The otherworldly, bioluminescent fauna of the twilight (or dysphotic) zone is astounding. A documentary series the Blue Planet features footage of creatures alien from our world on land from the way they were shaped, the way they moved, and the way they glowed.
Bioluminescence is a type of chemiluminescence which is when light is emitted during a chemical reaction and in chemiluminescence very little heat is emitted. This means that this sophisticated method of producing light is also very energy-efficient (perhaps someday technology will be mimicking these animals' efficiency). Bioluminescence Webpage gives a very clear and brief explanation: the chemical luciferin (either produced internally or obtained trough food) reacts with oxygen and is catalyzed by another chemical luciferase, resulting in the production of oxyluciferin and light from the atom going into an excited state.
So these animals use internal chemical reactions to make themselves glow brilliantly... but why? Here are a few thing bioluminescence is used for:
1. To see in the dark.
Very little light reaches the dysphotic zone in the ocean, so being able to glow can help animals see other organisms sharing the waters. Bioluminescence is different from bioflorescence which requires light to be absorbed from an external source in order to produce the glowing effect, so these animals wouldn't need to rely on external sources of light in their dark world. This might seem like an obvious reason, but the colour of the light also matters; many animals in the twilight zone glow blue or green, and that's because blue light will travel the farthest which can help the individual see more, whereas red lights do not travel as far, which can be helpful for finding food, but this also means that they would be more easily spotted by predators. Some animals have the ability to control their glow, but not all of them.
2. As a lure.
If you've seen Disney and Pixar's animated film Finding Nemo, then you've probably seen a enactment of the luring effect of a pretty little light on an angler fish... with some artistic license taken. The angler fish uses a little bioluminescent lure to attract smaller fish and other potential prey into its vicinity so that the angler fish can catch its prey by surprise.
3. For defense.
There are different ways in which bioluminescence is used in defense. Comb jellies/ctenophores have what is called sacrificial tags; when a part of one of the jellies is bitten off, the part remains glowing within the digestive tract of the predator, exposing the predator's location while a shrimp, Acanthephyra purpurea, ejects a blue bioluminescent cloud when it is attacked to protect itself (read more in Switek's article/post in the reference list). Other animals that can control the visibility of their bioluminescence can sometimes also use the glow to confuse predators. The glow of a firefly also warns predators of the bad-tasting chemical catalyst lucibufagens that the firefly is filled with.
Photonic camouflage is a unique type of bioluminescence, and the hatchetfish is one example. To avoid having a silouhette that can be spotted by predators, they adjust photophores on their stomachs which emit light to camouflage with the colour of the light/environment behind them.
p.s. Scroll to the bottom of the page for an interactive demonstration by feeing 10 bioluminescent fish... however, one has photonic camouflaging (it is the exact same colour as the background). Like the hatchetfish, this fish is very tricky. Try to find the camouflaged fish, and don't forget to share your result in the poll that will be up for the rest of the month (the fish will still be there after the month is up, though).
Hint: The fish will follow your mouse until you drop food in.
Hint: The fish will follow your mouse until you drop food in.
4. To communicate.
Although this is not their only method of communicating, fireflies are also an example of this one; when they are attracting a mate, males and females will flash signals with their bioluminescent abdomen to communicate with each other.
There is so much to bioluminescence this only scratches the surface of one of the most brilliant characteristics of a deep part of the ocean. If you're interested in further I found the websites in the reference to be great sources, and the the Blue Planet was awesome.
References
Bioluminescence Webpage. Chemistry of Bioluminescence.The Bioluminescence Webpage. Retrieved January 16, 2014 from http://biolum.eemb.ucsb.edu/chem/
'Docmo'. Animals the Glow: the Science of Bioluminescence. http://docmo.hubpages.com/hub/Animals-that-Glow-The-Science-of-Bioluminescence
National Science Foundation. December 30, 2011. What Glows Beneath: Illuminating Mysteries of the Unseen. Live Science. Retrieved January 16, 2014 from http://www.livescience.com/17685-glow-coral-oceans-nsf-bts.html
Office of Naval Research. Ocean Water: Optics. Office of Naval Research and Science Technology. Retrieved January 16, 2014 from http://www.onr.navy.mil/focus/ocean/water/optics1.htm
Scientific Amierican. Shining Examples: 10 Bioluminescent Creatures that Glow in Surprising Ways [Slide Show]. http://www.scientificamerican.com/slideshow/bioluminescent-avatar/
Switek, Brian. Glow Little Spewing Shrimp, Glow. January 23. 2012. Wired Science. Retrieved February 12, 2014 from http://www.wired.com/wiredscience/2012/01/glow-little-spewing-shrimp-glow/
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