Beetles whose flashes punctuate summer skies; killer fish that lure prey with an enticing light; algae that rat out their attackers with a telltale glow. These ominous organisms might seem like creatures from out of this world, but thanks to some clever chemistry, such beings are in fact abundant on our planet. Examples of creatures that generate their own light—a capability known as bioluminescence—are especially common in the ocean, where filmmaker James Cameron purportedly drew inspiration for the glimmering alien life in his new sci-fi flick Avatar.

Although it takes many guises in nature, bioluminescence serves the three basic purposes of "finding food, finding mates and defending against predators," says Edie Widder, co-founder, president and senior scientist at the Florida-based Ocean Research and Conservation Association (ORCA). Even in dim or dark environments where the sun's rays cannot penetrate, such as caves or much of the oceans, many animals still have eyes—oftentimes extra-large ones—in order to glean information from the few stray photons available. Some 80 to 90 percent of deepwater, oceanic life has developed the ability to produce light, taking advantage of the transparent, though predominantly dark, medium in which it lives.

Although bioluminescence is rare on land, with no recognized plant or vertebrate species producing such light, diverse critters from insect larvae to mushrooms to ooze-secreting earthworms have shown that they can shine. Overall, scientists think bioluminescence has independently evolved at least 40 different times across the animal, fungal and bacterial kingdoms, "which is a clear indication of the survival value of the trait," Widder says.

In most cases bioluminescence is generated when a light-emitting molecule, generically known as a luciferin, chemically reacts with oxygen in the presence of an enzyme, called a luciferase or a photoprotein. Over eons of evolution, a range of chemistries has emerged to crank out this efficient natural illumination, which has been dubbed "cold light" because it creates very little waste heat. This phenomenon is distinct from fluorescence, which occurs when an external light source excites a molecule that then reemits some of the absorbed energy. Many bioluminescent animals also display fluorescence, including jellyfish (whose green fluorescent proteins helped three scientists nab the Nobel Prize in Chemistry last year). And even inorganic materials—including the aptly named mineral fluorite—can exhibit a glow when placed in certain wavelengths of light.

As our limited knowledge about the deep sea's diverse inhabitants continues to broaden through projects like the Census of Marine Life, and field work reveals more of nature's as-yet-undocumented creatures, other examples of bioluminescence will surely come to light. "I do believe we have just scratched the surface of what is out there," says Karen Osborn, a postdoctoral fellow at the Scripps Institution of Oceanography at the University of California, San Diego. Given how often bioluminescence appears in earthly life, it seems a little less far-fetched to suppose that extraterrestrial life, as imagined in Avatar, might gleam, as well.

Slide Show: Bioluminescent Creatures