Various fluorescent fishes. Credit: Sparks et al 2014
In the ocean, longer wavelengths of light (i.e. red) are quickly filtered out, so that with increasing depth the red pigmentation of marinelife takes on increasingly inky tones. This provides excellent camouflage in the darkness and is why so many deeper-water creatures sport such vividly carmine hues. There are, however, fishes from these depths that can appear red even in the absence of ambient red light. They do so by absorbing the high-energy blue light which surrounds them and emitting a lower-energy red light, a process known as fluorescence. That reef fish were capable of this was entirely unknown until it was first documented in 2008, and now some newly published research is helping to shed light (pun!) on this poorly understood phenomenon. The source of the fluorescence are concentrations of guanine crystals contained within highly branched cells called iridophores. Through various histological and biophysical methodologies, it has been determined that at least some of the reef fishes emitting this red light are indeed able to detect it and even to behaviorally control its intensity—this implies that this red fluorescence is not merely some biochemical accident but, rather, an important part of how these species communicate with each other. Research conducted on Eviota gobies has shown that these fluorescent iridophores are capable of expanding and contracting in response to certain neurotransmitters and hormones, resulting in a dimming and brightening of the fluorescent light. For a cryptic goby like this, is this red coloration used for camouflage or for intraspecific communications?
Red fluorescent guanine crystals. Note that many fishes have non-fluorescent guanine crystals that impart a blue or silver hue instead. Credit: Michiels et al 2008
The contractile capabilities of a fluorescent iridophore in Eviota. Credit: Wucherer & Michiels 2012
Another group that appears to rely heavily on red fluorescence are the wrasses of the Tribe Pseudocheilini, which includes such familiar aquarium fishes as the Fairy Wrasses (Cirrhilabrus), Flasherwrasses (Paracheilinus), and the Sixline, Eightline and Mystery Wrasses of Pseudochelinus. Most species in this group occur in moderate depths (usually below 10 meters) where red ambient light is lacking, and the entire tribe is morphologically identifiable from a distinctive “double lens” in the cornea which acts as a “yellow intraocular filter”, enabling them to visualize longer wavelengths of light.
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