Ghost reef

Diving into the water, it is as if we are surrounded by ghosts. Massive coral colonies surround us, but they are all bone-white. These are not actually ghost corals, they are "bleached" corals.

Tom inspects a recently bleached coral on Dongsha.

Coral reefs are typically found in the deserts of the ocean - regions where there are few sources of nutrients to the surface waters where the corals live. The reefs that corals build are like oases in the desert, but corals can only do this with the help of symbionts. Even though corals themselves are animals, within their cells they host photosynthetic algae. Everyone wins - the symbiotic algae provide energy to the coral from the products of their photosynthesis, and the algae receive shelter within the coral and benefit from coral waste products.

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