BBC Wildlife recently reported that a master diver on Australia’s Great Barrier Reef observed an unusual form of communication between fish species. A large group of pale-colored surgeonfish rapidly changed from white to black as they were interacting with cleaner wrasse fish, a species known for removing and consuming parasites, dead skin and mucus from larger fish.

Marine experts assert that the pale coloration is a deliberate signal to the cleaner wrasse that the surgeonfish pose no threat and are ready for grooming. The darkening to black is thought to provide a high-contrast background, helping the cleaner wrasse spot any parasites and dead skin on the surgeonfish’s body.

Changing coloration in fish is a well-documented mechanism used in other settings for camouflage, mating rituals and as a stress response to threats or environmental changes.