very close, but was reduced to less than 10 percent at a separation of 10 metres and dropped to virtually zero by 20 metres.

The response

Co-author Dr Christopher Doropoulos of the CSIRO, Australia’s national science agency, said coral reproduction was fundamental to population resilience and evolution. 

“In the future we may need to help corals continue this key part of their lives. […] Ideally, the density of corals would be monitored at important locations and restoration carried out to return the density back to the levels required for successful reproduction,” he said.

Mumby has been actively involved in repairing damaged coral reefs: “Our work over the past five years on the Great Barrier Reef through the Reef Restoration and Adaptation Program is also helping to define these critical thresholds to help restoration practitioners set targets for density to help maintain coral populations.”