By Steve Oden
It was a gripping scene: Dory, the flighty and friendly blue tang, is moving hypnotically through darkness toward a faint glowing ball. As we watch the drama unfold we begin to see the ball is hanging just above a large mouth with fearsome sharp teeth. At the last minute, Nemo realizes her peril and saves her from becoming the sinister predator's latest meal. Exciting, yes, but might such a scenario really play out? The answer is "yes" and "no".
The cartoon fish was patterned after a real fish: a species of angler fish called a "black sea devil", genus Melanocetus. However, black sea devils live at depths much greater than where Dory or Nemo might dive and they are fairly small, only about 5 inches or so. Like other angler fish they use a modified dorsal fin to mimic something edible, luring their prey close enough to be engulfed by their oversized mouths. And yes, their lure does glow!
Many living things can produce light, a phenomenon called "bioluminescence". The light is a form of phosphorescence caused by a chemical reaction between oxygen and the protein "luciferin". In the presence of the enzyme "luciferase" the luciferin breaks down, or oxidizes, to release energy in the form of a softly glowing light. The myriad bioluminescent organisms in the oceans include phytoplankton, jellyfish, comb jellies, squid, and even small crustaceans. These organisms produce intermittent flashes of light triggered by external stimuli or timed releases of luciferin and luciferase. Common examples are the flashing light produced by fireflies on land or the flashing luminescence of certain marine algae (dinoflagellates) sometimes seen in the surf on a summer beach. Bacteria and fungi produce steady glows. A famous land example is the "foxfire" that is produced in forests by certain species of fungus. Interestingly, vertebrates do not produce their own light.
How, then, can a fish bioluminesce? In a fascinating example of symbiosis, certain marine fish (and some other animals) have specialized "light organs", pockets of tissue that harbor living colonies of bioluminescent bacteria. The soft glow produced by the black sea devil is due to bioluminescent bacteria harbored in a cozy pocket of tissue in a modified spine of the fish's dorsal fin. Another fish that produces a glow is the Atlantic midshipman, Porichthys plectrodon. It has a double row of light organs on its stomach, allowing it to camouflage itself from deep-water predators that find it difficult to discriminate between the fish's illuminated underside and the bright sky above. Often these light organs have flaps of skin or tissue that can act like eyelids, opening and closing to reveal or hide the light. This is seen in the so-called "flashlight fish" (Blepharon) which has lid-covered light organs allowing it to create a flashing light beneath each eye. It is thought that the flashing can disrupt a predator's ability to accurately gauge the fish's location when chasing it.
There are many examples of bioluminescence in the marine environment, and scientists aren't always certain of the advantage to the organism. For example, why would a marine alga that is food for lots of filter-feeders announce its presence by bioluminescing? One explanation is that the flashing startles fish that try to eat it. Other possible advantages for bioluminescence include attracting food or mates, warning possible attackers, or even using up excess oxygen. But what about the palolo worm in the South Pacific? It has a bioluminescent end that fills with eggs and breaks off to float to the surface, producing an impressive light display. Why would half of a worm light up? Maybe you can figure it out!
Steve Oden is a MSiC 6-12 educator.