The only glow-in-the-dark creature familiar to most of us is the lightning bug. A few other land creatures, such as glow worms and types of mushrooms also shine in the dark. This phenomenon is known as bioluminescence. Although rare among land animals, bioluminescence is widespread in the marine environment. Along with bacteria and algae, nearly every major group of marine animals has members that glow.
Who can forget that childhood experience with fireflies on a warm summer night? Although the illumination they produce conjures up images of magic and fairy tales, this delightful effect is nothing more than a biological process called bioluminescence, created by a chemical reaction taking place within a living organism’s body. It can be used for a number of purposes, including mating, hunting, camouflage, and communication. Though a firefly sighting may not be all that common in some reaches of the globe, marine bioluminescence occurs on a wide scale, and can be even more ethereal than that of its terrestrial counterparts.
Most people don’t realize that the tendency for creatures to emit light is actually more common in the sea than on land. In fact, marine bioluminescence becomes quite common in the depths of the sea where sunlight does not penetrate, with scientists estimating that as much as 90 percent of deep sea creatures use some form of bioluminescence! Marine biologists exploring great depths in submarines have long told tales of the peculiar twinkling lights they encounter in the deep sea, far from any natural light from above. Marine bioluminescence seems to be common across a diverse group of organisms, from alga that floats high in the water column to many species of deep sea fish and invertebrates.
Basic Facts of Bioluminescence
Bioluminescence is the light produced by a chemical reaction that occurs in an organism. It occurs at all depths in the ocean, but is most commonly observed at the surface. Bioluminescence is the only source of light in the deep ocean where sunlight does not penetrate. Amazingly, about ninety percent of the organisms that live in the ocean have the capability to produce light.
Four main uses for an organism to bioluminesce have been hypothesized. It can be used to evade predators, attract prey, communcate within their species, or advertise (Nealson, 1985). For example, the angler fish uses the Lure Effect (attracting prey). This fish has a dangling lure in which bioluminescent bacteria live. The lure hangs in front of its mouth; fish swim toward the light and may become food for the angler fish. Some fish use bioluminescence for mating signals or as territorial signals (intraspecies communication), and some use it to communicate interspecies (advertisement). Some organisms employ it for more than a single reason.
Most bioluminescence is blue for two reasons. First, blue-green light travels the farthest in water. Its wavelength is between 440-479 nm, which is mid-range in the spectrum of colors. Second, most organisms are sensitive to only blue light. They do not have the visual pigments to absorb the longer or shorter wavelengths. Red light, which has a long wavelength, is quickly absorbed as you descend in water- this is why underwater pictures appear blue. As with every rule, exceptions exist. Some cnidarians emit green light and one family of fish, the Malacosteids (known as the Loosejaws) emit and are able to see red light. The red light they produce is almost infrared and not visible to the human eye. This is a huge advantage to these fish because they can produce light to see their prey, but their prey can not see them!
Each luminescent organism has a unique flash. Factors that can vary are color, rise time, decay time, and total flash time (Nealson, 1985). Some organisms can emit light continuously, but most emit flashes with varying durations and brightness. The luminescence of one dinoflagellete lasts for 0.1 seconds and is visible to humans. Larger organisms, such as a jellyfish, can luminesce for tens of seconds.
In most multi-cellular organisms, the ability to produce light is controlled neurally. However, the transmitter that signals the change to take place is unknown in most organisms. Luminescence can also be induced by the presence of another luminescing organism.
A few characteristics are common to all bioluminescent reactions. All bioluminescent reactions occur in the presence of oxygen. Two types of chemicals are required- a luciferin and a luciferase (lucifer means light bringing). The luciferin is the basic substrate of the reaction and produces the light. The luciferase catalyzes the reaction. In the basic reaction, the luciferase catalyzes the oxidation of luciferin, which results in two products- light and inactive oxyluciferin. In most organisms, new luciferin must be brought to the system either by diet or internal synthesis for each reaction. Sometimes the luciferin and luciferase are bound together in one unit called a photoprotein. The photoprotein is then triggered when a particular ion is added to the system- frequently calcium. Most of the energy released in this reaction occurs in the form of light, therefore, bioluminescence is commonly called “cold light.”
