Corals under temperature stress blanch, expelling symbiotic algae that hide the white skeletons below them. But this coral bleaching is not the only phenomena that renders the tiny creatures ashen and lifeless: So-called white syndrome spreads across coral in the South Pacific in the wake of warming events. The two catastrophes can be distinguished by the ways they leave their palls of pale death. "White syndrome moves across in a band," says marine biologist John Bruno of the University of North Carolina at Chapel Hill, as if a pathogen is moving from neighbor to neighbor. On the other hand, "bleaching is a whole-colony phenomena." Now, a longitudinal study designed to assess the health of coral in Australia's Great Barrier Reef has revealed that thriving clusters of coral are more susceptible to the mysterious white syndrome than their less densely packed peers.

Bruno and his colleagues surveyed 48 colonies spread over 1,500 kilometers (932 miles) of the Great Barrier Reef every year for six years. The team measured general health and looked specifically for outbreaks of white syndrome. The cause of this affliction is unknown, but by comparing the health data they gathered with satellite measurements of ocean temperatures, the scientists were able to confirm that the disease tends to follow summers with higher than usual sea surface temperatures. They also discovered that only thriving reefs—where tiny coral polyps cover 50 percent or more of the ocean's bottom—suffered from the syndrome, according to their paper in PLoS Biology. "The irony is that the healthiest reefs are the ones where the disease outbreaks are occurring," Bruno says.

Because scientists do not know what causes white syndrome, it is impossible to say why denser reefs suffer more. But it is possible that, just like in human cities, proximity gives a pathogen a better chance of spreading. Corals compete with their neighbors, Bruno says, attacking them at night with special tendrils. "It could plausibly open up lesions that allow a bacteria to colonize the coral tissue," he says.

Determining the exact cause of white syndrome, which was first observed only about a decade ago, will require parsing the hundreds of bacteria that thrive in corals' mucus secretions. One thing is clear, however: rapidly increasing temperatures are subjecting the tiny animals to undue stress. "It is faster than they can acclimate," Bruno says. "Coral reefs are an ephemeral feature of the ocean. The concern is that we are going to lose the habitat that they create when they are super-healthy."