The ocean is still the most mysterious environment known to man, with vast changing conditions covering 70% of the world, reaching out from continental shorelines to the deep abyss. With the increase of depths comes the intense challenge of survival for organisms. Factors like decreasing temperatures, upwelling and downwelling of nutrients, increased pressure and overshadowing darkness beginning to consume areas past 200m. It is at this point known as the Mesopelagic zone, that species have become more adapted to the reduction of sunlight and living in complete pitch-black darkness. One of many adaptations of living in such conditions is the ability produce light through Bioluminescence. This is shown within a variety of organisms in the video below from National Geographic.

What is bioluminescence?

Bioluminescence is the ability to produce and emit light within an organisms’ body. The light is a result of a chemical reaction between Luciferin and Luciferase enzymes in species that inhabit both terrestrial and marine environments. The latter of which containing a higher abundance of species actively using bioluminescence in areas of the ocean such as the Mesopelagic zone. When induced together​ the energy levels of the components within the chemical reaction, particularly Luciferin, is pushed to a heightened​ state when in first contact with luciferase enzymes.  It is then diffused through oxidisation to a grounded state causing the emission of light in an organism’s cells.

Some species lack the adaption to create the chemical reaction so consume bacteria as an alternative method. These species retain the small organisms with specialised organs around their bodies, varying in species morphology. The bacteria are held to be beneficial within the organs.  The availability of food source helps cultivate the bacteria as the host sacrifices sugars from its cells to allow the reaction of bioluminescence from the bacteria, which is transported between the host’s gut and specialised organ. Luciferases enzymes can substitute the chemical reaction for some species, creating proteins known as photoproteins. Bioluminescence is triggered by compounds within different individual species bodies, such as calcium, magnesium and hormones.  This allows the advantage of storing large quantities of the cells until the organism reacts to stimuli.

Function of bioluminescence for predator/prey species

The Angler fish, Melanocetus murray, is an example of a predator species using bioluminescence to trap prey. The special organ of the species contains bioluminescence bacteria that are cultured by the fish and used to produce intervals of light in the dark zone. The specialised organ attached to the curved appendage from its head, shown in figure 1, signals intervals of light as the appendage swings back and forth as that of casting a fishing rod, hence the name. Creating the allusion of a smaller organism to prey on from a distance like squid. The specialised organ is recognised as a small copepod to consume, but once lured towards the angler fish it is the misled prey are snapped up by its ferocious looking mouth.  An unfortunate encounter in already bleak condition, but one that could have been avoided with the right defence mechanism.

Figure 1.
The Angler fish, Melanocetus murray

The use of bioluminescence can be a beneficial adaption for prey species to allude predators using the art of distraction to escape. Prey species like shrimp can secrete bioluminescent fluid in the presence of a predator. Blurring the vision of its impeding attacker with a cloud shield providing enough cover to manoeuvre away.  A similar method is used by the Green bomber worm, Swima bombiviridis, the polychaete species shown in figure 2, releases pockets of bioluminescent pellets that give off large flash of light within the darkness startling surrounding species including predators and prey. The display provides the opportunity to attract a second predator to combat the existing threat whilst an opportunity to escape.

Whilst cells of an organism can light up parts of their body, some species can detach parts of their body as a defence, like the common household pet the leopard gecko, when threatened or attack by its back end, it will release its tail. Species of squids can detach illuminate tentacles to avoid capture by predators. The detached tentacle can be used as a bait to allude predators from the squid’s position or to provide chance to swim away after a predator has attacked. Not always a signal for food or demise, the flashes of light in the dark zone can be an opportunity to find a mate. Bristle worms, Oxydromus flexuosus, use bioluminescence to attract the opposite sex during mating times of year. Alternately species use intervals of light in a sequence to attract a mate, as shown in ostracods.

Figure 2. Green bomber worm, Swima bombiviridis.

Why go towards the light? 

Bioluminescence can have multiple functions and purposes for species in the deep ocean, but for the ones mentioned, they are adapted to be to ward off threats or attract prey.  The decision to swim towards the light and suffer the chaos that unfolds for food or a mate is one taken at the risk of having the ability to survive an encounter. The surrounding darkness can be a daunting place for the ill-equipped but with the adaption of bioluminescence and other morphological advantages to survive. It is a case of one light to another, species in the ocean can make the most of light from the surface down to the great depths of the Mesopelagic zone.

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