To the human eye, it can be nearly impossible to tell some fish species apart, but these seemingly identical fish have no trouble identifying a sibling from a more distant relative.
Recent studies conducted at the University of Queensland Australia concluded that certain species, including damselfish, use ultraviolet facial patterns to distinguish between species. By studying two species of damselfish, Pomacentrus amboinensis and P. moluccensis, both of which are capable of seeing ultraviolet light, researcher Ulrike E. Siebeck was able to develop these new findings. Siebeck said, the distinct patterns in the scales around the eyes of the fish “are really fine, intricate patterns that we can't see at all."
Among her experiments, Siebeck facilitated a test where she placed fish of both species inside a glass chamber that had been set up with UV filters, and observed that P. amboinensis used the facial patterns to discriminate among the fish.
SOURCE: New York Times