Five main types of luciferins are known. Bacterial Luciferin is a reduced riboflavin phosphate and found in bacteria, some fish, and squid. Second, Dinoflagellate Luciferin is thought to be derived from chlorophyll because it has similar structure and is found in dinoflagellates and euphasiid shrimp. The third type, Vargulin, is found in the ostracod Vargula and is also used by the midshipman fish Poricthys. This is an interesting dietary link because the fish can not luminesce until they are fed luciferin bearing food. Fourth, Coelenterazine is the most common luciferin; it is found in many phyla- the radiolarians, ctenophores, cnidarians, squids, copepods, chaetognaths, and some fish and shrimp. The fifth is firefly luciferin, which requires ATP as a cofactor in its reactions.
The tiny flashes of bioluminescence made by dinoflagellates result from a chemical process inside their cells. The enzyme luciferase allows molecules of oxygen and luciferin to combine. In the process, light is given off. This luminescent light is different from the light we normally see.
The light we normally see is “hot” light. Light emitted from incandescent bulbs or the sun results from the glowing of objects (or gases) brought up to very high temperatures. Bioluminescent light is “cold” light. With bioluminescence, most of the cell’s energy goes into producing light, not heat.
With its small heat loss, bioluminescence is the most efficient method of light production known. The cell releases less than 1 percent of its energy as heat during bioluminescence. Compare this to other cell activities, which typically result in a 60 percent energy loss as heat, or to combustion in a gasoline engine, which results in a 75 percent energy loss as heat.
The dinoflagellates most commonly found in glowing surface water include Noctiluca (Latin: night light), Pyrocystis (Greek: fire bag), Peridium, Gonyaulax, and Gymnodinium. Gonyaulax is also responsible for causing certain red tides. Thus, these creatures cause a red tint by day and a bluish-green glow by night. Comb jellies, copepods, and jellyfish also add to surface water luminescence.
An enchanting experience with marine bioluminescence can be had by paddling through salt lagoons and bays where comb jellies happen to be migrating. Comb jellies are translucent, gelatinous creatures shaped like little footballs lined with eight rows of longitudinal cilia. Although they are similar to cnidarians, or what are commonly referred to as jellyfish, they are actually ctenophores. These odd, graceful creatures often display bioluminescence, creating beautiful blue-green trails in the water at night as small vessels dip their oars into the sea. If a bioluminescent alga is present, you may get to enjoy the rare thrill of watching your footprints glow as you walk down the beach under the inky blackness of the night sky.
One of the more fascinating examples of marine bioluminescence is found in deep sea anglerfish. These amazing fish have an elongated appendage protruding from their heads complete with a bioluminescent tip. When the illuminated tip flashes, it lures the prey closer to its awaiting jaws under the cover of darkness, and before the victim has a chance to realize it’s a trap, the anglerfish swiftly snaps it up. The anglerfish is just one incredible example of life adapting to its environment in the most clever of ways.
Most marine bioluminescence is in the blue and green ranges of color. These colors are the wavelengths of light that best penetrate sea water. However, other bioluminescent colors have also evolved among marine creatures, especially those in deeper water. These colors include red, pink, yellow, violet, and white light.
Luminescence among shallow-water fishes is limited, but many mid-water and deep-sea animals (see “The Deep-Sea,” Dive Training, June 1997) exhibit glowing lights. A wide diversity of bioluminescent functions takes place within these groups. Some of the most interesting functions lie within the realm of disguise and detection.
Light from the surface, although dim, can outline the bodies of mid-water fish. So to avoid detection, many mid-water dwellers have luminescent bellies. They camouflage themselves by matching their belly lights with the intensity of light from above. Unfortunately, this clever deception is only a thin disguise to some predators. The hatchet fish is equipped with a special yellow eye filter that allows it to easily detect blue-green luminescent bellies.
Most of the fishes who produce bioluminescence in the blue and green ranges see these colors. However, they may not see red light. The fishes Aristomias, Pachystomias, and Malacosteus use this to their advantage. First, their eyes can detect red light, and second, they emit red light by which to hunt. Their red hunting lights are invisible to their prey.
Many sea creatures, such as prawns, are red. Red light absorbs rather than reflects green and blue light. As a result, this renders them invisible to predators hunting with blue or green lights. However, under the red light of Pachystomias, red prawns light up brightly and become easy prey.
