People have long been fascinated by organisms that can produce light. Aristotle, who was both a scientist and a 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 immersion 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 bioluminescent fireflies, algae that create glow-in-the-dark bays, small crustaceans with elaborate courtship displays, and deep-sea fish and coral. But despite its widespread occurrence, scientists still don't know when and where it first emerged, or what its original function is.
As marine biologists who specialize in deep-sea habitats, we know that bioluminescence is especially common in the ocean. This indicates that light production can give organisms from across the tree of life a fitness advantage that increases their chances of survival.
Our research focuses on octocorals - soft corals, such as sea fans, that have tree-like shapes and occur in various places in the world's oceans. They are a diverse and ancient group of animals that includes 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 nurseries for many other animals, including fish and sharks.
All octocorals use the same chemical reaction to produce bioluminescence. A 2022 study determined the evolutionary relationships between these corals. These genetic connections, and the fact that fossils of octocorals exist, make these animals an ideal focus to investigate when bioluminescence appeared and how it spread over geological time.
Testing for bioluminescence at sea
More than a decade ago, we began testing the ability of various octocoral species to exhibit bioluminescence. To produce the glowing light, corals must be stimulated physically or chemically.
Bioluminescence first piqued our curiosity during a 2014 research cruise on the R/V Celtic Explorer across the Whittard Canyon off the southwest coast of Ireland. We took a tissue sample from a bamboo coral, collected from the deep seabed by a remote-controlled vehicle.
The vehicle had manipulator arms that allowed the pilot to collect coral specimens and place them in sample containers to keep the organisms alive and protected while the vehicle surfaced. After this monster came aboard the ship, we used forceps to extract a single coral polyp in a dimly lit room and saw a flash of blue light.
Since then, we've worked with collaborators at the Monterey Bay Aquarium Research Institute and Tohoku University to document which species can glow, either on the ship after collection or while we observe them on the seafloor using cameras in low light. Combined with previously published data, we now know that bioluminescence occurs in approximately 60 coral species. There are probably many more waiting to be discovered.
When and why bioluminescence emerged
In a study published in April 2024, we presented the oldest record in geological time for bioluminescence on Earth. We have shown that this chemical reaction evolved several millennia earlier than previous estimates, around the time that life on Earth was rapidly diversifying more than 540 million years ago in a period called the Cambrian Explosion. We determined this by mapping the presence of bioluminescence on the octocoral tree of life, a graphical tool biologists use to show evolutionary relationships between species.
Bioluminescence may have initially evolved to reduce free radicals - chemically unstable atoms that can damage cells. However, at some point it evolved into a form of communication.
Our results indicate that light signaling was the earliest form of communication in the oceans, and we know that some animals that could detect light evolved during the Cambrian. Our research indicates that interactions involving light occurred between species at a time when animals were rapidly diversifying and occupying new habitats.
Gaining and losing light
We continue to test corals for bioluminescence in various ways. One key component involved in light production in corals and other animals is an enzyme called luciferase. Using DNA sequence data, we are developing a test for the genetic potential for bioluminescence, making 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 a common ancestor of all octocorals. Why corals that cannot produce light have retained these genes is a mystery.
Do they produce very low-level light that scientists cannot detect with current methods? Or are their luciferase genes non-functional? Further research could reveal why certain octocorals appear to have lost the ability for bioluminescence, and how this loss may have affected their survival in different habitats.
Our recent results show that many corals that live in shallow waters but evolved from deep-water ancestors retained the capacity for bioluminescence. It is possible that some corals have lost this ability over time, as it became less useful in shallower ocean environments with more light.
We are also investigating how bioluminescence has evolved in other creatures, including shrimp that migrate up from deep water to feed during the day and return to deep water at night. These animals are exposed to changing light conditions and produce light in multiple, unique ways.
A notable example: Some shrimp regurgitate light-producing chemicals, creating a luminous beam to ward off predators. They also have external bioluminescent light organs along their bodies that produce blue light.
Studying these types of creatures improves our understanding of how different amounts of light in the environment, including light produced by organisms, influence the evolution of bioluminescence and affect 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 implies that this ability has contributed significantly to their survival. Furthermore, our findings support the idea that bioluminescence has been a crucial form of communication over geological time for many species of animals, especially in the deep sea.
This research has provided us with new ideas about the early evolution and communication of animals. Light signaling gave animals a new way to communicate in a rapidly changing time, with new predators and a more complex landscape emerging. Increased sensory capabilities in the ocean could have been valuable under these conditions. Perhaps bioluminescence is a missing piece of the puzzle that has not yet received full attention in studies of the origins and evolution of animals in deep time.
This article is republished from The Conversation, an independent nonprofit organization providing facts and trusted analysis to help you understand our complex world. It was written by: Danielle DeLeo, Florida International University and Andrea Quattrini, Smithsonian attitude Read more: Andrea Quattrini receives funding from the Smithsonian Institution, the National Oceanic Atmospheric Administration Office of Ocean Exploration, and the National Science Foundation. Danielle DeLeo does not work for, consult with, own stock in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
