Animals that live by day and in places where sunlight reaches unhindered rarely notice, but among nocturnal animals, or among those that live in dark places, a phenomenon occurs with relatively unusual frequency that does not stop surprise us. In biology it is called bioluminescence , the ability of some organisms to emit light with their own body.
The biochemical and physiological mechanisms responsible for the same function may be different between different groups of living beings. In most of the large groups, the genes and proteins involved in the bioluminescence process are unrelated and evolved independently . It is estimated that bioluminescence, throughout the long and extensive history of the evolution of life, may have arisen up to 30 times.
However, the most common is that the process happens thanks to the chemical oxidation reaction of a reagent, which in each group can be different, but whose family is called luciferin —from the Latin, ‘that carry light’—, and through the action of an enzyme, which is called luciferase —the ending ‘-ase’ indicates that it is an enzyme, a kind of biological catalyst—. These chemical reactions emit photons, which ultimately make up light. There are bacteria, dinoflagellates and even bioluminescent fungi. And of course, there are also several groups of animals with that ability. Of these, the best known example is that of the fireflies, whose name comes from the Latin lucerna , which means ‘oil’ or ‘lamp’.
The fireflies, lamps in the forest
Among the fireflies , only the females, wingless —wingless—, are capable of shining, and they do so through the final region of the abdomen. The winged males flutter at night looking for those tiny points of light that illuminate the forest, like stars that show sailors the way home.
But although they are capable of emitting light, the name ‘oil lamp’ does not do them justice . A lamp burns a fuel, generally an oil or other type of fat that impregnates a cotton wick, and its flame is what lights up in the dark, like a tiny torch. However, the chemical process that allows fireflies to glow is different.
In a lamp, light and heat is emitted by the combustion of oil, which is an oxidation reaction. Fireflies also take advantage, as has already been explained, of oxidation to fulfill their most famous function, but instead of doing it by combustion, they use their own metabolism, with very low heat emission .
fish of the deep
In the abyssal zones of the sea , in the dark abyss where the sun does not reach, there is a strange ecosystem composed mainly of animals and microorganisms that feed on everything that precipitates from the parts where there is more light, and other animals that eat these first. Much of these animals are blind, but not all. In fact, some fish use the light they emit as a form of communication, claim, threat, or as bait to attract their prey.
Some do not emit light by themselves, but instead contain organs under their skin in which they maintain colonies of luminous bacteria in symbiosis. Although, many others, such as the frogfish of the genus Porichthys, do have their own light-emitting mechanism.
Among these luminous fish are the viper fish, the dragon fish, the demon fish , and many other animals with no less terrifying names, which live up to their nomenclature with their spooky appearance. Creatures that seem designed by HR Giger or taken from a novel by HP Lovecraft, and that could populate the nightmares of those who have seen them… if it weren’t for their size, most would fit in the palm of a hand .
Creatures of an almost alien aspect, that populate a world completely different from the one we know; a world of water and darkness, with a pressure that would crush a human being; a hostile world in which any creature can become prey to a larger one. Big eyes that distinguish colors, and huge mouths, with long and irregular teeth, transparent, and sharp as needles.
Terrors hidden in the darkness, masked by the dazzling brilliance of dancing lights, beautiful and attractive, that stand out in the dark.
Jellyfish with their own light
Fish are not the only inhabitants of the sea capable of emitting light. Certain species of jellyfish have a very particular bioluminescence ; They emit intermittent flashes of light that come from specialized cells located in the endoderm—the internal tissue of the jellyfish. Thanks to the fact that its body is practically transparent, it acts as a diffuser, and that light is visible from the outside, highlighting the bright points, and drawing the entire body of the jellyfish in the dark.
More specifically, light sources are often found at the base of the tentacles and along the inner edge of the umbel. It is assumed that the function of bioluminescence in jellyfish has a function of defense and dissuasion from possible predators.
A surprising peculiarity of jellyfish is the ability to emit light without the need for oxygen. As the luciferin reaction is an oxidation, this is a fascinating evolutionary feat. The luciferin reaction in jellyfish — called coelenterazine — is mediated by calcium ions. Animals, in the presence of oxygen, accumulate a peroxidized and stable version of the reagent —peroxycoelenterazine— , and then only need calcium to emit light, even without oxygen in the medium.
the vampire squid
Among cephalopods there are also cases of bioluminescence, perhaps the most fascinating being the vampire squid ( Vampyroteuthis infernalis ). Despite its name, this cephalopod, which is neither an octopus nor a squid, has notable differences from them. On the one hand, the number of arms; it has eight and not ten, it can regenerate them if it loses them, the inner part is covered with spines instead of suckers, and they are connected to each other by a membrane. Additionally, it has two long retractable filaments that are unique among cephalopods.
The luminescent organs of the vampire squid are located under the skin, called photophores , they are located mostly at the end of the arms. The skin has a series of transparent, lens-shaped cells that stretch or compress thanks to skin muscles, opening or closing the passage of light.
On the one hand, this animal can use light as a bait to attract its prey. However, perhaps the most surprising thing about the vampire squid is its defense system. Instead of ink, it emits a slimy, sticky substance, loaded with bioluminescent particles . When attacked, the vampire squid takes the opportunity to wrap its arms around itself, exposing a black and spiny surface to avoid being devoured, at the same time, it emits this shiny mucus, which adheres to the hunter, dazzling it, giving it away to other possible prey. , and making it, at the same time, an easy prey for other predators to see.
Ctenophores, lights and rainbows
Almost half of the ctenophore species are bioluminescent. Although this strange group of animals does not contain more than 170 known species.
Alien-looking, these animals are reminiscent of the creatures in James Cameron’s famous 1989 film The Abyss . In addition to emitting their own light through bioluminescence reactions, their eight rows of locomotive cilia seem to project the colors of the rainbow .
Although many claim that these rows of changing colors are also the product of bioluminescence, however, it is not a phenomenon of light emission, but of diffraction. The same type of diffraction that Newton discovered by passing a beam of white light through a prism. Indeed, these cilia diffract the white light that the ctenophore receives from the environment , changing its color depending on the orientation of the cilia —and it also changes depending on where you look at it from.
The bioluminescence of ctenophores is dim, blue or greenish in color, and is only visible when in great darkness.
References:
Mills, C. E. 2010. Ctenophores – some notes from an expert [University of Washington].
O’Day, W. T. et al. 1974. Aristostomias scintillans (Malacosteidae): A deep-sea fish with visual pigments apparently adapted to its own bioluminescence. Vision Research, 14(7), 545-550. DOI: 10.1016/0042-6989(74)90044-3
Robison, BH et al. 2003. Light Production by the Arm Tips of the Deep-Sea Cephalopod Vampyroteuthis infernalis. The Biological Bulletin, 205(2), 102-109. DOI: 10.2307/1543231
Woodland Hastings, J. 1995. 48 – Bioluminescence. En N. Sperelakis (Ed.), Cell Physiology Source Book (pp. 665-681). Academic Press. DOI: 10.1016/B978-0-12-656970-4.50054-3
