Some species of luminescent fish live in deep waters because bioluminescence allows them to camouflage themselves, communicate, attract prey, or repel predators in the darkness of the abyss.
At great depths, the pressure can reach several hundred times that at the surface, an extreme situation for living organisms. To survive, luminescent fish possess specific biological structures, such as soft and flexible tissues, capable of withstanding this enormous compression without damage. Their bodies are often devoid of air-filled cavities, which prevents them from being crushed under the intense pressure. At these depths, their proteins and cell membranes have also evolved to function effectively despite significant physical constraints. Some fish also exhibit a compact shape, thereby reducing their exposure and vulnerability to these extreme forces. It is these remarkable adaptations that allow them to navigate easily where few other animals can survive.
At great depths, some fish use an astonishing phenomenon called counter-illumination. Essentially, their belly produces a soft light, mimicking that from the surface. When a predator looks up from below, this faint light makes the fish almost invisible, blending in with the slight halo coming from above. Other species employ a more subtle camouflage by precisely adjusting the color and intensity of their light according to their immediate environment. As a result, they completely evade detection by their predators, who see nothing but darkness all around. Convenient, right?
In the dark depths of the ocean, some fish use their bioluminescence to seduce and attract a mate. In a few species, females send light signals in the form of precise flashes to signal their presence. Males respond with their own illumination, creating a discreet and effective visual language. This strategy helps quickly identify individuals of the same species in total darkness, thus avoiding awkward misunderstandings. Essentially, the clearer and more precisely a fish lights up according to the right rhythm, the better its chances of reproducing!
Some species of fish living in the dark depths use bioluminescent lures to attract their prey. The coolest example is probably the abyssal anglerfish: it has a kind of little glowing fishing rod fixed to the top of its head, which it gently wiggles to entice its victims. Curious, the prey approaches this attractive light point, unaware, thinking it has found an easy meal. And then, bam, the anglerfish opens its jaws wide and quickly sucks in the careless prey. Other species illuminate certain parts of their bodies to mimic small marine organisms, thereby encouraging the prey to come close enough to become the next bite.
Some deep-sea fish use their light as a deterrent weapon. When they spot a predator, these fish can suddenly trigger powerful light flashes to dazzle or disorient the attacker. Others project a luminous cloud into the water or release bioluminescent fluids to confuse predators, who temporarily lose their visual bearings. This brief moment of confusion allows the target fish to slip away discreetly and avoid being an improvised snack.
Bioluminescence represents the most widespread source of light in the oceans; it is estimated that more than 75% of deep-sea marine animals are capable of emitting a form of light.
The black dragonfish (Idiacanthus atlanticus) uses a red light (rarely perceived by other marine species) as a discreet torch to illuminate its prey without being detected.
Some species of deep-sea fish use a bioluminescence technique called 'counter-illumination' by illuminating their bellies, which allows them to blend in with the faint light coming from the surface, thereby becoming nearly invisible to their predators below.
The famous lanternfish (family Myctophidae) are so numerous that they could account for over 65% of the deep-sea biomass, making them one of the most abundant groups of vertebrates on Earth.
Most bioluminescent fish are found at depths greater than approximately 200 meters, an area commonly referred to as the "mesopelagic zone" and beyond. At these depths, sunlight becomes almost entirely absent, which promotes the evolution of bioluminescence.
Bioluminescence itself is not dangerous for humans, as it is neither caused by radioactivity nor by toxic chemicals harmful to humans. However, some luminescent fish may have defenses or dangerous characteristics independent of their ability to produce light, such as spines or mildly irritating substances.
No, bioluminescence does not exist in all species living in deep waters. Only certain species have the necessary biological adaptations to produce this luminescence. It is a very specific and complex evolutionary adaptation that has emerged in certain groups to ensure their survival.
Bioluminescence is generally very energy-efficient. Luminescent fish use chemical reactions that produce very little heat and a cold light, without wasting much energy, making this method very suitable for their limited energy environment in the abyss.
The bioluminescence of deep-sea fish does not primarily serve to help them see better. On the contrary, it mainly acts as a means of camouflage, communication, or prey attraction in the darkness of the ocean depths.
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