Sometimes I wonder about the space, extraterrestrial bodies and possibilities of living on another planet. There is something so fascinating about the dark and starry sky that it would be a tragedy to not go beyond the atmosphere’s limit. But I am curious to know, too, why our focus predominates on discovering the limits beyond Earth, rather than searching for what we already have? So many funds are dedicated to space discoveries, and only few are meant for the oceans. Why? After all, it is estimated that ~90% of marine species are waiting to be described, and can you tell me where they hide?

You were right – they are found in the abyss. We have this perception that the deep is inhabited by giant squids, occasional megalodons and odd monsters with pointy teeth or big eyes and, possibly, with a lamp hanging from their head, just like we saw in the movie Finding Nemo. Even though this is partially true, it does not necessarily mean that animals are enormous, scary and ready to come up to the surface to eat us. Deep sea animals have adopted different feeding and reproductive strategies, as well as other types of behavior in order to survive in such an inhospitable environment.

Before entering into complete darkness, there is a layer of water mass that stretches from about 200 to 1000 meters in depth called the “twilight zone.” Many open water inhabitants vertically migrate into these waters and those below -1000 meters during daytime, in order to avoid being seen by their predators found near the surface. The light here is dimmer and not as much directional as above the -200 meters. It is difficult to be seen, indeed, but it is also difficult to locate food, attract a mate and defend oneself against other predators, hence it is necessary to build some adaptations. The most entrancing is certainly bioluminescence.

Biochemically speaking, bioluminescence is an exergonic reaction that produces light instead of heat. So, basically, it involves the production of sufficient energy to excite a single state molecule that will generate a visible photon as it goes back down into its ground state. The primary mechanism consists in the breakdown of a peroxide bond, found within the substrate luciferin, by the enzyme luciferase. For example in bacteria, a reduced flavin mononucleotide (FMNH₂) and a long-chain aliphatic aldehyde (R-CHO) are oxidized by molecular oxygen and luciferase. The aldehyde is consumed during the reaction but is continuously synthesized by the bacteria, which results in persistent glow.

In some cases, accessory proteins serve as secondary emitters that shift the violet-blue colour of the bioluminescent emissions towards longer wavelengths (green, yellow, orange, red). An example is that of the comb jellyfish Beroe forskalii that you can see here. Another one is that of the lightless loosejaw Malacosteus niger, that can emit red light in order to be recognized by another individual of the same species, of which the retina has accessory pigments that can absorb it. In addition, it seems that the emittance of red light is also a strategy to illuminate preys that are blind to red light.

The luminescent molecules can be released directly into the water column, producing clouds of light to distract or blind the predator, as in the case of some crustaceans, squid, fish and jellies. Or they can be retained within special cells called photocytes, which along with accessory structures, form photophores (or light organs). Thus the waveband and angular distribution of light emitted can be adjusted by muscles and optical components that reflect or refract light.

The most common use of bioluminescence is probably a defensive mechanism against predators. Along with light clouds, some organisms are able to mark their primary predators with extracellular luminous mucilage that makes them noticeable to secondary predators (“Aha! You’ll die with me!”). At times, caught organisms produce flash displays that attract secondary predators, and by doing so, they get the chance to escape from the primary predators’ mouth. For instance, the copepod Acartia tonsa grazes the dinoflagellate Lingulodinium polyedrum that illuminates the copepod, therefore drawing the attention of its fish predator.

Within the twilight zone specifically, fish, squids and crustaceans have evolved a great technique to camouflage themselves. Their opaque silhouettes are replaced by bioluminescence of the same colour, intensity and angular distribution as that of the ambient light that penetrates into the depths. Since many predators have upward-looking eyes, their bodies are in this way hardly visible.

At last, special symbiosis has been observed between some bacteria and fish and squids. The bioluminescent bacteria permits the host to be visible to other co-specimens, or scare predators away, or attract preys, while the host provides an optimal habitat.

“In the ocean, bioluminescence is the rule rather than the exception.” – Edith Widder

In conclusion, there are many hypotheses on the evolution of this spectacular process; but for the sake of understanding the basics first, I have decided that I will return back to this topic sometime in the future. Maybe I will also be able to find more literature regarding the behavioural mechanisms and other ethological aspects.

The deep is certainly one of the most curious and overwhelmingly surprising environments on Earth. In the past, it was thought to be one of the most static areas, too, but we have the evidence now, that the Deep sea is far from being static. The questions do not only concern the species richness, but also the dynamics of biological, physical and chemical processes. Who knows if we will be able to discover them all.

“Every drop of knowledge sparks a light, illuminating an ocean of darkness teeming on the edge of brilliance.” – C. N. Hamilton