Fish are starting to show their true colors—and they’re really glowing. In a first extensive survey, researchers have identified more than 180 species of fish that sport fluorescent coatings—showing that this phenomenon is widespread and could potentially allow individuals to communicate with each other.

Glowing, or biofluorescence, is a phenomenon in which special proteins absorb incoming light and re-emit it at a longer wavelength, giving off brilliant green, orange, and red colors. Although marine organisms such as corals and jellyfish glow, the effect had not been widely observed in fish, and scientists had spotted only red coloration. But discovering new fluorescent proteins could have practical benefits. The isolation of a green fluorescent protein from jellyfish, for example, has provided a revolutionary tool for biological imaging, enabling safer and easier viewing of everything from the innards of the brain to the AIDS virus.

In the new study, reported today in PLOS ONE, ichthyologist John Sparks of the American Museum of Natural History in New York City and his colleagues explain that they had not originally set out to look at fish. They were studying the biofluorescence of corals when an eel swam in front of their specially filtered camera. Unexpectedly, the creature glowed green. Their curiosity piqued, the researchers set about to see just how common fluorescence was among fish. Accompanied by professional photographers and filmmakers, the team conducted four expeditions—to locations in the Bahamas and the Solomon Islands—and visited public aquariums to examine a range of fish species. Using waterproofed, high-intensity blue lights—which match the predominant color underwater—the researchers stimulated the fish’s natural fluorescence. Although the resulting light show may have been invisible to human eyes, yellow filters enabled the team to see and film previously hidden colors.

“We found out that biofluorescence is much more widespread in terms of colors and fluorescent patterns than we ever anticipated,” Sparks says. The team spotted biofluorescence in about 180 species, including cartilaginous fish (like sharks) and their bony counterparts (such as eels and lizardfish). The team believes the occurrence of fluorescence in so many different species indicates that, among fish, the capacity to glow may have evolved a number of times.

Biofluorescence was especially common in species that—to our eyes at least—are well-camouflaged in their coral reef environments. Instead of blending in, however, the species became highly noticeable when viewed through the yellow filters. Perhaps unsurprisingly, many of the same biofluorescent fish had their own yellow filters within their eyes, allowing them to see their peers. Because these fish often displayed species-specific patterns, the researchers propose that fluorescence may serve in communication or even mating displays, while keeping the fish camouflaged against predators.

Nico Michiels, a zoologist at the University of Tübingen in Germany who was not involved in this research, notes that the study is the first to present fluorescence data from a large number of fish species in the context of the fish tree of life. Although he agrees that the distribution of green and red fluorescence among fish does hint at possible uses in communication, he cautions against assuming that the biofluorescence is functional. He says that the need for artificial lights to detect the weak fluorescence in many species casts doubt on the usefulness of the coloration in the fish’s dimly lit natural environments.

Bioluminescence expert Steven Haddock of California’s Monterey Bay Aquarium Research Institute in Moss Landing—who also was not involved in the study—agrees. Many natural materials fluoresce because of their structure, he notes. Even humans have fluorescent features—such as in our skin, fingernails, and eye lenses—but these have no specific role. More studies are needed to determine whether fluorescence in specific fish does have a function, he says. However, he gives Sparks and colleagues credit for paving the way toward interesting questions about the natural functions of fluorescence.

Sparks and colleagues intend to try to answer those questions: “For now we want to try and get at the functional aspect of it,” Sparks says. He and his team plan to key in on some of the groups with more diverse patterns to determine whether fluorescence does play a role in communication.

*Correction, 8 January, 9:24 p.m.: This item originally stated that bioflourescent proteins re-emit light at a shorter wavelength. This has been corrected to "longer wavelength".