Reef fish get riled when intruders glow red

Even in the blue underwater realm, fish can see red in more ways than one. Reef fish that see glowing red patches on a rival aggressively bite and put on extra threatening displays, researchers say.

Red wavelengths from sunshine can’t penetrate water much deeper than 10 or 20 meters, failing to reach far into the realm where more than 40 small Cirrhilabrus fish species called fairy wrasses dart along reefs. But shorter, bluer wavelengths can plunge deep, where they are absorbed by the scales and some of the fin rays in both males and females of the C. solorensis fairy wrasse. The fish then fluoresce, emitting their own red light in distinctive his-and-hers swaths across their bodies.

The first behavioral test of whether a red-fluorescing fish cares about the red glow shows that the coloring sparks aggression among males, says Nico Michiels of the University of Tübingen in Germany. Males seeing the full red glory of a mock intruder responded with attempted bites and menacing displays, Michiels and his colleagues report May 28 in the Proceedings of the Royal Society B.

When Michiels and colleagues reported in 2008 that a fish could glow red, some other researchers were skeptical that the fluorescence mattered to the fish, he says.  Fish vision supposedly excelled in the blue and green wavelengths that dominate the animals’ habitats, and any reds wouldn’t be visible from afar.

For those very reasons, however, Michiels proposed that red fluorescence might offer a discreet channel of communication unlikely to catch the attention of unintended eavesdroppers such as distant rivals or predators. That notion is very difficult to test. “You need to know to know everything about the bystanders and everybody who is looking at these fish,” he laments.

Since then, researchers have described other fish that glow red, and research on eyes has turned up evidence that certain fish should be able to detect red.  

For a behavioral test of what the fish themselves think of the color, Michiels and his colleagues turned to a fairy wrasse species with strong, deep red fluorescence. Team leader Tobias Gerlach, also at Tübingen, had to treat the active, rather nervous fish very gently so that he could show males their mirror images. Males responded as if the fish in the mirror were intruders. When a male saw the mirror in full color, he reacted more menacingly, with more display postures and attempted bites, than he did when the researchers used a filter to remove the red wavelengths.

Testing fish behavior is important for understanding how fluorescence evolved and how animals use it, says marine biologist David Gruber of the City University of New York and the American Museum of Natural History in New York City. Red and green are the main colors he and his colleagues reported in January among more than 180 kinds of fish known to fluoresce. These colors could be important in camouflage as well as in signaling, says Gruber. For fish hiding among reef corals and algae, both of which can fluoresce, adding some emitted light of their own could help them blend in.