Why do some organisms glow?

SALT LAKE CITY -- Bioluminescent organisms, that is life that literally glows, exist in large variety, from bacteria to mushrooms, worms, fireflies, jellyfish and sharks. But how, and why, do they do it?

Bioluminescent organisms create light through a process in which two or more compounds are brought together to create light. It’s very similar to combining the chemicals inside a glow stick to create light.

One of the substances is called luciferin, which is a light-producing pigment. When you see a bioluminescent organism glowing, it is the activated luciferin that you’re seeing. Luciferin reacts with oxygen to create light, according to Steve Haddock, a research scientist and adjunct associate professor at Monterey Bay Aquarium Research Institute. An enzyme called luciferase allows oxygen and luciferin to interact, producing photons of light that escape from the cell.

In some species, a charged ion is needed to complete the reaction, and sometimes adenosine triphosphate (ATP) must be included to complete the process. Adenosine triphosphate is found in most living organism, including humans. It’s a very specialized molecule whose job it is to transport and store energy.

But that's not the only way to get glowing. Some bioluminescent animals don’t create their own light but instead host bioluminescent bacteria, which they keep stored inside themselves in an organ, according to Tracy Wilson, author and researcher for HowStuffWorks.com, who has written extensively about bioluminescence. These bacteria constantly produce light.

There are times when constantly glowing can be disadvantageous - like when you're being hunted by other fish. To get around that problem, some species can control the amount of light emitted by pulling the bacteria inside of themselves or by covering their glow-spots with flaps of skin, like closing an eyelid.

Some dinoflaggelates (small marine plankton) obtain their energy through photosynthesis and they have luciferin very similar to chlorophyll, according to Wilson. Their luciferin absorbs sunlight in a photosynthetic manner during the day and releases the light back during the night.

The majority of the world’s bioluminescent organisms live in the ocean and most of those exist at a particular zone, called the disphotic (poorly-lit) zone, which exists from 660-3,300 feet. As its name suggests, very little light reaches this deep into the ocean. Scientists estimate that 90 percent of marine life living in the disphotic zone is bioluminescent.

But even animals that aren’t bioluminescent benefit from the light given off by those creatures that are. In the lower depths of the disphotic zone there is more light from bioluminescent creatures than from sunlight. Studies from marine biologists show that bioluminescent light can be seen from more than 300 feet away.

But why bother to glow at all? Bioluminescent organisms use their gift for a variety of reasons, according to Wilson.

Communication

Some species, such as fireflies, use bioluminescence to communicate to each other. In the case of fireflies, this communication is a mating signal. It’s believed that some bacteria also use bioluminescent capabilities to communicate between each other, though scientists don’t yet know what it is they’re communicating.

Hunting and locating foodMany bioluminescent animals use the light they create the same way people use flashlights: to help them find their way and hunt for prey.

There are many fish species that live in the disphotic zone that aren’t bioluminescent. They lack the ability to search for prey with their own light, but they don’t need to. They are able to see and prey upon the animals that are bioluminescent.

Attracting prey

Other bioluminescent animals have evolved to use their bioluminescence to act as a lure. One such animal is the angler fish , which has abioluminescent appendage that it dangles into the darkness to attract passing fish. When a fish comes to investigate, the angler fish attacks it.

Camouflage

Given the above scenarios of fish being hunted and captured because they are glowing in a dark environment, it seems counter-intuitive that some species of fish use bioluminescence as camouflage. In the ocean’s darkest depths, a fish has a difficult time seeing any objects beneath it, but it can more easily see fish that are above it because they are silhouetted against the light — small as it may be. Some species have developed areas of light on their undersides, which blurs their outlines and helps them blend into light coming from above.

Self-defense

In the same way that a squid can release a cloud of ink when it’s threatened, some animals release a cloud of bioluminescent fluid, and they escape into the darkness during the predator’s confusion.

One of the most ubiquitous bioluminescent organisms are dinoflagellates, which are single-celled plankton that live on the ocean’s surface. They glow when they are disturbed. This is why — in zones containing dinoflagellates — a ship’s wake glows or a rock skipped across the sea’s surface will produce glowing rings where the rock struck. Scientists believe that this originated as a defense mechanism against small fish feeding on the plankton. The glow attracts larger fish that would arrive and eat the fish that were feeding on the plankton.

Bait and switch

The cookie-cutter shark has a bioluminescent underside, except for one small patch, which looks like the silhouette of a smaller fish seen from below. Predators swimming below the shark see this, think it’s a small fish and swim up to attack. How does that help? When the predator approaches, the cookie-cutter shark turns the tables on its attacker, attacking it first, usually taking one bite from its predator-turned-prey, then fleeing.

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