Can Corals Toughen Up After a Warming Crisis? In 2010, record warm ocean temperatures across much of the tropics led to catastrophic coral bleaching. The devastation approached the scale of a 1998 calamity in which 16 percent of the world's reefs died. Coral bleaching occurs when unusually warm, acidic or polluted water puts such stress on the corals that they expel their symbiotic algae. The coral loses its vibrant color, turning skeleton white, and if unfavorable conditions persist, the polyp starves to death. According to research published online in the journal PLoS One, however, the bleaching of 2010 may actually have yielded evidence that the world's reefs are responding to climate change better than had been hoped. Usually the corals hardest hit by warming oceans are fast-growing branching varieties like Acropora (staghorn and elkhorn corals) and Pocillopora (cauliflower and brush corals). These corals bleach most rapidly and are usually least able to rebound. Slow-growing, massive corals like Porites (finger coral) are typically more resistant to bleaching. When researchers analyzed sites in Southeast Asia, however, they found that branching corals had not been hard-hit in Singapore and Malaysia in 2010. In fact, in Singapore only 5 percent of Acropora and 12 percent of Pocillopora colonies died. And massive corals fared worse there, in a reversal of the usual pattern. Meanwhile, in Pulau Weh, Indonesia, 94 percent of Acropora and 87 percent of Pocillopora colonies died off. Overall mortality rates across all coral species was much lower in Singapore and Malaysia, too. Only 4 to 15 percent of the colonies died there, while about 45 percent of the colonies in Indonesia died. One possible explanation for the difference in reef responses to the 2010 heat stress is that reefs in Singapore and Malaysia suffered severe bleaching in 1998 while colonies in Indonesia were not affected. The researchers therefore propose that once a coral colony is hit by warm waters, it will be more resistant to bleaching in the future. It's only a hypothesis for now, and a controversial one at that, but the results of the study do suggest that branching corals may have a fighting chance in the future. It's promising news for coral diversity and the $375 billion reef economy.