Bioluminescence in the Deep Sea
Creatures that live beyond the twilight zone spend their lives in an expanse of darkness. But although there may be no sunlight, many organisms have used this feature to their advantage.
These twinkling, flashing, pulsating lights are caused by the process of bioluminescence - the result of a chemical reaction that produces light energy within the body of these firefly squid. It is a common feature of many marine organisms - a single adaptation uniting vastly different creatures, from bacteria and algae to cephalopods and sharks.
There are 1,500 species of fish alone that are able to luminesce. And the fact that so many different organisms have this ability is evidence of convergent evolution - in which bioluminescence has evolved multiple times on at least 40 separate occasions, according to scientists.
This suggests that down in the deep sea, it is a highly beneficial adaptation to have, contributing to the success of organisms by allowing them to communicate, find a mate, and attract prey, among other useful behaviours. Take a look at some examples of bioluminescent organisms.
This deep-dwelling cephalopod uses bioluminescent lures on the tips of its tentacles in order to attract small prey. Vampire squid are also able to secrete a cloud of bioluminescence to deter predators - this is their alternative to the ink used by shallow water species, for black ink would be ineffective in the dark depths.
Anglerfish are unable to create their own bioluminescence. Instead, they form a symbiotic relationship by allowing glowing bacteria - known as Photobacteria - to inhabit their fleshy lure, or 'esca'.
The green bomber worm releases bioluminescent bombs in order to distract its many predators. To achieve this affect, it explodes its light producing photophores, and then swims away while the predator instead attacks the alluring lights.
THE CHEMISTRY OF BIOLUMINESCENCE
In most cases, an organism itself will contain the chemicals needed to luminesce. In order for bioluminescence to occur, a species must contain a molecule called luciferin. When luciferin undergoes a chemical reaction with oxygen, it produces light. Organisms are able to regulate their chemistry in order to control when they light up, as well as how bright and what colour. This is useful as different situations will benefit from differences in the way the light is expressed.
There are various different forms of luciferin, which vary slightly depending on the organism. Many animals will also produce luciferase, a catalyst that helps to speed up the reaction and allows for fast flashing bursts of light to be produced.
Some organisms have pre-packaged bioluminescent bombs called photoproteins. In these, the luciferin is bundled with oxygen, so that it is ready to light up very quickly upon the addition of a certain ion. The intensity of light can be controlled by how many of these ions are allowed to enter the photoprotein.
WHY IS BIOLUMINESCENCE BLUE?
One common feature that is present in almost all bioluminescent organisms is the colour of their lights. Blue. But it isn't mere coincidence that all these creatures have evolved to emit the same colour light. It is because blue light travels furthest in water, with its intermediate wavelength. Therefore it is advantageous to light up in the most visible colour, in order to be noticed in the abyss.
Visible lights with very long wavelengths, like red, or very short wavelengths, like purple, are absorbed faster and filtered out. The very same phenomenon is responsible for the ocean appearing blue.
Some creatures can use this feature of different coloured light to their advantage, like the bloody-belly comb jelly, which lights up red to make itself invisible to predators.