Sometimes good fish go bad. But it's not their fault.

In a three-year effort to understand the effects of known stressors such as overfishing and nutrient pollution on coral reefs, scientists made a totally unexpected finding: A normally healthy interaction between fish and coral had turned deadly.

In typical conditions, parrotfish--like many other species--are essential to the health of coral reefs, nibbling at them to remove algae while causing no permanent damage. However, a new study conducted by UC Santa Barbara field ecologist Deron Burkepile and colleagues in the Florida Keys found that 62 percent of corals weakened by nitrogen and phosphorus pollution died when parrotfish bit them. The results appear in the journal Nature Communications.

"Normally benign predation by the parrotfish turned into coral murder," said Burkepile, an associate professor in UCSB's Department of Ecology, Evolution, and Marine Biology. "But it's not the parrotfishes; they're like the reef janitors, keeping it clean. Those extra nutrients--nutrient pollution--turn parrotfishes into an actual source of mortality by facilitating pathogens in the wounds left by their bites. Excess nutrients turn a coral accomplice into a coral killer."