The Sea Aglow

By Karen Steidinger

Have you ever considered using the glowing, or luminescent organs of certain fishes as bait? Well, the natives of the Banda Islands do. And did you know that during World War II the Japanese mixed dried, ground luminescent animals with water to produce a dim light for reading maps in darkness? These are just two interesting tales of bioluminescence-production of light by plants and animals. The real marvel of light under the sea lies in the plants and animals themselves: how, where, and why the light is produced.

These pictures are modified from an article in National Geographic Magazine by Dr. Paul Zaul from July 1960. Theman in the picture had twenty bucketfuls of water poured over his head (left) from Phosphorescent Bay in Puerto Rico, an area known for abundant bioluminescent dinoflagellates. When lights were extinguished, this man was literally "glowing in the dark" (right).

Bioluminescence in the sea was once thought to be produced by the friction of salts in seawater or by the element phosphorus and, thus, was called phosphorescence. Today it is realized that certain animals have special light-producing organs called photophores and glands that emit light by a chemical reaction involving a light-producing substance called luciferin, an enzyme called luciferase, salts, oxygen, water, and an energy-carrying substance. In some animals, the photophores actually harbor bacteria that produce the light. Other organisms capable of luminescence, in addition to bacteria and animals in the sea, belong to Protista, a kingdom of organisms placed by scientists between bacteria and true land plants and animals. Certain dinoflagellates and radiolarians, both one-celled protists, are bioluminescent.

With the exception of insects and fungi, most luminescent organisms occur in the sea. They range from single-celled dinoflagellates and bacteria to anglerfishes with ornate luminescent lures. In between there are light-producing sponges, jellyfish, ctenophores (comb jellies), sea pens, squid, copepods, shrimp, shrimp-like animals, clams, worms, brittle starfish, and sea squirts.

Some luminescent squid, copepods, and fishes have glands that secrete luminous substances or bacteria. When released into the seawater, these substances produce clouds of sparks and are thought to function in distracting predators. The parchment tube worm known scientifically as Chaetocterus produces a luminous slime that possibly serves to attract prey into its tube. This form of bioluminescence is known as extracellular; however, most bioluminescent phenomena are intracellular processes, meaning the animals produce light from within their bodies.

Light organs can be simple or complex. Complex photophores often involve a layer of light-producing cells, a reflector, a layer of pigment to prevent the light passing into the body and being wasted, a lens or lenses, and a cornea somewhat like the human eye. In such cases, a beam of light can be focused and controlled by nerve impulses. A blood sinus provides oxygen for the process. Light from photophores has been known to travel up to 30-60 feet. In other types of photophores, luminescent bacteria produce light continuously, and in order to control light emission in a series of flashes, the animals have developed shutters, screens, and rotating photophores that disappear into pouches. Luminescent bacteria are generally not carried over from generation to generation through the host egg; instead, they must occupy the host animal after it has hatched. The light produced is in the visual spectrum. Dinoflagellates, jellyfish, lantern fish, and euphausids, which are shrimp-like animals, produce a blue light, while comb jellies produce a blue-green light. Luminescent flashes of comb jellies and jellyfish are among the brightest so far recorded.

Light organs can be concentrated around the eyes, mouth, or inside the body, they can hang above the head, or they can be scattered over the whole animal. Some fishes have a linear row of photophores down their body, which several authors say look like the lighted cabins in a passenger ship.

Most luminescent animals produce one or two colors of light, but one deep sea squid produces four colors-white, deep blue, sky blue, and red, and each color is located in a different part of its body. The color is thought to be controlled by color screens in front of the photophores or by differences in the chemical makeup of the luciferin.

It is speculated by researchers that this efficient flashing of cold light, with little release of heat, can function as a protective device, a lure for food, recognition signals among males and females, or as a light for vision. In one instance, a visual function is excluded-the majority of luminescent copepods are blind. To illustrate the function of signals, the males of a species of marine worm are attracted to the flashing females during their spawning season; a flirtatious account perhaps, but this ritual is necessary for the survival of the species.

In bacteria, luminescence is continuous. However, in certain one-celled organisms and animals, the light is discontinuous, appearing as flashes that require some triggering mechanism. These flashes can be triggered by touch, motion, light, electrical shock, or chemicals. If a sea pen is touched in one area, it can cause that area to glow and, in turn, may cause a wave of luminescence over the whole animal. The sea squirt with the scientific name Pyrosoma is comprised of closely packed individual animals forming a hollow cylinder, and light from one colony can stimulate another colony to glow. However, both natural and artificial light often inhibits luminescent processes by affecting the nervous system or the light-producing cells in the animal.

The intensity and frequency of bioluminescence is greatly reduced in seawater more than 3,000 feet deep, but with improved equipment, flashes from animals have been recorded from depths of 11,250 feet. Bioluminescent flashes in the deep sea must be a startling sight for those researchers and adventurers who frequent such depths in small submarines. But for those of us who walk the beaches or fish at night, bioluminescent dinoflagellate blooms in surface waters still bring a gasp of marvel. The sea aglow, whether at the surface or thousands of feet deep, is indeed an awesome sight.