The UV vision may act as a secret channel of communication because it is invisible to the fishes' predators, the researchers speculate.

“We observed that certain fish had very distinctive ultraviolet markings on their faces – and we wondered what they were using them for,” says lead author, Dr Ulrike Siebeck of The Vision Centre and The University of Queensland (UQ).

In a series of carefully controlled experiments, the team exposed male Ambon damselfish to males of the same and a different species (with similar overall body coloration but different UV facial patterns) in conditions in which the UV markings could and could not be seen.

They observed that there was only a difference in territorial reaction when the UV facial patterns could be seen, demonstrating that these patterns are necessary for the fish to discriminate between their own and another species.