Seeing Red

Reef fish, once thought to be unable to see red wavelengths, not only fluoresce deep red, but males of some species react to seeing their own bioluminescent pattern.

By | August 1, 2014

NOW YOU SEE ME: A filter that blocks red-wavelength light (bottom) masks the fluorescent pattern (top) and aggressive behavior of a male fairy wrasse.TOBIAS GERLACH


The paper
T. Gerlach et al., “Fairy wrasses perceive and respond to their deep red fluorescent coloration,” Proc R Soc B, doi:10.1098/rspb.2014.0787, 2014.

The context
Below about 20 meters depth, little to no red light penetrates ocean water. In 2008, Nico Michiels of the University of Tübingen in Germany and his colleagues demonstrated that, inexplicably, several species of fish emit unique red fluorescent patterns on their bodies, and some are able to see this wavelength. Michiels’s team next wanted to determine whether this has any functional significance.

The experiment
In an aquarium lit by monochromatic blue light, male fairy wrasses (Cirrhilabrus solorensis) responded to their reflections in a mirror by bristling their fins defensively or trying to bite their image. When the researchers placed a filter on the mirror that absorbed red wavelengths, the fish were less aggressive, demonstrating that they can perceive and react to the fluorescence.

The setting
Ambient marine light at the depth where these fish live still has small amounts of red light, but the blue light used in the experiment does not. So the laboratory setting may be exaggerating the behavioral influence of the red fluorescence, says Gil Rosenthal of Texas A&M University, who was not involved in the work. Without studying wrasses in their natural reef environment—a very challenging feat—it will be difficult to gauge this.

The implication
Reef fish may have much wider ranges of vision and behavioral responses than previously thought. The study is “indicating that red fluorescence is an important part of intraspecific communication,” says Michiels. Rosenthal notes that the results also raise questions about why reef fish evolved red fluorescence in the first place. 


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