Some fish from the deep sea produce light through a phenomenon called bioluminescence. They host luminescent bacteria or possess specialized cells that emit light through a chemical reaction, which can be used to attract prey, communicate, or camouflage themselves.
In the marine depths where sunlight does not penetrate, some fish use bioluminescence to attract prey, escape predators, or find a mate. For example, the classic trick: creating a small glowing lure at the end of a filament, like the famous anglerfish does. Others use a subtle method called counter-illumination, which involves producing light on their belly similar to that coming from above, making their silhouette invisible from below. This is useful for dodging attacks. Some predators suddenly light up their surroundings to surprise their meals. In short, in these dark depths, producing light is often vital for eating, avoiding being eaten, and meeting other marine creatures.
In bioluminescent fish, everything relies on a simple yet effective chemical reaction: the interaction of a molecule called luciferin with a protein, luciferase. When these two elements meet, it produces a chemical reaction, thus releasing cold light (it hardly heats up!). Some species also use symbiotic bacteria housed in small special organs called photophores. These bacteria do the job of producing light for them. To control the intensity or blinking, fish have specialized structures that allow them to regulate the influx of oxygen or temporarily mask the emitted light. Practical and efficient, plain and simple.
Many deep-sea fish have particularly large eyes, sometimes enormous, to capture the little light available. Those that live in very deep water often have eyes specifically adapted to perceive blue or green light, the colors that penetrate best into the ocean depths. Other species, on the contrary, have completely abandoned sight: what good are eyes in total darkness? These fish have developed highly advanced senses for vibrations or movements in the water, thanks to multiple highly sensitive sensory organs. Some adopt reduced or nearly transparent pigmentation, or even ultra-matt black skin that absorbs the slightest bit of light, in order to remain unnoticed by predators or prey. Finally, many abyssal animals drastically reduce their metabolism: they move slowly, breathe slowly, and conserve their energy as much as possible in this cold, food-poor, and completely dark environment.
In the total darkness of the depths, some fish use bioluminescence to communicate. With these light signals, they can attract a partner for reproduction or mark their territory. Other creatures, a bit more cunning, take advantage of this to naively lure their future prey with a small, fascinating glow. Even more fascinating, some fish have developed techniques to confuse predators: by illuminating their bellies from below, they erase their silhouette against the faint light that timidly comes from above. And then, there are those predators that shine to imitate prey and deceive other hunters — a perfect trick underwater!
Some fish use bioluminescence to camouflage themselves, a strategy called 'counter-illumination.' They produce light under their bellies so that their silhouettes disappear by perfectly blending in with the ambient light coming from above.
The vampire squid (Vampyroteuthis infernalis), despite its unsettling name, is a relatively harmless animal. It has the astonishing ability to project a luminous substance in the form of a shimmering cloud to disorient its attackers or escape easily.
The color of bioluminescent emissions varies from one marine species to another: some primarily emit blue light, a color that propagates efficiently in the ocean depths, while others emit red light to communicate discreetly, as red light is nearly invisible to most marine creatures.
Marine bioluminescence accounts for about 90% of the total bioluminescence observable on Earth. This high presence is primarily explained by the need to attract prey, communicate, or evade predators in the dark environment of the ocean depths.
Some deep-sea fish use bioluminescence as a lure to attract their prey. For example, the abyssal anglerfish uses a luminous appendage located on its head to lure its prey, while others emit bright light to startle or disorient small organisms before hunting them.
Generally, bioluminescence is an evolutionary advantage, but it can also be risky. A bioluminescent fish is potentially visible to nearby predators. As a result, many fish can turn off or temporarily mask their light to avoid being easily detected.
Besides certain fish, various marine organisms such as jellyfish, squids, plankton (like dinoflagellates), certain marine worms, and even bacteria can also produce light through bioluminescence.
Researchers use submarines, remotely operated robots, or specially designed fishing nets to preserve bioluminescent capabilities. In the laboratory, they then study the biochemical and genetic functioning of light-producing organs to better understand these unique adaptations.
No, not all abyssal fish have the ability to bioluminesce. However, many species, such as the abyssal anglerfish or lanternfish, possess specific light-producing organs (photophores) to survive in their dark habitat.
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