Bleaching harms coral fertility

Bleaching events caused by rising sea temperatures have a detrimental long-term impact on the fertility of coral, a study has found.

Bleached coral near the Panamanian coast. Credit: Raphael Ritson-Williams

Coral bleaching occurs when high water temperatures or UV light stresses the coral to the point where it loses its symbiotic algal partner, zooxanthellae, that provide corals with their colour.

"Even corals that didn't bleach aren't reproducing at the levels they should," said Professor Don Levitan, biologist at the Florida State University who led the study.

Most corals reproduce by releasing sperm and eggs into the ocean during brief annual spawning events. The chance of sperm finding and fertilising an egg depends on corals spawning in close proximity and in synchrony with each other.

In their study of reef-building corals in the Caribbean, Levitan's team found that the species living in shallower water experienced near total reproductive failure, whereas species living in deeper water were about half as likely to spawn.

"The remarkable finding from this study was that the reduction in spawning persisted for three additional years, long after the corals had regained their symbiotic partners and regained their normal appearance," Levitan said.

Levitan's team has been studying the same reef system off the coast of Panama since 1996. Since then, the corals have been exposed to two bleaching events. On average, it takes coral three to four years to recover from bleaching.

"Even if we can fix what's killing these corals, it's going to be hard for coral populations to recover, because the surviving corals might not successfully produce enough offspring to repopulate reefs," he said.

The worldwide decrease in coral abundance in combination with long-term reductions in spawning and reproduction following bleaching events put reef-building corals in a difficult situation. Eggs might be released, but never fertilised.

The coral in that region is critical to building reefs, a crucial part of local ecosystems.

In future work, Levitan said his team would like to examine the quality of gamete production and also determine if corals that have already bleached are more or less likely to bleach again