From the eerie glow of fireflies dancing in the night to the mesmerizing brilliance of the New Zealand glow worms, bioluminescence in the animal kingdom has long intrigued humans. But it is not just on land that animals are able to light themselves up, as when phytoplankton bloom it can sometimes seem like the whole ocean is lit up. In fact, this might not be too far from the truth.
Researchers have found that bioluminescence is way more common in marine fishes than previously imagined. Using a massive variety of methods to achieve their flashy abilities, from commandeering glowing bacteria to evolving specialized light-producing organs, many species of fish have developed this ability. But this wide variety of processes also hints that there may be multiple different evolutionary origins. A new study has now shown that the ability to bioluminesce has in fact evolved independently an impressive 27 times.
“When things evolve independently multiples times, we can infer that the feature is useful,” explains W. Leo Smith, who co-authored the study looking into the origin of glowing fish published in PLOS One, in a statement. “You have this whole habitat where everything that's not living at the top or bottom of the ocean or along the edges – nearly every vertebrate living in the open water – around 80 percent of those fish species are bioluminescent. So this tells us bioluminescence is almost a requirement for fishes to be successful.”
Bristlemouth fish are around 7.6 centimeters (3 inches) long, and the most numerous vertebrate on the planet. NOAA Photo Library/Flickr CC BY 2.0
The ability to glow is used for a variety of reasons, from signaling other fish to attracting prey to camouflage via "counter-illumination." This is when the bottom of the fish glows to match the light coming from the surface above. The use of bioluminescence is particularly beneficial to those fish that live in the “deep scattering layer” of the ocean, the layer at which huge amounts of ocean biomass live. This biomass includes the bristlemouth fish, which is thought to be the most abundant vertebrate species on Earth, numbering around an astonishing quadrillion individuals. And they also glow.
Not only does it seem that bioluminescence is highly advantageous to marine fish, but it also seems that it could be a driver of diversity. The researchers found that whenever an evolutionary line of fish developed the ability to produce light, often soon thereafter they would branch into numerous new species.
“Many fish proliferate species when they evolve this trait – they differentiate, but we don't know why,” says Smith. “In the ocean, there are no physical barriers to separate groups of deep sea fishes, so why are there so many species of anglerfishes, for example? When they start using bioluminescence for species recognition, they diversify into a lot more species.”
Main image: NOAA Photo Library/Flickr CC BY 2.0
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