If you've seen one damselfish, you've seen them all.

That may be true for people, who have a difficult time telling some damselfish species apart. But the fish themselves see it differently, according to a study in Current Biology. They can use ultraviolet facial patterns to tell one species from another.

Ulrike E. Siebeck of the University of Queensland in Australia and colleagues studied Pomacentrus amboinensis and P. moluccensis, two species of damselfish capable of seeing light at the ultraviolet end of the spectrum.

They are also highly territorial as evidenced by P. amboinensis males. For example, the males will chase off unfamiliar members of their species because they are seen as competitors. But they go easier on P. moluccensis intruders.

To people, the two species of reef fish look practically identical. But under UV light they are revealed to have distinctly different patterns in the scales around the eyes. “These are really fine, intricate patterns that we can't see at all,” Siebeck said.

The question for her and her colleagues was whether the patterns, and the ability to see them, had an effect on behaviour.

In a series of experiments in which, among other things, they placed fish inside a glass chamber equipped with UV filters, they showed that P. amboinensis used the patterns to discriminate between the two species.

The work provides support for the idea, suggested by others, that the ultraviolet part of the spectrum may be a way for some species to communicate secretly, in ways invisible to those that cannot see UV.