At the deepest depths of the ocean, where sunlight can no longer penetrate, there is a complex ecosystem of organisms functioning in complete darkness. The creatures of the deep sea have evolved impressive ways of dealing with the darkness that engulfs them, and one method in particular, bioluminescence, is present at remarkably high frequencies. This chemical process involves the oxidation of a light emitting molecule, luciferin, by an enzyme catalyst, which could be either a luciferase or photoprotein . There are many functions of bioluminescence that various species of marine life have adapted to fit their needs for survival. In fact, bioluminescent abilities have independently evolved 27 times . Because this characteristic has been estimated to evolve so many times, the ability to produce light has spread through the branches of the tree of life, clearly proving itself advantageous in many ways.
In Blue Planet, Season 1, Episode 2: The Deep, David Attenborough introduces the phenomenon of deep sea bioluminescence. At 12:35 minutes into the film he says, “a deep sea angler fish flashes in the darkness. The light is generated by bacteria that live permanently inside the lure, which attracts prey to [its] murderous teeth.” The angler fish generates an alluring glimmer of light through a process of bacterially-mediated symbiosis , while other deep sea bioluminescent creatures, like the hatchet fish, produce light endogenously, without the help of a partner.
Luminescence occurs by one of two different methods of light production. Some fish species with intrinsic bioluminescence, like the hatchet fish, “show their own luciferin-luciferase system in specialized light organs” . The hatchet fish utilizes bioluminescent photophores on its ventral underside to mimic the light coming from the water’s surface above, thus making it undetectable to the predators beneath. Fish species that do not possess these intrinsic mechanisms, such as the angler fish, are able to produce light by hosting bioluminescent bacterial species in specialized light organs in a symbiotic relationship, through which they gain the advantage of mystifying their prey in exchange for protection and nutrients the bacteria can garner from the angler fish.
Another way bioluminescence is used is to escape predators. Oplophorid shrimp secrete bioluminescence from light emitting organs called photophores as a mode of predatory defense. By spewing out a blue luminescence, they are able to distract their potential predator and flee into the darkness. Recent studies have shown that some photophore-bearing species have an additional photopigment with spectral sensitivity maxima in the near ultraviolet spectrum . This could provide these deep-sea shrimp with the potential to differentiate between their different modes of bioluminescence.
by Marion Muller and Natalie Bauer
Blue Planet, Season 1, Episode 2, The Deep, starting at approximately 12:35
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