Shining Examples: 10 Bioluminescent Creatures that Glow in Surprising Ways [Slide Show]
A wide range of organisms generate their own light to seek mates, sustenance and survival--inspiring researchers and moviemakers alike
Credits: JAMES JORDAN/FLICKR
BONUS: LUMINOUS BEINGS ARE WE? No mammals have been shown to produce bioluminescence—at least in an intended, substrate-and-enzyme-based visual display like fireflies and many denizens of the deep—and humans are no exception. As a July
PLoS ONE paper shows, however, the human body does throw off a slight glimmer, though at a luminosity about one thousandth of what the naked eye can see. Researchers used a special supercold camera to capture this so-called ultraweak biophoton emission. In study participants, the shimmer increased over the course of the day and peaked in the late afternoon, implying that our quasi-bioluminescence may be linked to the circadian rhythm, the 24-hour cycle of biological activity in our bodies.
This enigmatic glow does not appear tied to other well-known ebbs and flows, such as body temperature, which crests during the night. Nor can it be explained by skin surface temperatures: The warmest readings emanated from the participants' shoulder and neck regions, although their faces shone the brightest, according to the paper. Its authors believe these human lumens stem from the production of free radicals, or energetic molecules, from normal metabolic actions. Although our own subdued light show plays no discernible role in our daily lives, idioms about one being "aglow" might now claim a more literal basis. MASAKI KOBAYASHI/DAISUKE KIKUCHI/HITOSHI OKAMURA/PLOS ONE
UNDERWATER "NIGHT VISION" Most oceanic creatures bioluminesce in blue—and to a lesser extent green—because these short wavelengths of light travel farther in water than longer, redder wavelengths. For the same reason, sea life has adapted to register these colors and often does not possess the visual pigments needed to see reds, oranges and yellows. Taking advantage of—and exception to—this pelagic norm is the deep-sea family of predators known as loosejaw dragonfish. The fish emit red light from special organs just below their eyes, which have evolved to detect this crimson glow. In effect, this red bioluminescence gives the carnivores short-range "night vision" that they can use to sneak up on unsuspecting prey lurking in the perpetual gloom.
SPARKLING SLIME Dozens of earthworm species from all over the world can secrete a glowing slime, thought to startle predators. This particular worm,
Diplocardia longa, is found in sandy soils in southern Georgia in the U.S. and can stretch to over half a meter in length. To collect and study these creepy crawlies, University of Georgia professor emeritus of biochemistry and molecular biology John Wampler and colleagues first tried digging the worms out of their U-shaped burrows. They then vibrated the ground using so-called worm grunting. When these methods failed, locals advised the researchers to just poke the worms in their holes with wire grass, and sure enough D. longa would wriggle on out. "To study the bioluminescence mechanism we collected thousands of worms this way, one by one," Wampler says. It turns out that chloragocytes—the cells in earthworms that produce the bioluminescent ooze are part of a system that sequesters toxins in the earthworm's body, much like a liver. MILTON J. CORMIER/JAMES M. ANDERSON AND JOHN E. WAMPLER/BIOLUMINESCENCE LABORATORY, UNIVERSITY OF GEORGIA
PHOTONIC CAMOUFLAGE Even in ocean depths where sunlight barely penetrates, the faint silhouette that a fish throws to predators beneath it in the water column can make it an easy target. Accordingly, many fish, crustaceans and squid have developed bioluminescent "counterillumination" abilities. Light-emitting organs called photophores line their undersides. These creatures can adjust the light output of these organs to match the light their eyes receive from above to help eliminate their shadows. The hatchetfish, shown here, is one such species equipped with an underbody that lights up to camouflage it from hungry eyes below.
FLY FISHING--WITH A FLASHLIGHT Before a short adult life as a gnat, larvae in the genus
Arachnocampa spend months as carnivorous glowworms in caves or sheltered areas using light as a lure. In the top image, a hungry New Zealand glowworm, Arachnocampa luminosa, lays a trap. From its nest on a cave ceiling, the glowworm dangles several dozen "fishing lines," each studded with evenly spaced, sticky droplets of mucus. The worm then churns out bioluminescence from organs on its posterior, attracting passing insects. These duped bugs get snagged in the gummy threads, and the glowworm hauls in its catch. In the bottom image, a host of glowworms light up the Waitomo Caves in New Zealand's north island, where visitors can take tours to see bioluminescence in action. SPELLBOUND GLOWWORM AND CAVE TOURS, WAITOMO, NZ
BLUE TIDE When plankton called dinoflagellates grow too numerous near shore, the single-celled algae can stain the water a reddish-brown, causing so-called red tides that are often toxic to people and fish alike. Certain dinoflagellates species also produce bioluminescence, and when night falls at the beach, the teeming algae can make the shallows glow an electric blue, as captured here by photographer Phil Hart in the Gippsland Lakes in southern Australia.
