Glowing Sea Creatures Have Been Lighting Up the Oceans for More Than Half a Billion Years

Many creatures light up the world’s waters. From microorganisms that turn shoreline waves electric blue at night to anglerfish deep down who use glowing lures to entice prey, a varied array of marine creatures emit light in a natural phenomenon known as bioluminescence.

Now, a new study has found the earliest example of bioluminescence yet, pushing its origins back into the earliest days of animal life. Around 540 million years ago, soft, branching animals called octocorals began to light up the oceans.

The study, published Tuesday in Proceedings of the Royal Society B by biologist and National Museum of Natural History research associate Danielle DeLeo and colleagues, uses the relationships of octocorals in our modern oceans to work backward and unravel when bioluminescence first appeared in these frond-like animals.

In the seas today, octocorals take a variety of forms such as sea pens, sea fans and soft corals. “Octocorals are soft-bodied corals that have tree-like shapes,” DeLeo says. The creatures get their name from the symmetry of their bodies, arranged in eight parts. Often, “octocorals can create coral gardens and animal forests in the oceans, particularly in the deep sea, which can provide homes and even nursery habitats for an array of other animals,” she adds.

Researchers focused on octocorals for the new study because most octocoral species are luminous. Just as in all bioluminescent organisms, the enzyme luciferase interacts with the compound luciferin to create light among the octocorals.

So far, zoologists have estimated that the ability to be luminous evolved around 100 different times among animals. Exactly when living things began to bioluminesce, however, has been challenging to track. Up until now, the oldest known example of bioluminescence was in 267-million-year-old crustaceans. DeLeo and co-authors expected that the ability likely went even further back in time. Octocorals, with their many luminous species, seemed like a good group to look into for more evidence.

“Because of their mainly soft bodies, octocorals don’t fossilize as well as the hard, stony corals,” DeLeo says. Sometimes all experts have to work with are small, hard parts of the coral structure called sclerites. The critical biochemical reaction can’t be directly observed in fossils, either, and so experts have had to rely on other lines of evidence to explore the origins of biological glow. In the case of the octocorals, looking at their family tree and where they settle down in the sea has helped DeLeo and co-authors identify when the animals began to brighten up the oceans.

To follow the evolution of bioluminescence in octocorals, DeLeo and colleagues looked to the genetic similarities among octocorals and their relatives. The 3,500 or so species of octocorals belong to a larger group called Anthozoa that includes corals and sea anemones. The zoologists looked for which species generate biological light to determine how many times the ability evolved among the animals. “We were able to use genetic similarities to decipher the evolutionary relationships of the corals,” DeLeo says, using coral fossils to help calibrate the timeline of when the various octocoral groups appeared. The researchers also tracked the depth bioluminescent octocorals live at to work out whether emitting light evolved in the shallows or in the darker depths of the oceans. And with such an assessment of modern octocorals, the researchers estimated what the ancestral octocoral might have been like and when it lived.

Bioluminescence is so common in octocorals, with shared biological mechanisms, that the last common ancestor of the entire group must have been bioluminescent, the researchers propose. The ancestor lived near the beginning of the Cambrian, around 542 million years ago, when life on Earth was blooming in new ways and new forms of animal were rapidly diversifying. DeLeo and colleagues also expect that the earliest glowing octocorals originated in shallow waters before later expanding into the deep sea.

“I feel this paper presents a compelling case for one origin of bioluminescence in octocorals by the Cambrian,” says University of California, Santa Barbara, zoologist Emily Lau, who was not involved in the study. Even accounting for unknowns or groups of octocorals that bioluminescence has not been documented in yet, the available data indicates that the ability originated once in the group hundreds of millions of years ago.

The study also raises new questions. “I’m excited about this finding,” Lau says, partly because the study reveals the puzzle of how organisms like octocorals obtain the biological compound needed to bioluminesce. Even though luminous marine organisms need a substance called coelenterazine to bioluminesce, she says, few produce it themselves. Some animals obtain it through their diet instead. In the case of the earliest octocorals, then, were the glowing fronds making their own coelenterazine, or were they consuming some other organism to obtain the needed compound? Now that the timeline for bioluminescence has been refined for the octocorals, researchers can start diving deeper into how the ability changed through the eons.

Evolving the ability to glow in the shallows likely opened the pathway for octocorals to expand further and further into the depths with time. “The most common octocoral families found throughout the dark, deep sea are known to bioluminesce,” DeLeo says, hinting that the ability was important for the animals growing in the dark.

Ancient octocorals likely gained bioluminescence ability because of a biological quirk. “Bioluminescent light was likely a byproduct of a more ancient antioxidant-like pathway,” DeLeo says, or the evolution of a way for animals to mitigate damage to their bodies. As the pathways evolved and became more refined, DeLeo says, circumstances still favored bioluminescence.

“Bioluminescence can help animals evade predation, attract prey and even communicate with other individuals,” DeLeo says. A biological quirk more than half a billion years ago would eventually have the seas glowing from the shallows to the depths.

Riley Black | | READ MORE

Riley Black is the author of The Last Days of the Dinosaurs and many other books. She is a science correspondent for Smithsonian magazine covering fossils and natural history, and she writes about the prehistoric past for a variety of publications.