Bioluminescence on jellyfish

Many oceanic species can glow using marine bioluminescence

July 21 2021

In the ocean, the availability of light affects the distribution of marine species in a major way. In the upper layer of the ocean (the photic zone), sunlight is received during the day. However, at nighttime this light disappears, leaving the area in dim light or darkness.

As you keep descending in the aquatic layers, sunlight levels decrease. The first light to be absorbed is red light followed by yellow and green lights, leaving only blue light. By the time 200 meters of depth have been reached, the ocean is in permanent twilight (disphotic zone). When we reach 1000 metres of depth blue light fades out leaving the ocean in absolute darkness, in what is called the aphotic or midnight zone.

The emission of light by organisms is called bioluminescence. Less than 20% of the emitted light generates heat or thermal radiation, therefore it is considered cold light. This chemiluminescent reaction is an active process of communication, preying or camouflage that some marine organisms use to their advantage. Examples of passive luminescent reactions would be phosphorescence or fluorescence. In these processes, photons are absorbed and then re-emitted by a structure at a different wavelength ( for example blue light hits an animal on its surface and that light is re-emitted as a different color like orange, red, or green).



How the chemical process works

Let's get a bit technical for a minute, to understand how ocean bioluminescence works. The chemical reaction that produces bioluminescence needs two chemical compounds.

On one side, luciferin which is the compound that produces the light. This would be the substrate in a chemical reaction and the resulting bioluminescent color is the consequence of how the molecules are arranged. Luciferin can be synthesized on its own by some organisms. Others however are not able to synthesize it on their own and need to absorb it through other organisms with which they have a symbiotic relationship or in the form of food.

The second chemical compound needed for bioluminescence would be luciferase, which is an enzyme and it is the catalyst in the chemical reaction. The interaction between luciferase and oxidized luciferin ultimately creates light. Some bioluminescent reactions do not involve an enzyme (in this case luciferase). Instead, they use a chemical compound named photoprotein. To produce light, photoproteins need another agent in combination with luciferin and oxygen. Usually this agent is calcium ion. Photoproteins have only been recently identified by biologists and their chemical properties are still being studied.



The purpose of bioluminescence

Marine animals use bioluminescence to communicate, find their prey, as camouflage and other activities. This behavior is not limited to marine species, as there are also bioluminescent organisms on dry land such as fireflies, some types of mushrooms or even some insects like beetles.

Producing light is actually a very useful characteristic, especially for deep sea fish. This trait has been in constant evolution and is mostly seen underwater at lower sea levels, where it is often the only source of light produced. In pitch black conditions, a single flash of light can be perceived from a distance of approximately 80 metres.

An impressive 76% of ocean animals can produce their own light or host bioluminescent bacteria that do. Most of these marine species are found at deep levels, where 90% of fish observed below the 500 meter mark were bioluminescent. At higher levels, less than 2.5% of marine animals possess this trait.

Most bioluminescent organisms are found in marine waters instead of freshwater. There are different reasons why this is so. Marine habitats have been around for a very long time, as opposed to freshwater habitats which are, in comparison, quite new. They don’t have the biodiversity that can be found in oceans and evolution is a process that takes millions of years.

Also, freshwater habitats are quite different to the ocean and the species found in freshwater would not benefit as much from bioluminescence. For instance, freshwater habitats are often muddy and other traits, like the sensitive whiskers on catfish, would be a more useful adaptation to the murky environment

There are some species that do have bioluminescence in freshwaters, like the "Latia neritoides", which is a snail found in New Zealand that secretes a glowing mucus when it feels attacked.

Bioluminescence is not only a fascinating characteristic of marine species. The high number of animals with this capability indicates that it plays an important role in the Earths’ ecosystem. With the majority of life on Earth being aquatic, the extent of its capabilities are still unknown and awaiting to be discovered in the near future.





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