Although the best known example of bioluminescence is the firefly, there are few terrestrial organisms that exhibit this quality. In contrast, it is in the immensity of the oceans, in the depths of the sea where light cannot penetrate, that this phenomenon manifests itself in all its splendour, to the point that it is estimated that more than three quarters of the living beings that inhabit this environment glow in the perpetual darkness. More than a few of these luminous creatures exhibit surprising adaptations.
This ability of certain creatures to produce light through a chemical reaction arose in the marine environment on multiple occasions and in different organisms, evolving independently. This theory agrees with the recent discovery, published last August, which explains that the unique biofluorescence exhibited by certain species of catsharks is generated by a novel family of metabolites (chemical substances produced in the body itself, during processes such as respiration or digestion) not identified until recently, and which are present exclusively in the dark stripes of the skin of these sharks—specifically the chain catshark (Scyliorhinus retifer) and the swell shark (Cephaloscyllium ventriosum).
The result is a biofluorescence that, apparently, can only be detected by other specimens of the same species. For this reason, biologists assume that its main functions are those of communication and identification and the search for potential mates.
In the marine environment, blue radiation is the colour that transmits best and reaches the greatest distance, while red radiation is rapidly absorbed. This means that almost all organisms living in this environment only perceive the green and blue regions of the spectrum, and therefore their bioluminescence is restricted to that region. This is also the reason why many marine species have red pigmentation, which renders them invisible to their predators.
A notable exception is the deep sea dragon fish of the Malacosteidae family, which are capable of producing a red bioluminescence only for their eyes, which have evolved to be able to perceive this colour. This is something that they achieve through a unique adaptation of their photophore organs (where bioluminescence is generated) that are found on their faces. Within them, the mechanism of light production is in principle analogous to that of other bioluminescent organisms: they produce blue radiation that is, however, absorbed by a fluorescent molecule that reemits this radiation in the range of red wavelengths. What’s more, the photophore is covered by a membrane that acts as a filter and only allows the passage of red light. In this way they can sweep their deep water surroundings with a light that is invisible to all the other creatures and hone in on their prey without them being aware of it.
The firefly squid (Watasenia scintillans) is equipped with a complete set of hundreds of tiny photophores that cover its entire body and tentacles and emit an intense blue light. Like many bioluminescent organisms, the firefly squid is capable of controlling these photophores to emit continuously or in flashes and to light up by regions —in unison or sequentially— to create a wide variety of patterns and light signals.
Biologists believe this species uses them for different purposes, including to communicate with their fellow firefly squids. In fact, scientific studies indicate that, as with terrestrial fireflies, females choose one male or another according to their particular symphony of light. This would support the fact that this squid species is the only one capable of seeing in colour, having large eyes equipped with the three types of photoreceptor cones and a sophisticated retina.
In addition, this pattern of hundreds of flashing dots of light also serves as camouflage, hiding and distorting its silhouette. While some light signals serve to distract its predators, others are used as a lure to attract its prey.
Green bomber worm
The swimming worm Swima bombiviridis, which lives deep in the sea, has the extraordinary ability to drop luminescent “bombs”. These projectiles are modified regions of their gills, like sacks or spheres, filled with a fluid capable of producing luminescence. When they come loose, the “bomb” is activated and causes an intense burst of light that lasts several seconds before fading little by little, a distraction for its predators that the marine worm takes advantage of to escape by swimming in the opposite direction to the light explosion.
This mechanism of distraction and escapism is similar to the one used by the squid Octopeuthis deletron, endowed with tentacles that it is able to release when it feels sufficient pressure on them, this is to say, when something catches them. When this happens, the released limb contorts for a dozen seconds, while emitting an intense bioluminescence that confuses the attacker and gives the injured squid a chance to escape.
Hawaiian bobtail squid
The Hawaiian bobtail squid (Euprynma scolotes) is one of the most sophisticated examples of symbiotic bioluminescence, that is, when it is not the organism itself that produces light through its metabolism, but rather acts as a host for bioluminescent bacteria that colonize its phosphors in exchange for protection and food. In the case of the Hawaiian bobtail squid and its symbiont, the bacteria Vibrio fischeria, these photophores are distributed along the entire mantle of the cephalopod. During the day, this Pacific Ocean squid remains buried under the sandy seabed; when it gets dark, it ascends the water column in search of food.
It is then that its network of photophores comes into action. The light they emit is equivalent to that filtered by the moon, achieving a counter-illumination that allows it to “erase” its silhouette and thus hide its presence from potential predators. Like almost all symbiotically luminescent organisms, at birth the squid lacks these bacteria, so during the first hours of life it secretes a mucus that captures them.
In the case of the Hawaiian bobtail squid, it has been learned that its symbiotic relationship with its bacteria goes even further, since they also control the circadian rhythms of the cephalopod: the bacteria are only activated and light up at dusk, and it seems that it is this particular light emitted by the bacteria—with a certain wavelength— that triggers the mechanisms that regulate the squid’s internal clock.
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