A lot of reef fish possess brilliant red coloration.  While this in and of itself doesn’t suggest anything about fish’s ability to view red spectra (620-740nm), the fact that some reef fish can fluorescence red, especially during nuptial courtship, raises the question of whether this color serves a purpose.  If marine fish can’t see red, what’s the point of the magnificent red pigmentation when males are “showboating?”

Researchers from the University of Tübingen, Germany, experimented with male solar fairy wrasse (Cirrhilabrus solorensis) to find out if the fluorescent deep red (650-700nm) colors the males display during courtship (“flashing”) was visible to other male solar wrasses.  For this experiment, the scientists employed the “mirror trick” many marine aquarists have used to distract aggressive fish; they lowered three different types of mirrors into aquariums housing terminal phase male solar wrasses.  One mirror was a standard mirror.  Another was treated with filters to absorb red light (600nm+). The third was treated with filters to absorb all light except red (380–600nm).

The reflections of the male wrasses seen through various filtered mirrors

The result?  Displaying males placed next to standard mirrors demonstrated the expected aggression towards their reflections.  However, displaying males placed next to the red-masked mirrors exhibited no aggression at all.  The fairy wrasses can indeed see red light!  Fact is, red light is a crucial visual cue for fairy wrasse behavior.  When solar wrasses do not see red on other male solar wrasses, they show no aggression.

Furthermore, males placed next to mirrors with the 380-600nm filters also exhibited no aggression.  This third finding tells us that red by itself is not enough to elicit aggressive responses.  Fairy wrasses need to see the full spectrum outline of other fish to determine whether it is a rival or not.  This is not surprising since aquarist have learned that fish tend to be more aggressive towards other fish with similar body shapes.  Besides, many other marine fish fluorescence red so if a fairy wrasse went spastic simply against anything with red colors, it would be one perpetually angry fish.

The next time you use the “red flashlight” trick to catch fairy wrasses at night, the wrasse probably sees everything.  Then again, the trick may still work (aquarists’ anecdotal experiences support that it does) because you may be confusing the heck out of the wrasse.

The study is published in the current issue of the Proceeds of the Royal Society B.