Bioluminescence, or the ability of an organism to create light, is one of nature’s most amazing phenomena, seemingly drawn more from science fiction than science and natural history. While relatively rare on land, bioluminescence is very common in the ocean, at least in the pelagic zone (the water column), where 80 percent of the animals that live between 200 and 1,000 meters (656 and 3,280 feet) depth are bioluminescent. Bioluminescence is most common among fish, squid, and what we call the gelatinous zooplankton – jellyfish, siphonophores, comb jellies, and other animals that are mostly made of water.
While usually blue in color, because this is the light that travels best through the water, bioluminescence can range from nearly violet to green-yellow (and very occasionally red). All bioluminescent organisms use a reaction between an enzyme and a substrate to make light, but different species use different chemicals in the process, suggesting that the ability to make light may have evolved independently many times.
Deep-ocean environments are almost completely dark; yet light is still important in these environments. Thus, bioluminescence may provide a survival advantage in the darkness of the deep sea, helping organisms find food, assisting in reproductive processes, and providing defensive mechanisms...but we don’t really know the main purpose or function of bioluminescence. In fact, although many marine species are able to produce this “living light,” much about bioluminescence remains a mystery. For example, scientists have yet to learn why bioluminescence is common in the ocean water column but not in freshwater systems or on land or even how bioluminescence evolved.
Part of the problem is that bioluminescent organisms are difficult to observe: turning on bright lights can cause mobile animals to move away and may permanently blind light-sensitive sight organs. In addition, transparent and camouflaged organisms may be virtually invisible even with strong lights, and many types of bioluminescence can’t be seen under ordinary visible light. Additionally, collecting samples of these organisms is incredibly difficult.
Thus, bioluminescence is a subject with many more questions than answers.
Living Light (pdf, 1.2 MB) – Bioluminescence 2009: Living Light on the Deep-sea Floor