Scientists at the Georgia Institute of Technology said these "mutualistic" inch-long fish called gobies respond to the chemical signals from the coral in a matter of minutes.
"These little fish would come out and mow the seaweed off so it didn't touch the coral," biology Profess Mark Hay said. "This takes place very rapidly, which means it must be very important to both the coral and the fish. The coral releases a chemical and the fish respond right away."
Gobies spend their entire lives in the crevices of specific corals, receiving protection from their own predators while removing threats to the corals, the researchers said.
This symbiotic relationship between the fish and the coral in which they live is the first known example of one species chemically signaling another species to remove competitors, a university release said.
The researchers studied Acropora nasuta, a species of coral important to reef ecosystems because it grows rapidly and provides much of the structure for reefs.
"This species of coral is recruiting inch-long bodyguards," Hay said. "There is a careful and nuanced dance of the odors that makes all this happen. The fish have evolved to cue on the odor released into the water by the coral, and they very quickly take care of the problem."