Though they look like rocks, corals are actually living organisms. Or, more accurately, communities of organisms. Each coral lives in symbiosis with the algae that resides inside it. But according to biologist Andréa Grottoli, corals are more complicated than this, and the relationship is actually more of a ménage à trois. There’s the coral, the algae, and the coral’s microbiome. “I started thinking about the coral as a three-way symbiosis,” says Grottoli, who studies coral at Ohio State University.