|Tom inspects a recently bleached coral on Dongsha.|
However, the symbiosis can go awry. Under certain conditions, typically when exposed to unusually warm temperatures, the symbiotic algae begin to produce chemicals that are toxic to the coral host. In response, the coral kicks out the symbionts. This is "coral bleaching" - the vibrant colors of coral colonies are actually caused by the symbiotic algae; once the algae are gone, the colony appears white because we are actually seeing the calcium carbonate skeleton through the translucent coral tissue. This is bad news for corals: without their symbionts, corals often starve and the colony dies. But bleaching is not always a death sentence - corals can acquire more symbionts from the seawater around them, and sometimes the coral colony fully recovers.
Sadly, we saw on Dongsha yesterday that most of the massive coral colonies are bleaching right now. They are still hanging on - with a close look at the colony surface, we could see that the coral animal is still alive. The question is whether they can hang on long enough to acquire new symbionts. With more than four weeks of our expedition remaining, we will continue to monitor these corals, and try to figure out why they bleached in the first place. Stay tuned for more - and cross your fingers that the corals here on Dongsha can recover!
- Tom DeCarlo
Joint Program in Oceanography
Woods Hole Oceanographic Institution