Marine fish boast an incredible array of bright and exotic colors, at least in shallower waters. Long-wavelength sunlight, corresponding to red and orange, is rapidly absorbed by the ocean. After around 10-20 meters very little red or orange light is able to penetrate through the water, meaning that red marine fish dwelling in deeper environments will appear grey. Shorter wavelengths, in the blue and green range, are able to penetrate to a much greater depth; this has created a bias towards blue and yellow colors of reef fish. It was therefore also assumed that marine fishes are less able to detect longer wavelengths of light.

Some fish, however, are able to display wavelengths that are otherwise absent in the environment through fluorescence, meaning that they stand out from the crowd. Fluorescent pigments are able to absorb shorter wavelengths of light and then re-emit photons at longer wavelengths. This means that even in an absence of red/orange light in the environment fish may appear these colors, creating striking color contrasts. Although this much is known, scientists have struggled to discern the role of this long-wavelength fluorescence in marine fish.

Intrigued by these organisms, scientist Tobias Gerlach from the University of Tübingen and colleagues turned to a species of Indo-Pacific reef fish called the fairy wrasse (Cirrhilabrus solorensis) to find out more about the purpose of this red fluorescence. The study can be found in the open-access journal Proceedings of the Royal Society B.

Fairy wrasses represent an ideal test subject because they can be found at depths of up to 65 meters, which is much deeper than red sunlight is able to penetrate. They also display a marked red fluorescent body and the males and females differ dramatically in appearance. When viewed under white light male fairy wrasses appear yellow and purple, but shine a blue light on them and a dramatically different coloration is seen.

Top image: male fairy wrasse under white light. Bottom image: male fairy wrasse under blue light. Image credit: Gerlach et al., Proceedings of the Royal Society B.

The team wanted to test the hypothesis that fairy wrasses are able to perceive red fluorescence and elicit behavioral responses towards it. They collected fairy wrasses from an ornamental fish trader and set them up in individual aquariums under constant blue light, representative of the environment within which they live. In order to investigate how these fish respond to other males the team placed mirrors inside the tank. This is because they are unable to recognize their own reflection and thus the image in the mirror is perceived as another male, which could potentially be a competitor. Sure enough, preliminary tests showed that the fish reacted aggressively towards their reflections.

In order to manipulate the image on the mirror, the researchers covered the mirror with different color filters which could block different wavelengths of light. Intriguingly, the researchers found that the males elicited less agonistic behaviors towards mirror images where the red fluorescence had been masked when compared with control treatments. They therefore concluded that these fish are able to perceive the red fluorescence, and that this fluorescence affects male-male interactions.

Top image: Fairy wrasse under blue light. Bottom image: Fairy wrasse with NoRed filter. Image credit: Gerlach et al., Proceedings of the Royal Society B. 

So why does this red fluorescence make these males more aggro? One possibility raised is that less fluorescent males may be perceived as less of a threat. Fairy wrasses start off as females and then transition into males, but it takes a while to develop the red fluorescence. The males may have therefore decided that the less fluorescent fish were not fully competent rivals. Whether females use this red fluorescence as a means of mate selection remains to be elucidated.

Another question that you may be asking- why red? It may be that since red is so poorly transmitted through water, it is ideal for these fish that communicate socially or sexually at short distances. They can display their colorful bodies to potential mates, yet remain hidden from predators.