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• 9 Bioluminescence T he title of this book relates to bioluminescence, or biological light, so it is most appropriate to include a section on this dazzling subject. Hank Compton more eloquently referred to it as “fire.” On land, where light is abundant, few organisms produce bioluminescence. The few terrestrial examples are fireflies and glowworms. In the sea, where light at the surface is quickly absorbed, many different kinds of vertebrates and invertebrates produce light. Bioluminescence is seen in at least 5 superorders of teleosts (bony fishes), some sharks, squid, crustaceans, and other invertebrates . The ability to produce light has been noted in at least 45 families of fishes. Most are teleosts with only 2 elasmobranch (cartilaginous fishes) families doing so. Light producers live at all depths from the surface to the bottom of ocean trenches. The strongest bioluminescence is no brighter than moonlight, however, so at shallow depths it is significant only at night or in very murky conditions. Thus, there are only a few shallow-water luminescent fishes, including slipmouths, midshipmen, and flashlightfishes. In contrast, in the deep ocean bioluminescence is enormously important because it is the only light that most of the animals ever see. Most bioluminescent fishes inhabit the mesopelagic zone and are found between 300 and 1000 m. In some localities, up to 66% of the fish species and more than 50% of the individuals of mesopelagic fishes are bioluminescent. The lanternfish family has more bioluminescent genera and species than any other family, but the spookfishes, grenadiers, batfishes, seadevils, and stomiatoids have numerous light-bearing representatives. The light organs and modes of light production are more diverse in the fishes than in any of the other marine animals. Bioluminescence occurs when a protein (luciferin) is combined with oxygen in the presence of an enzyme (luciferase). The resultant chemical energy is converted into light energy with almost none lost to heat. Most bioluminescence associated with marine organisms is blue-green, although red and yellow wavelengths have been reported. The luminescent organs, called photophores, are located in various places. Many deep-sea fishes display rows of photophores along their sides or on their bellies that produce their own light (self-luminescence). Other species may have depressions containing colonies of bioluminescent bacteria that perform the chemical reaction and produce light (bacterial luminescence). There are about 65 genera with bacterial photophores and 130 genera that are self-luminous. Self-luminous fishes commonly possess a series of light organs along the ventral surface of the body that direct the light downward. A few, however, have some photophores along the dorsal surface, and some stomiiformes have an abundance of small, simple organs along the dorsal surface that enable the fish to be silhouetted in light. Some have light organs in the mouth as well as on the jaws. There may be more than 1000 photophores per individual and more than one type of light organ on an individual. Light organs vary from very small, unpigmented structures (such as those on the back and fins of the stomiiformes) to very complex structures with the glandular portion surrounded by an efficient reflector that directs the light through a lens. This lens can then concentrate the beam as it is emitted. Some have irislike structures that control the amount of light emitted. In most self-luminous, bony fishes the photophores are innervated, that is, they appear to be under the control of the nervous system. Although most luminous fishes are self-luminous, many depend onsymbioticbacterianurturedinspecialgland-likestructures.Most shallow-water luminous fishes have only bacterial photophores, usually in the region of the eyes or gut, but some deep-sea fishes McKee_Book 1.indb 9 10/15/13 12:46 PM 10 • bioluminescence may have bacterial light organs in other parts of the body. Luminescent structures can be associated with a variety of body parts. Grenadiers, some cods, and certain hakes and roughies are examples of groups with luminescent structures at or around the anus on the ventral surface. Slipmouths have a bacterial light gland around the esophagus, a lens that concentrates the light and directs it into the gas bladder, and a pigmented shutter that can conceal the light. One species can beam luminescence from the mouth as well as from the ventral surface and can direct light as a discrete beam. The lantern eyes bear bacterial light organs beneath their eyes. Deep-sea squid exhibit bioluminescent spots on their tentacles and encircling their eyes. Some produce a bioluminescent “ink” they can discharge into the water...

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