Bioluminescence can be defined as the production and the emission of light by a living organism. It is one of the natural phenomena that have successfully amazed our species for ages -so much that this amazement has even led to mythological stories from the ancient times about glowing scary creatures in the depths of the ocean. One example for this could be quoted from the source of the Jewish theology, the Talmud:
“The body of the Leviathan, especially his eyes, possesses great illuminating power. By his needing a light doth shine, and his eyes are like the eyelids of the morning.”
In this ancient text, Leviathan is believed to be a primordial sea serpent which has, as described, luminous eyes. From this piece of mythological text, it can be understood that our species’ curiosity for luminous beings in nature has started very long time ago. The ancient science was even seen as fantasy throughout history. Travelers from 18th-19th century wrote about glowing waters of the ocean, which was dismissed as fantasy.
Figure I: Dalatias licha, also known as the Kitefin shark, is the largest vertebrate known to possess luminous abilities [1].
What is Bioluminescence?
In the deep sea, where there is no sunlight at sight, everything is thought to be pitch black. While this is correct, the deep sea is not truly devoid of light. Instead, the organisms down below have found a way to glow themselves through bioluminescence either by harnessing bioluminescent bacteria or initiating a chemical reaction within their bodies to form bioluminescent properties. One would think that the darkness of the oceans’ floor is the cause of bioluminescence, but it is observed that the proportion of animals that glow is similar at all levels of the ocean [3]. Hence, unlike the popular belief, bioluminescence in the sea is not a unique property of an organism, in contrast, it is a widespread adaptation in the sea where more than 75% of organisms are bioluminescent [3]. The question is, why is bioluminescence so common in the ocean? It has been observed that many marine organisms use bioluminescence as a defense mechanism to scare away predators, and some organisms like red crown jellyfish Atolla wyvillei, seen in figure III, uses its luminescent properties as flashing lights when faced with a predator to potentially alarm other predators that might eat what is attacking the jelly. This defense mechanism is often called as a “burglar alarm”[4].
Figure III: Atolla wyvillei flashing its bioluminescent light [5].
Other organisms like the female anglerfish use their symbiotic bioluminescence as a trap to lure prey in. Other usage of the bioluminescence adaptation in marine life is communication. Humboldt squid, Dosidicus gigas, have been observed to produce different luminous patterns when faced with different situations. Some scientists thought that this patterned luminescence might be a way for the squid to communicate with other Humboldt squids in the group [4, 6].
Figure IV: Flickering patterns of Dosidicus gigas [7].
In marine environments, bioluminescence is abundantly used for communication and defense, however, the majority of luminous organisms’ evolutionary origins and timing of such adaptations remain enigmatic. Bioluminescence is defined as the chemical reaction between the substrate luciferin and the enzyme luciferase in the presence of molecular oxygen which results in the emission of light in an organism [8]. Among lineages that share a common evolutionary origin of bioluminescence, luciferins and luciferases are identical and homologous (with exceptions in some marine organisms). A recent study suggests that the ability to bioluminesce has evolved independently more than 100 times across the tree of life.One recent research has used a phylogenomic approach and ancestral state reconstruction, thus, provided evidence for a single origin of bioluminescence in Octocorallia and inferred the age of occurrence into Cambrian era: approximately 540 million years ago. This data stands as the earliest timing of emergence of bioluminescence in marine environment.Octocorallia are a major anthozoan group of marine luminous organisms. Anthozoan Cnidarians are highly diverse, and its species can be found throughout the worlds’ oceansfrom the shallow water to the bottom of the sea [8,9].
An organism has two options when producing bioluminescence:
Organisms that use intrinsic bioluminescence mix specific chemicals in their own bodies to produce photons of various colors and intensities [10]. Meanwhile, symbiotic bioluminescence requires the organism to cultivate specific bacteria in specialized organs.The female anglerfish’s bioluminescent properties can also be given as an example of symbiotic bioluminescence [10].
The bioluminescence mechanisms vary greatly; however, instead of explaining them, I will rather ask the question “Why is bioluminescence so common in the sea?” again. This question is also followed by “What initiates bioluminescence?”. According to various scientists who study bioluminescence, it is not the darkness of the sea that initiates the bioluminescence itself. If it was, then why is bioluminescence so rare on land even though most of the land ecosystems are in dark half of the time? Some believe that it is the toxicity, since some byproducts of bioluminescence reactions can be toxic to the organism; and this toxicity is mostly removed from the organism in the sea with flowing water. Nevertheless, this does not explain the cause either. Freshwater bioluminescence is basically non-existent even though freshwater also flows through the organism and cleanses it. Some researchers believe it might be explained with time since life in the ocean is much more ancient than life on land or life in freshwater. So perhaps it is not that life on land and freshwater do not have bioluminescence; it is that they do not have bioluminescence yet.
References
[3]J. Woodland Hastings, “Bioluminescence,” Cell Physiology Source Book, pp. 665–681, Jan. 1995, doi: 10.1016/B978-0-12-656970-4.50054-3.
[4]J. Vacquié-Garcia, J. Mallefet, F. Bailleul, B. Picard, and C. Guinet, “Marine Bioluminescence: Measurement by a Classical Light Sensor and Related Foraging Behavior of a Deep Diving Predator,” Photochem Photobiol, vol. 93, no. 5, pp. 1312–1319, Oct. 2017, doi: 10.1111/PHP.12776.
[6]B. P. Burford and B. H. Robison, “Bioluminescent backlighting illuminates the complex visual signals of a social squid in the deep sea,” Proc Natl Acad Sci U S A, vol. 117, no. 15, pp. 8524–8531, Apr. 2020, doi: 10.1073/PNAS.1920875117/SUPPL_FILE/PNAS.1920875117.SM01.MP4.
[8]J. Lee, “Perspectives on Bioluminescence Mechanisms,” Photochem Photobiol, vol. 93, no. 2, pp. 389–404, Mar. 2017, doi: 10.1111/PHP.12650.
[9]D. M. DeLeo, M. Bessho-Uehara, S. H. D. Haddock, C. S. McFadden, and A. M. Quattrini, “Evolution of bioluminescence in Anthozoa with emphasis on Octocorallia,” Proceedings of the Royal Society B, vol. 291, no. 2021, Apr. 2024, doi: 10.1098/RSPB.2023.2626.
[10]S. H. D. Haddock, M. A. Moline, and J. F. Case, “Bioluminescence in the sea,” Ann Rev Mar Sci, vol. 2, no. 1, pp. 443–493, Jan. 2010, doi: 10.1146/ANNUREV-MARINE-120308-081028/1.
Figure References
[1]“The Kitefin Shark Is The World’s Largest-Known Luminous Vertebrate Book Review and Ratings by Kids – Emily Moulin.” Accessed: Apr. 28, 2024. [Online]. Available: https://www.dogonews.com/2021/4/8/the-kitefin-shark-is-the-worlds-largest-known-luminous-vertebrate
[2]“Can bioluminescence cause the St. Lawrence to glow? – Baleines en direct.” Accessed: Apr. 28, 2024. [Online]. Available: https://baleinesendirect.org/en/can-bioluminescence-cause-the-st-lawrence-to-glow/
[5] “Real Monstrosities: Atolla Jellyfish.” Accessed: Apr. 28, 2024. [Online]. Available: http://www.realmonstrosities.com/2011/12/atolla-jellyfish.html
[7]“Deep-Sea Squids Glow to Communicate in the Dark | Science| Smithsonian Magazine.” Accessed: Apr. 28, 2024. [Online]. Available: https://www.smithsonianmag.com/science-nature/humboldt-squid-glow-communicate-dark-180974576/
