Paper: Sparks JS, Schelly RC, Smith WL, Davis MP, Tchernov D, et al. (2014) The Covert World of Fish Biofluorescence: A Phylogenetically Widespread and Phenotypically Variable Phenomenon. PLoS ONE 9(1): e83259. doi:10.1371/journal.pone.0083259
If you have experience diving beneath the ocean surface, you know that as you descend, colors change. Water filters out the different wavelengths of visible sunlight, like a rainbow with its colors slowly getting erased. The vibrant reds and oranges are the first to go, while the blues and violets cling on for a bit, creating a nearly-monochromatic deep blue realm; then, the ocean literally becomes a place that has never seen the light of day.
As different colored light becomes limited, opportunities are created for organisms to exploit fluorescence to produce visual contrast and patterns. This is where it gets tricky. Biofluorescence is easily confused with bioluminescence, but they are very different things. Bioluminescence is like a DIY glow-in-the-dark. The organism emits its own light, which human eyes can see without any special filters. Biofluorescence is more like reflecting light, not creating light. This means that in order to biofluoresce, at least a little bit of light is necessary from a different source. To see fluorescence, a special filter is needed on your dive goggles, typically with a yellowish-tint.
Biofluorescence of coral is well studied, so Sparks et al. aimed to investigate the little known details regarding the impact of biofluorescence on the other creatures that thrive in coral reef habitats, specifically the 8,000+ species of fishes. What they found was shocking. Not only is biofluorescence widespread throughout the tree of life for all fishes, it is particularly common and both genetically and environmentally variable in marine lineages. This widespread and previously unrecognized phenomenon gives new insight into the evolution of marine fishes and changes how we think light/visual systems work in the marine environment.
In the initial surveys, Sparks et al. had already identified 16 orders, 50 families, 105 genera, and 180+ species of biofluorescent fishes. They also gained enough new information to reconstruct the phylogenetic branch for ray-finned fishes.
Why would fish fluoresce? How can fluorescence be seen?
In order to exploit fluorescence and use it for your advantage, you need to be able to see it. Shallow marine fish are visual animals and conveniently many have yellow lens filters on their eyes, which could enable an enhanced perception of biofluorescence in the ocean.
As mentioned earlier, there have been many studies about corals and biofluorescence. Some organisms associated with coral produce their own food via photosynthesis, like algae. Photosynthesis relies on the presence of chlorophyll, which happens to fluoresce red. Red fluorescence also can be a result of the reaction of particular proteins. This means that if an animal lives on a coral reef and it can also fluoresce red, it can camouflage itself.
Sparks et al. show us that fishes don’t only fluoresce red. They can be green, orange, or in patterns of multiple colors to help with camouflage. As if the impressive list of new finds wasn’t long enough, the team also found that related species that look nearly identical under white light (ie. “normal” sunlight) can have considerable variation in their glow colors and patterns. And, it turns out, this widespread biofluorescence not only acts as great camouflage, but it could also be a way for fish to communicate with each other, as is suspected with the mantis shrimp Lysiosquillina glabriuscula.
Do deep-sea creatures biofluoresce if there is no light?
One could think that from an evolutionary perspective, it does not make sense to be capable of reflecting light, if there is no light to reflect. However, there are a few instances of biofluorescence in deep-water organisms that live in places where sunlight cannot penetrate.
Potentially, the fluorescence in deep-sea fish could be a result of ancestral relations to shallow water relatives. Or maybe, it is used in conjunction with bioluminescence, the DIY glow-in-the-dark mentioned earlier.
The bottom line
Biofluorescence in fishes is far more common than previously thought and is broadly distributed throughout a diverse group of fish species. It is used for communication (like attracting mates), predator evasion, and maybe prey attraction leading to predation.
There have been far too few studies about biofluorescence and the field is ripe for scientific analysis. For instance, the repeated independent evolution of fluorescence throughout the phylogenetic tree is curious, but its importance in the diversification of marine fishes has yet to be explored.