Many tropical marine reef fishes fluoresce red and are capable of seeing other fish that fluoresce the same colours.
According to a study published in the journal BMC Ecology, a number of tropical marine reef fishes are able to fluoresce a red colour, challenging the long held view that red light is of little importance to marine fish.
In waters less than 10m/33' deep, the light is predominantly blue-green as seawater selectively absorbs the red wavelengths in the sunlight downwelling from the surface.
The eyes of reef fish are therefore adapted for this shorter wavelength light by special visual pigments. The current view is that, due to the lack of red ambient light in shallow waters, the eyes of fish found there are not sensitive to red and the colour has largely been considered irrelevant.
However, this new study, which was led by Nico Michiels of the University of Tubingen in Germany, has reported 32 reef fishes from 16 genera and five families that showed pronounced red fluorescence under natural daylight conditions at depths where downwelling red light is virtually absent.
Private communication systemMichiels said that the study showed that red fluorescence was widespread among marine fish, and said that the team's findings challenged the notion that red light is of no importance to marine fish, and called for a reassessment of its role in fish visual ecology.
Said Michiels: "We believe red fluorescence may be part of a private communication system in fish.
"Red fluorescence is at the borderline of what is visible to many marine fish, and due to rapid attenuation of red light by water, even those that can see red will be able to see it over short distances only.
"Fluorescent eye rings may function as an indicator of presence or reveal the direction of gaze."
He believes that the several features indicate that fluorescence is used for private communication in small, benthic, pair- or group-living fishes.
SpectrometryThe findings came after Michiels' team dived below the depth at which red light penetrates and took photographs of reef fish using special underwater cameras equipped with red filters to block certain wavelengths of light.
This revealed widespread fluorescence in many fish, algae and invertebrates, including some such as polychaetes, sponges and feather stars in which fluorescence has never been recorded before.
They then analysed the tissues of the fish using fluorescence microscopy and spectrometry to clarify that guanine crystals were the main mechanism for the fluorescence.
The results showed that red fluorescence was found to be particularly strong in the fins that were involved in signalling with members of the same species.
An additional analysis on the goby Eviota pellucida showed that it was capable of seeing its own fluorescence, suggesting that it has uses in signalling and communication.
For more information see the paper: Nico K. Michiels, Nils Anthes, Nathan S. Hart, Juergen Herler, Alfred J. Meixner, Frank Schleifenbaum, Gregor Schulte, Ulrike E. Siebeck, Dennis Sprenger and Matthias F. Wucherer (In press) - Red fluorescence in reef fish: a novel signaling mechanism? BMC Ecology.