Luminescent Organism

an organism capable of emitting light. Most terrestrial luminescent animals are arthropods, for example, fireflies, the cucujo click beetle of tropical America, larvae of fungus gnats (family Ceroplatidae), springtails, and myriapods. Also several species of earthworms are luminescent, and there are many different kinds of light-emitting marine animals.

Luminescent unicellular organisms include many dinoflagel-lates (for example, noctilucas) and naked flagellates, which frequently cause luminescence of the sea. Radiolarians also emit light. Luminescent coelenterates include many jellyfishes, hy-drozoans, siphonophorans, and sea pens. Light-emitting cteno-phores include nemertines of the genus Emplectonema, pelagic polychaetes of the family Tomopteridae, and several bottom-dwelling polychaetes, including forms that swarm during the mating period.

There are many luminescent species of pelagic crustaceans, for example, ostracods, copepods, amphipods, mysidaceans, euphausids, and decapods. Light-emitting mollusks include some pelagic nudibranchiates, heteropods, and pteropods; the bivalve Pholas dactylus; several cuttlefishes; and squids. The squid Watasenia emits a bright blue light. Luminescence is particularly developed in deep-sea squids. For example, in Thaumatolampas diadema photophores on various parts of the body emit dark blue, light blue, white, and red light. Among echinoderms, luminescence is characteristic of many brittle stars and some starfishes, sea cucumbers, and sea lilies. There is a large number of light-emitting pelagic tunicates, including Salpae, Appendiculariae, and Pyrosomatidae. The latter are among the most intensely luminescent organisms. Light-emitting organs are also found in many fishes, especially deep-sea species.

Many coelenterates and some mollusks elaborate luminescent slime. The deepwater shrimp Acanthephyra and the squid Heteroteuthis discharge a cloud of luminescent liquid to hide them from predators. In shallow-water fishes and cephalopod mollusks luminescence is most commonly caused by accumulations of light-emitting symbiotic bacteria, whereas self-luminescence is typical of deep-sea organisms.

In some animals the ability to emit light makes it possible for an animal to recognize or locate a member of the opposite sex (fireflies, some deepwater animals). Animals are often recognized by the location of their luminescent organs and by the color of the light emitted. In some species luminescence serves to frighten off predators or to attract prey. For example, some fishes attract their prey by using “flashlights” situated at the end of a long protuberance on the head (anglers), inside the mouth (Galatheathauma), or at the tip of a filiform tail (Saccopharynx).

Among microorganisms, the capacity to luminesce is observed in many fungi and some bacteria (about 20 species inhabiting mainly seas). Unlike animals, fungi and bacteria glow continuously. Luminescent bacteria often multiply in meat or fish at a low temperature but do not cause decay or form toxic substances. In unicellular organisms the biological role of luminescence is not clear; it is assumed that in these species luminescence is an accompanying phenomenon of oxidative metabolism. (For information on the biochemical nature of luminescence seeBIOLUMINESCENCE.)