Imagine a world 540 million years ago. No land masses had yet formed, and life teemed within the vast oceans. Among the earliest animals were the octocorals, a diverse group that included soft corals, sea pens, and sea fans. A new study, published in the Proceedings of the Royal Society B: Biological Sciences, reveals a surprising secret about these ancient creatures: they were likely bioluminescent, capable of producing their own light.
This discovery pushes back the previously known origin of bioluminescence in animals by a staggering 300 million years. Before this study, the earliest evidence pointed to small crustaceans called ostracods, which emerged around 267 million years ago. Bioluminescence, the fascinating ability of living organisms to emit light through chemical reactions, has captivated scientists for centuries. It’s a dazzling phenomenon used for a variety of purposes, from attracting mates to luring prey and even communication. But its evolutionary origins remained shrouded in mystery.
“Nobody quite knows why it first evolved in animals,” said Andrea Quattrini, curator of corals at the Smithsonian’s National Museum of Natural History and senior author of the study. Understanding when this ability first appeared is crucial to unlocking the bigger question: why did bioluminescence evolve in the first place?
Octocorals: A Glowing Lineage
Quattrini and lead author Danielle DeLeo, a research associate at the museum, focused their investigation on octocorals. These ancient animals are known for their bioluminescence, but the exact function of their light-emitting capabilities remains unclear. Some species only glow when disturbed, leaving scientists to ponder the evolutionary advantage of such a seemingly sporadic light show.
“We wanted to pinpoint the emergence of bioluminescence,” explained DeLeo. “Octocorals are one of the oldest groups known to possess this trait, so we wondered – when did they acquire this ability?”
Luckily, Quattrini and her collaborators had already built a detailed evolutionary tree, or phylogeny, of octocorals in 2022. This extensive map, based on the genetic data of 185 species, provided a solid foundation for their research.
Dating the Light: Fossils and Phylogeny
The next step involved incorporating fossils. By placing two octocoral fossils of known ages within the evolutionary tree based on their physical characteristics, the researchers could estimate when different lineages diverged. This approach allowed them to reconstruct the branching history of the octocorals.
Then, they mapped the branches containing living bioluminescent species. With the dated phylogeny at hand, they employed statistical techniques for ancestral state reconstruction.
“Based on the bioluminescent species alive today, we can use statistics to infer whether their ancestors likely possessed this trait,” explained Quattrini. “The more living bioluminescent species share a branch, the higher the probability that their ancestors also had the ability.”
A Deeply Rooted Innovation
Multiple statistical methods yielded the same remarkable conclusion: the common ancestor of all octocorals, dating back 540 million years, was very likely bioluminescent. This finding dwarfs the previous record holder, the ostracod crustacean, by a significant margin.
The high prevalence of bioluminescence among the thousands of living octocoral species suggests its importance in their evolutionary success. While the specific purpose of their light show remains a mystery, the fact that the trait has persisted for so long underscores its significance for their survival and fitness.
Illuminating the Future of Research
Now that the ancestral origin of bioluminescence in octocorals is established, DeLeo and Quattrini are delving deeper. They aim to create a comprehensive record of which species within the group retain this ability and which have lost it. This information could shed light on the ecological factors influencing bioluminescence and its potential functions.
DeLeo envisions a genetic test to identify species with functional luciferase genes, the key enzyme involved in bioluminescence. This would be a faster and more efficient method for determining a species’ bioluminescent potential.
Beyond Origins: Conservation and Knowledge
This research not only illuminates the evolutionary history of bioluminescence but also provides valuable insights for modern conservation efforts. Octocorals face significant threats from climate change, resource extraction activities like fishing and oil drilling, and emerging challenges like deep-sea mining. Understanding their evolutionary history is crucial for informing effective conservation strategies.
This study aligns with the Smithsonian’s Ocean Science Center’s mission to expand knowledge of the ocean and share it with the world. While the mystery of why bioluminescence first evolved remains unsolved, DeLeo and Quattrini acknowledge the possibility of even older origins being discovered in the future. Their groundbreaking research adds a crucial chapter to the story of bioluminescence, one that began in the depths of time with the ancient glow of the octocorals.
Source: Smithsonian