Tropical Coral Reefs Fail at Sex After Bleaching Events

Most are aware that bleaching events in tropical corals can cause short-term impacts and devastate reefs. Now, though, scientists have found that there are longer-term effects. They've discovered that bleaching events due to higher temperatures cause corals to have a slower rate of reproduction.

Most corals reproduce by releasing sperm and eggs into the ocean during brief annual spawning events. The chance of a sperm finding and fertilizing an egg, though, depends on corals spawning in close proximity and spawning at the same time.

In order to better understand coral spawning behavior, scientists examined corals in the Caribbean that had experienced a bleaching event. Bleaching events occur when ocean waters warm and the corals eject their zooxanthellae, which are beneficial photosynthesizes that allow the corals to convert sunlight to energy. Because zooxanthellae give corals their vibrant hues, ejecting them means that the corals turn white. In the end, the scientists found that species living in shallow water experienced near total reproductive failure, while the species in deeper water was about half as likely to spawn.

"The remarkable finding from this study was that reproduction in spawning persisted for three additional years, long after the corals had regained their symbiotic partners and regained their normal appearance," said Don Levitan, one of the researchers, in a news release. "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."

The findings reveal that corals are in far more danger than expected. While corals recover from bleaching events, it seems that they have a hard time reproducing afterward, which means that rebuilding reefs after a bleaching event could take time.

Currently, the scientists plan to examine the quality of gamete production and determine whether corals that have bleached once are more or less likely to bleach again. This could allow researchers to better assess the health of reefs and, in consequence, better protect them.

The findings are published in the journal Marine Ecology Progress.