Matt Davis

Matt Davis

Matt Davis

Matt Davis

Rene Martin

Kirsten Jensen

Leo Smith

Leo Smith

Matt Davis

Rene Martin

Leo Smith

Matt Davis

Bioluminescent animals have the power of light. Sometimes they emit a bright glow from one specialized body part, trying to attract prey or mates. Sometimes they radiate dimly on the undersides of their bodies for camouflage counter-illumination, hiding their shadows on the seafloor by matching the light levels coming from the surface. As weird as it sounds, bioluminescence turns out to be an incredibly beneficial adaptation. A new study shows that it has evolved no less than 27 times in biological history, for countless reasons.

A group of zoologists described how bioluminescence evolved in the journal PLoS One, noting that 80 percent of glowing animals live in the oceans. Only a few land animals emit light, and they are all arthropods like fireflies and millipedes. There are only two ways that animals start radiating. Either they have intrinsic bioluminescence, mixing chemicals in their bodies to regulate the color and intensity of light, or they have symbiotic bioluminescence, cultivating colonies of glowing bacteria in specialized organs or pouches. The question that intrigued the researchers was why so many animals adapted to their environments by starting to glow.

University of Kansas evolutionary biologist Leo Smith, who contributed to the study, told Ars that fish use their built-in lights for many different reasons. In coastal areas, fish use patterns of flashing lights to "communicate during mating," which is important because they live in sandy areas where visibility is low. Like aquatic ravers on the prowl, these fish also use light patterns to recognize each other in areas where many species swim together. Fish that live in the deeper ocean also flash their mates, but they mainly use bioluminescence like flashlights to find prey.

Among "true deep-sea fishes," the main use is camouflage, said Smith. These deep-sea fish, who live their entire lives without ever seeing land, also tend to have intrinsic bioluminescence. They don't depend on finding bacteria to help them glitter. "These are fishes that are found anywhere from 1,000 feet down to near the bottom of the ocean," Smith explained. "It could be luck, but it could just be that these successful fishes benefited from evolving the ability to produce their own light."

Bioluminescence has evolved 27 different times in a range of species, but what sparked that first adaptation? It's hard to say for sure, because both communication and camouflage are crucial adaptations. Smith explained:

Ponyfishes only glow during mating season, so communication was first. A group like the tube shoulders project a bioluminescent ink (think glowy octopus ink) to avoid predation, so that was just "camouflage." In the case of the lanternfishes, it appears as though it originally evolved for camouflage, but then the myctophid lanternfishes further modified the system to be used for communication. In cases where they use it for both communication and camouflage, I would lean toward thinking that camouflage was first, but that is more of a hunch than a "truth" or a finding of our work.

Many glowing fish groups are called "species rich" because they tend to diverge into new species very quickly. Smith and his colleagues believe this is probably because they're using species-specific light patterns to recognize mates and communicate. If a group of fish finds a nice ecological niche that works, and it has a very obvious way of communicating with its immediate group, members of that group of fish will mate only with each other and rapidly diverge from their neighbors. These animals, explained Smith, "found multiple niches in the same environment and have been able to create and maintain the species barriers because of the species-specific bioluminescent patterns." The easier it is to recognize mates and comrades, the easier it is for new species to form—even in crowded conditions where lots of other species are swimming around. Cautioned Smith, "To be clear, that is our hypothesis, not a fact per se, but there seems to be a lot of evidence to support that."

Considered in this light, bioluminescence is basically the best feature a fish could ever have. It's a disguise, a food-finding tool, and even a way to get some undersea love action. No wonder it has evolved so many times over the past few hundred million years.

PLoS One, 2016. DOI: 10.1371/journal.pone.0155154

Listing image by Matt Davis