Because red light downwelling from the sun is selectively absorbed by seawater, scientists have long assumed that red light is of little importance to reef fishes in shallower waters less than 33 ft. (10 m) deep where blue-green light dominates.



Now a team of researchers has discovered that 32 different reef fishes from 16 genera and five different families have the ability to display red fluorescence in daytime conditions and in waters without natural red light from the sun. Heads, eyes, and fins are the body regions that fluoresce most intensely, and it is surmised that the flashing of color is important in silent underwater communication between aquatic species.

Led by Nico K. Michiels at the University of Tubingen in Germany, the team of researchers also demonstrated red fluorescence in stony corals, macroalgae, polychaete worms, sponges, and feather stars.

Cryptic Signals?

Because many of the fishes and other organisms show cryptic coloration in other parts of the visible spectrum, Michiels says that red fluorescence may be "used as a private communication mechanism in small, benthic, pair or group-living fishes."

"Our findings challenge the notion that red light is of no importance to marine fish, calling for a reassessment of of its role in fish visual ecology in subsurface marine environments," the authors conclude in a paper just released in the Journal ''BMC Ecology.''

Many differences were found between families and genera. Eye rings and parts of the head and thorax most commonly glowed in fishes, with Triplefin Blennies (Trypterygidiidae) able to signal with brightly fluorescing dorsal fins. Some Pseudocheilinus and Paracheilinus wrasses show a "whole-body glow."

Methods Used

The Michiels' team from Tubingen, Austria, and Australia, equipped themselves with dive masks and camera lenses with red filters blocking wavelengths below 600 nanometers. Diving deeper than 15m, where natural red light does not penetrate, they found a world glowing with red. "By emitting a colour that is lacking from the environment, a fish contrasts more against its background," said Michiels.

"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." This seems to support the theory that fluorescence may be most important in communication between members of a species living in close proximity on the reef.

The functionality of sessile invertebrates showing red fluorescence remains to be studied.

Mechanisms Involved

According to the study:

"Dissection revealed that red fluorescence was associated with guanine crystals in pipefish, triplefins, blennies and gobies. Guanine crystals are produced by iridophores and are well known as the source of silvery reflection and iridescence in bony fish. However, they have never been described to show strong red fluorescence....

"Preparations of crystals maintained strong fluorescence after prolonged storage in a dried or liquid form, allowing us to confirm fluorescence in preserved gobies collected up to 5 years before. This is in striking contrast to reflective red pigmentation, which bleaches out within hours after fixation."

Warning to Divers

The study's authors cautioned divers to use great caution in diving with red masks:

"Diving with a red mask is similar to night diving, with dramatically reduced light intensities and viewing distances. Disorientation becomes a serious problem. Moreover, it takes several minutes to adapt to the darkness. Staying in a small, familiar area and moving slowly and carefully is crucial. Furthermore, it is essential to take a torch to read equipment. Dials, indicators and computer backlights are either reflectant or luminesce blue or green, making them effectively illegible at depth in the absence of a local white (red-containing) light source.

"To circumvent this problem, we also used the Oceanic DataMask which has a built-in dive computer that can be read irrespective of any filter attached to the front. Because of these unfamiliar restrictions, we recommend that only experienced divers use this procedure and that only one partner in a buddy team uses a red mask at any given time. We also recommend attaching filters in such a

way that they can be instantly removed without having to change to a spare mask, which one should carry nevertheless."