Ever wish you had a secret code that you could use to communicate with a select few? Researchers have found that one little breed of fish actually has one. The Ambon damselfish (Pomacentrus amboinensis) can see detailed ultraviolet (UV) patterns on their fellow fishes—and detect the lack of these lines in other similar species, according to a study published online February 25 in Current Biology.

The findings are the first to show an animal "that is able to discriminate between fine-scale UV patterns using only their short-wavelength receptors (UV cones)," the researchers wrote in their study. These fish seem to use the UV cues to distinguish their own from other similar-looking species.

Some fish have been known to be able to see light in the UV spectrum, but their UV vision was thought to be fuzzy at best. "Researchers have been assuming for a long time that UV vision is not very good—and that it is only useful for detecting the presence and absence of UV light or objects in front of UV bright backgrounds," Ulrike Siebeck, a postdoctoral fellow at the University of Queensland in Australia and lead author of the study, said in a prepared statement.

Siebeck and her colleagues used the Ambon damselfish's natural behavior to try out their UV-detection abilities. The males, which usually live amongst a group of females, are fiercely defensive of their territory and will attack intruders of the same species—but will usually not go after those of other species, as the latter present no direct mating competition.  The nearly identical-looking lemon damselfish (P. moluccensis) has slightly different UV-sensitive markings on its face than its aggressive cousin Ambon.*

The researchers found that when UV light was present (making visible the markings on both species), the Ambon would attack intruding brethren. But when the UV light was removed from the environment, the fish no longer went after one species more often than the other. To firm up the findings, the team trained the fish to swim toward a particular UV pattern among a variety printed on a white sheet of paper. The fish excelled at the task, demonstrating that it is not just the presence or absence of UV cues that the fish detect but rather that they can see details in this spectrum, too.

"The exciting thing is that we can show that these fish can tell the difference between intricate UV patterns—something that was not expected," Siebeck said.

That these fish can be particularly attuned to UV rays is a bit of a surprise. Short-wave rays on the UV end of the spectrum are known to disperse quickly in both water and air. So these shallow-water marine fish still have to get relatively close (a few body lengths) to one another to detect their peers' UV patterns, but their ability to see these markings is as clear as, well, daylight.

"If you think about it in simple terms, fish have to be inconspicuous if they want to go undetected by their predators and prey, but at the same time, they have to be conspicuous if they want to attract the attention of potential mates," Siebeck said. "Using UV patterns to do this is a clever way to maximize both at the same time—they are still inconspicuous to predators but very conspicuous to other fish with UV vision."

But just what are these damselfish seeing when they detect the ultraviolet patterns on a fellow fish—and what are we missing out on by being blind to this spectrum? "It is hard to imagine how the fish look to each other," Siebeck explained in an email to ScientificAmerican.com. She gives the example of a green field with red flowers in it: "To a colorblind person the flowers would be invisible—but to a person with color vision the flowers would be very conspicuous. So we are all like a colorblind person when we look at the fish." Humans and other longer-living creatures block UV light for at least one good reason: It can be damaging to the eyes—a lesser concern in the shorter life of a damselfish.

Image courtesy of Ulrike Siebeck/University of Queensland

*Erratum (2/25/10): This sentence was changed after publication. It originally stated that the lemon damselfish does not have UV-sensitive markings.