While conducting research in the lagoon on Mo’orea, French Polynesia, scientists noticed the fish droppings on the coral. They could easily have overlooked what is a routine occurrence on the reef, but were curious what effect this might have when combined with rising ocean temperatures.

“If corals are healthy and they’re not stressed out, then that poop is not a bad thing,” says Deron Burkepile, a professor in the ecology, evolution, and marine biology department at the University of California, Santa Barbara. It’s potentially even beneficial, providing important nutrients to the reef denizens.

“But when corals are stressed out, like during a marine heatwave,” he says, “this natural, relatively benign interaction becomes something pretty malignant.”