People have long been fascinated by organisms that can produce light. Aristotle, a scientist as well as philosopher, wrote the first detailed descriptions of what he called “cold light” more than 2,000 years ago. More recently, pioneering researchers such as World War II Army veteran Emmett Chappelle and deep submersible pilot Edith Widder have advanced the study of this phenomenon with new technologies.
At least 94 living organisms produce their own light through a chemical reaction in their bodies, an ability called bioluminescence. Examples include glowing fireflies, algae that form “glow-in-the-dark” bays, small crustaceans that perform complex courtship displays, and deep-sea fish and corals. But despite its widespread occurrence, scientists do not yet know when or where it first appeared or its original function.
As marine biologists specializing in deep-sea habitats, we know that bioluminescence is especially common in the oceans. This suggests that light production may provide a fitness advantage to organisms across the tree of life that increases their chances of survival.
Our research focuses on octocorals, which are soft-bodied corals like sea fans that have tree-like shapes and are found in various parts of the world’s oceans. They are a very diverse and ancient group of animals, containing approximately 3,500 species, many of which are bioluminescent.
Octocorals can create coral gardens and animal forests in the oceans, especially in the deep sea. These communities provide homes and breeding habitat for many other animals, including fish and sharks.
All octocols use the same chemical reaction to bioluminescence. A study conducted in 2022 determined the evolutionary relationships between these corals. These genetic connections and the availability of octocol fossils make these animals an ideal focus for investigating when bioluminescence emerged and how it spread through geological time.
Bioluminescence testing at sea
More than a decade ago, we began testing the ability of different types of octocol to bioluminescence. To produce glowing light, corals must be physically or chemically stimulated.
Bioluminescence first sparked our curiosity in 2014 during a research trip aboard the R/V Celtic Explorer over the Whittard Canyon on the southwest coast of Ireland. We were taking tissue samples from bamboo coral collected from the deep sea floor with a remote-controlled vehicle.
The vehicle had manipulator arms that allowed the pilot to collect coral samples and place them in sampling containers to keep the organisms alive and protected once the vehicle surfaced. After this sample came aboard, we picked up a single coral polyp using forceps in a low-lit room and saw a flash of blue light.
Since then, working with collaborators from the Monterey Bay Aquarium Research Institute and Tohoku University, we have recorded which species can glow after being collected while observing them on board or on the seafloor using low-light cameras. Combined with previously published records, we now know that bioluminescence occurs in approximately 60 coral species. Probably many more are waiting to be discovered.
When and why did bioluminescence appear?
In a study published in April 2024, we presented the oldest record in geological time of bioluminescence on Earth. We show that this chemical reaction developed several thousand years earlier than previously thought, when life on Earth was rapidly diversifying 540 million years ago during the so-called Cambrian Explosion. We determined this by mapping the presence of bioluminescence on the octagonal tree of life, a graphical tool that biologists use to illustrate evolutionary relationships between species.
Originally, bioluminescence may have evolved to reduce free radicals, chemically unstable atoms that could damage cells. But at some point this turned into a form of communication.
Our results show that light signaling is the oldest form of communication in the oceans, and we know that some animals that can detect light evolved during the Cambrian period. Our research shows that light-related interactions between species occurred at a time when animals were rapidly diversifying and occupying new habitats.
Gaining and losing light
We continue to test corals for their bioluminescent abilities in a variety of ways. One of the main components involved in light production in corals and other animals is an enzyme called luciferase. Using DNA sequence data, we are developing a test for bioluminescence genetic potential that will make it easier and less invasive for us to study this trait.
We have preliminary evidence that non-bioluminescent octocorals still have homologous luciferase genes (genetic instructions passed down from the common ancestor of all octocorals). It is a mystery why corals that cannot produce light retain these genes.
Do they produce very low levels of light that scientists cannot detect with current methods? Or are the luciferase genes dysfunctional? Further studies may show why some octocorals appear to have lost the ability to bioluminescence and how this loss may have affected their survival in different habitats.
Our latest results show that many corals that live in shallow waters but are derived from deep-water ancestors retain the ability to bioluminescence. It is possible that some corals may have lost this ability over time, making them less useful in shallow ocean environments where there is more light.
We are also investigating how bioluminescence has evolved in other creatures, including shrimps that migrate upstream from deep waters to feed during the day and return to deeper waters at night. These animals are exposed to changing light conditions and produce light in many unique ways.
In one notable example, some shrimp vomit light-emitting chemicals, creating a glowing vomit to ward off predators. They also have external bioluminescent light organs that produce blue light throughout their bodies.
Studying creatures like these improves our understanding of how different amounts of light in the environment, including the light produced by organisms, affects the evolution of bioluminescence and affects the organisms’ vision. This may provide insight into how bioluminescence influenced eye evolution and vision around 540 million years ago, when life on Earth was diversifying.
The fact that corals have been able to produce light for hundreds of millions of years suggests that this ability contributes significantly to their survival. Additionally, our findings support the idea that bioluminescence has been a critical form of communication for many animal species throughout geological time, especially in the deep sea.
This research sparked new ideas for us about early animal evolution and communication. Light signaling gave animals a new method of communication in a rapidly changing time when new predators and a more complex environment were emerging. Increased sensory capabilities in the ocean could be valuable in these conditions. Perhaps bioluminescence is a missing piece of the puzzle that has not yet received full attention in studies of the origin and deep-time evolution of animals.
This article is republished from The Conversation, an independent, nonprofit news organization providing facts and authoritative analysis to help you understand our complex world. Written by Danielle DeLeo Florida International University and Andrea Quattrini, Smithsonian Institution
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Andrea Quattrini receives funding from the Smithsonian Institution, the National Oceanic Atmospheric Administration Office of Ocean Research, and the National Science Foundation.
Danielle DeLeo does not work for, consult for, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond her academic duties.