Out at sea, dinoflagellates use bioluminescence as a sort of "burglar alarm": when disturbed, the plankton flash or light up, essentially creating a glowing trail that leads right to their assailant. This silent signal alerts predators higher up in the food chain about the dinoflagellates' nemesis. "[The burglar alarm] is a scream for help," Widder says. "The best chance you have when you're getting attacked is to attract something bigger than what is eating you." PHIL HART
EFFULGENT FUNGI Mushrooms gleam in forests all over the world, from the
Mycena lucentipes species seen here, described in Brazil last year, to the honey and jack o' lantern mushrooms that emit a greenish "fox fire" glow in woodlands. Researchers have now documented more than 70 species of bioluminescent fungi, although the exact purpose of the 'shrooms' bioluminescence remains mysterious. For species in which just the spore-containing cap shines, the glow may help get the attention of nocturnal bugs that then aid in spore dispersal, similar to brightly colored fruit that draws in frugivores to spread pollen and seeds. Other species with radiant mycelia, or threadlike, vegetative parts, may deploy bioluminescence defensively to attract the predators of the insects that dine on the mushrooms, says Dennis Desjardins, a biologist at San Francisco State University. A third possibility, given the fact that the mushrooms keep pumping out photons around the clock, is that the light show may just be a by-product of normal metabolism and serve no function, Desjardins says. Ongoing genetic studies by Desjardins and his colleagues seek to uncover the evolutionary origins of fungal bioluminescence, which has emerged in very few of the Fungi kingdom's 85,000 or so members. CASSIUS V. STEVANI/IQ-USP, BRAZIL
GELATINOUS GLOW Comb jellies, technically known as ctenophores, are a phylum of seafaring organism characterized by their use of small hairs, or cilia, for aquatic locomotion. Almost all of these blobby beings also bioluminesce, and they provide yet another example of defensive lighting with so-called "sacrificial tags". Chunks of ctenophores bitten off by predators will keep glowing in the predator's translucent guts, highlighting the gobbler in the ocean's gloom. "This makes it risky to be a predator of [the ctenophore]," says Steve Haddock, a marine biologist at the Monterey Bay Aquarium Research Institute and a co-author of a review of bioluminescence set for publication in the January issue of the
Annual Review of Marine Science. Many ctenophore diners have responded over the generations by developing black- and red-pigmented digestive tracts to hide this glowing evidence of their last meal.
A common myth about comb jellies is that their bodies can produce an undulating rainbow of colors, as some photographs and movies appear to attest. Most ctenophores, however, can generate just greens and blues, although their cilia can diffract light, creating multicolored hues. MARSH YOUNGBLUTH/NOAA Advertisement
BOMBS AWAY! A paper published in
Science this August made waves when it announced the discovery of five previously unknown species of sea worm that launch liquid-filled, bioluminescent capsules that burst into green light for several seconds. Scientists think these "bombs" distract piscine predators; they have seen similar countermeasures deployed by both squid and brittle stars.
Other defensive uses of marine bioluminescence include "smoke screens" of glowing fluid to mask getaways and vengeful sea cucumbers whose sticky, glowing skin rubs off onto their predators, marking them for easier identification by their devouring enemies. "There are a huge number of different [bioluminescence-based] defensive systems," says Edie Widder of the Ocean Research & Conservation Association. KAREN OSBORNE
KILLUMINATION Hundreds of meters down in deep, pitch-black ocean waters, monstrous-looking anglerfish wave about bioluminescent lures, called
esca, to temp prey into swimming within striking distance. Like fireflies, these common deepwater fish may use the lighting effects in mate selection, as well.
The anglerfish is unusual among bioluminescent creatures in that it does not make its own light chemically; rather, it hosts colonies of symbiotic, light-producing bacteria in its fleshy lure. In anglerfish and other environs, bacteria rely on bioluminescence to achieve communication, sometimes via "quorum sensing" in which the single-celled organisms coordinate their gene expression and behavior based on the size and density of their group. STEVE HADDOCK
FLYING WITH FIRE The power to make its own light distinguishes the life—and death—of the familiar firefly. Also commonly called lightning bugs, these species have developed unique call-and-response patterns of flashes between courting, airborne males and the females that watch from leafy perches. But danger lurks in this bioluminescent Morse code: Female fireflies in the
Photuris genus, for example, mimic the flash responses of females in the Photinus genus, tricking love-seeking, smaller Photinus males into becoming a light meal.
The insects' flashiness can also be a saving light in the dark. Recent research published in August in Animal Behaviour supports the idea that fireflies' glowing rear abdomens serve as warning signals to predatory bats that the bugs don't make for good eating. (Bats use both vision and echolocation to snare their food.) Known as aposematism, this overt advertisement by a prey species of toxicity or unsavoriness makes evolutionary sense for fireflies. "It's hard to envision how fireflies would get away with being so conspicuous without some form of chemical defense," says lead author Paul Moosman, assistant professor of biology at the Virginia Military Institute. To prove the point, Moosman and his colleagues probed bats' fecal pellets and learned that the flying mammals ate surprisingly few fireflies given their overlapping periods of nocturnal activity. Caged bats actually coughed, shook their heads and wiped their snouts, the paper says, when they sampled mealworms covered in "homogenized" firefly, which "is science talk for a firefly smoothie," Moosman says. JAMES JORDAN/FLICKR Advertisement