One of the most alluring visual feasts in the movie Avatar was its alien biosphere of glowing plants and animals. Nearly every living thing on the moon Pandora seemed to shimmer and sparkle—sometimes in response to touch, other times as an expression of emotion. It’s something that separates this magical world of make-believe from the real world here on Earth.
Or is it? While bioluminescent organisms are perhaps not as common in the real world as they are in science fiction, they do exist, in a surprising variety of places. I first encountered them at night on a dock near my childhood home in Seattle. Initially the waters of Puget Sound seemed dark, but dipping a hand revealed a luminous surprise—tiny glowing bits appeared, like underwater sparks, wherever my hand disturbed the water. Then I saw a glowing fish swim by, leaving a luminous trail. The fish wasn’t actually glowing; rather, it was causing tiny bioluminescent dinoflagellates to glow as it passed them. This may be a defense mechanism for the dinoflagellates. Since any movement by their predators causes them to glow, this light may attract other, larger predators that could then do away with the danger.
Image courtesy of Jeremy Marr
Christie Lynn, a graduate student in cell and molecular biology at the University of Hawaii, points out that dinoflagellates aren’t the only creatures in the sea that glow. Indeed, at depths of greater than 1,000 meters, where no light from the surface can reach, it has been estimated that nearly 90 percent of creatures emit some kind of light. These aren’t just microorganisms: Fish, squid, jellyfish, and shrimp are also commonly bioluminescent at these depths.
Their lights have a variety of purposes: Camouflage, attracting mates, attracting (or distracting) prey have all been observed. In animals with nervous systems, in most cases, neural activity initiates the bioluminescence. But in the velvet belly lantern shark, Lynn says, researchers found that the glowing was not caused by nerve cells. Instead, it seemed, certain hormones controlled the glow: Melatonin and prolactin turned it on, and a hormone called Alpha-MSH turned it off. This makes some sense, as melatonin is activated by darkness (it helps control sleeping behavior in humans). This species of shark uses glowing as a form of camouflage. It swims around 500 meters below the surface, and its glowing belly, matched to the dim light filtering down from above, make it less visible from below.
Bioluminescence isn’t limited to the deep, dark portions of the ocean. Lucas Brouwers is a graduate student in Molecular Mechanisms of Disease in Nijmegen, the Netherlands. He blogs about a coral that is ordinarily a dull shade of brown, but glows in a vivid rainbow of colors under certain conditions. The coral’s glow is due to the dinoflagellates living inside it in a symbiotic relationship. Bioluminescence is most commonly a yellow-green color, whether in fireflies or phytoplankton, so naturally researchers have been interested in how the wide array of colors exhibited by these corals evolved.
Using a very clever technique, Steven F. Field and Mikhail V. Matz of the University of Texas at Austin reconstructed the evolution of the proteins responsible for the coral dinoflagellates’ luminescence—all on a petri dish. Their results were published last September in the Journal of Molecular Biology and Evolution. Field and Matz examined all the different possible mutations of the bacterial genome between a green ancestor and modern red-glowing bacteria, using 20,000 different cell cultures. Through a process of elimination, they identified 20 critical mutations in the genome that were responsible for the variety of colors we see today. Interestingly, these mutations are epistatic: That is, individually, they don’t result in much difference, but combined, they result in the vibrant, bold colors of the coral’s glow, ranging from blue to red. The researchers were even able to illustrate these genetic relationships by using colonies of the host bacteria to construct a living phylogenetic tree.
Of course, the creature many of us associate with bioluminescence doesn’t live in the oceans at all. Zen Faulkes, a biologist at the University of Texas–Pan-American, uncovered a study about the glowing “firefly,” actually one of several glowing beetles. When I first saw fireflies after I moved to the southern US, I wondered how that glow could possibly be beneficial. Wouldn’t it attract predators? A team led by Paul R. Moosman studied how insect-eating bats respond to the glowing fireflies. They found relatively few remnants of fireflies in bat droppings, and caged bats rejected pieces of fireflies as food. Perhaps the glow of a firefly serves as a signal to potential predators that they are distasteful or poisonous, just like red berries signal danger to herbivores. Indeed, the researchers did find that some bats attacked glowing lures less than non-glowing lures, although Faulkes says the results weren’t conclusive for all bat species that were studied.
So while it’s possible that one purpose of the glow of fireflies is as a warning for predators, clearly much remains to be learned about the function of bioluminescence.
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