In the dark, cold expanse of the deep ocean, bioluminescent creatures light up their world like stars in an inky sky. According to a new study, the ability to create light may have evolved independently 27 times in marine ray-finned fishes. If you count sharks and rays, it's evolved 29 times. And being able to turn on a personal light switch might actually make a group more likely to evolve a large number of diverse species.
The findings, reported Wednesday in PLOS, may change the way we think about bioluminescence as an evolutionary quirk. Previous research had estimated that the trait, which allows animals to create their own light through myriad chemical reactions, evolved just 40 times in the entire tree of life, including things like fireflies and fungi that live on land. Now that seems likely to be a gross underestimation. In the deep sea, nearly everything glows — and according to an analysis of the marine family tree, those lights didn't come from a handful of common ancestors. They turned on independently, using different chemical strategies, over and over again.
"Sitting there in the dark in a submarine, everything is just flashing around you," said study author John Sparks, curator-in-charge of the American Museum of Natural History's Department of Ichthyology. For now, available cameras can't adequately capture the light show (something Sparks and his colleagues are close to fixing), so their research relies on the analysis of creatures trawled up from the depths.
It's not quite the same. Many of the fish will glow in the lab, but although their heads are often covered with intricate arrangements of light-producing organs, they'll simply turn them all on at once. Sparks isn't convinced this is natural behavior — he thinks they use the lights to produce complex signals down in the deep. He hopes to study those particular creatures in their natural stomping grounds one day, because they may help explain why bioluminescence evolved so many times.
It's these critters — the ones that seem to use bioluminescence for something other than camouflage, based on the positioning of their lights — that have enjoyed the most species diversity.
"In the ocean, there are no physical barriers to keep different groups from reproducing with one another," Sparks said. "So you have to ask how reproductive isolation comes about." For new species to evolve, groups need to be isolated enough from one another to develop unique traits. Based on the findings of the new study, he thinks that bioluminescence may drive a lot of the deep sea's diversity. The number of species in a lineage seem to boom after the evolution of bioluminescence — and then again when it's adapted for a secondary use.
The lantern fish makes a good example. One of its lineages has illuminating organs perfectly placed to make it blend in with the downwelling light — which makes sense, because it's thought that all bioluminescent organs first evolved as camouflage. But over the past 150 million years or so, another lineage has let its lights migrate to form unique patterns on its head and sides.
"It's a unique arrangement of little lights on the head, so it produces a unique signal," Sparks said, meaning they can be used for communication and reproduction.
The camouflage group has far fewer unique species than the related branch with more ostentatious lights. This pattern was repeated across the lineages studied by Sparks and his colleagues. It's possible that the evolution of light patterns allowed animals to distinguish themselves from one another and form groups isolated enough to speciate.
Sparks and the rest of the team hope to perfect a deep-sea camera that would allow them to capture signal flashes in the wild.
"Right now, we have no idea how they flash," he said. "And when something evolves so many times, you know it's obviously very important."