by Octocorals can create coral gardens and animal forests in the oceans, especially in the deep sea. These communities provide homes and nursery habitats for many other animals, including fish and sharks.
All octacorals use the same chemical reaction to bioluminesce. A 2022 study determined the evolutionary relationships between these corals. These genetic links, and the fact that octocoral fossils exist, make these animals an ideal focus for investigating when bioluminescence originated and how it spread over geologic time.
Test for bioluminescence at sea
More than ten years ago, we started testing the ability of different octocoric species to bioluminesce. To produce the glowing light, corals must be physically or chemically stimulated.
Bioluminescence first piqued our curiosity during a 2014 research cruise on the R/V Celtic Explorer over the Whittard Canyon off the southwest coast of Ireland. We were taking a tissue sample of bamboo coral collected from the deep sea floor by a remote vehicle.
The vehicle had manipulator arms that allowed the pilot to collect coral specimens and place them in sampling containers to keep the organisms alive and protected as the vehicle surfaced. After this sample arrived on board the ship, we used forceps to remove one coral polyp from it in a dimly lit room and saw the flash of blue light.
Since then, we have worked with collaborators from the Monterey Bay Aquarium Research Institute and Tohoku University to record the species that can glow, either on the ship after collection or when we observe them on the seabed using low-light cameras. Combined with previously published records, we now know that bioluminescence occurs in about 60 coral species. There are probably many more waiting to be discovered.
When and why did bioluminescence emerge
In a study published in April 2024, we presented the oldest record in geological time for bioluminescence in the world. We showed that this chemical reaction occurred several thousand years earlier than previously estimated, around the time when life on Earth changed rapidly over 540 million years ago in a period known as the Cambrian Explosion. We determined this by mapping the presence of bioluminescence on the octachored tree of life, a graphical tool used by biologists to show evolutionary relationships among species.
Originally, bioluminescence may have evolved to reduce free radicals – chemically unstable atoms that can damage cells. However, at some point, it changed to a form of communication.
Our results suggest that light signaling is the earliest form of communication in the oceans, and we know that some animals that could detect light appeared during the Cambrian period. Our research shows that light-related interactions among species occurred during a period when animals were rapidly diversifying and occupying new habitats.
Gaining and losing light
We are continuing to test corals for bioluminescent capabilities in a variety of ways. One key component involved in producing light 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 that will make it easier and less invasive for us to study this trait.
We have preliminary evidence that non-biclonal 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 light at a very low level that scientists cannot detect with current methods? Or are their luciferase genes non-functional? Further studies may reveal why certain octocorals appear to have lost the ability to bioluminesce, and how this loss may affect their survival in different habitats.
Our recent findings show that many corals that live in shallow waters but evolved from deep-water ancestors have retained the ability to bioluminesce. Some corals may have lost this ability over time because it was less useful in shallower ocean settings with more light.
We are also investigating how bioluminescence has evolved in other creatures, including shrimp that move 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 many unique ways.
As one notable example, some shrimp vomit chemicals that make light, creating a luminous spew to deter predators. They also have external bioluminescent light organs along their bodies that produce blue light.
Studying such creatures improves our understanding of how different amounts of light in the environment, including light produced by organisms, affect the evolution of bioluminescence and affect the vision of organisms. This can provide insight into the impact of bioluminescence on the evolution of eyes 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 contributed greatly to their survival. Furthermore, our results support the idea that bioluminescence is a critical form of communication through geological time for many types of animals, especially in the deep sea.
This research has inspired us with new ideas about the evolution and early communication of animals. Light signaling provided a new way for animals to communicate in a rapidly changing time, when new predators and a more complex landscape were emerging. Increased sensory capacity in the ocean may be valuable in these conditions. Perhaps bioluminescence is a missing piece of the answer that has not yet been given full attention in studies of the origin and evolution of animals in deep time.
This article is republished from The Conversation, a non-profit, independent news organization that brings you facts and analysis to help you make sense of our complex world.
It was 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 Atmospheric Administration Office of Ocean Exploration, and the National Science Foundation.
Danielle DeLeo does not work for, consult with, or own shares in, or receive funding from, any company or organization that would benefit from this article, and has disclosed no relevant connections beyond their academic appointment.
