I. Description
Bioluminescence is the ability of certain organisms to ; generate visible light as a result of a chemical reaction.
Although different animals use different chemicals, these are all variants of two chemicals: the substrate is called luciferin and the enzyme is luciferase. The chemical production of light is very efficient and does not generate heat like all other light sources.
Bioluminescence has a number of important biological and ecological functions. Light emission is used to avoid or confuse predators (e.g., shrimp and squid), attract prey (e.g., angler and flashlight fishes), and help attract mates (e.g., lanternfish and sea firefleas). Many organisms use bioluminescence for predator avoidance using a mechanism called counterillumination, in which the color and intensity of sunlight or moonlight entering the water column is matched to reduce the animal’s silhouette as it appears from below.
II. Occurrence of Bioluminescence
Although rare on land, bioluminescence is extremely widespread in the marine environment and it would be difficult to find a location where it does not exist. It occurs in all oceans, all latitudes, and at all depths. Bioluminescent systems are believed to have evolved independently at least 30 different times.
In the sea, the photic zone occurs, in ideal conditions, only tens to hundreds of meters into the water column. Therefore, light that occurs below the photic zone is generated by the organisms themselves. In fact, most bioluminescence occurs in the very deep ocean, and more than 90% of marine organisms from 300 to 3000 feet glow—a tribute to the importance of bioluminescence to a wide variety of organisms that inhabit waters where sunlight does not penetrate. However, a number of invertebrate species that occur within the photic zone also exhibit bioluminescence.
Among the thousands of bioluminescent marine organisms, the majority are planktonic. There are also enormous numbers of odier species, including lanternfish (Myctophidae), squid, decapod shrimp (e.g., the oplophorids and sergestids), krill, cope-pods, ostracods, amphipods, and gelatinous zooplankton. The benttooth bristlemouth (Cylcothone spp.) bioluminesees and may be one of the most abundant vertebrates on earth. These creatures are vastly important components of marine ecosystems and help form the basis of many marine food webs.
Near the surface, bioluminescence is often caused by di-noflagellates, single-celled algae. The bioluminescent flash this alga produces when physically agitated is probably related to predator avoidance. The flash may cause flight in copepod predators because the flash may have attracted the attention of something larger that might eat the copepod. Frequently found in vast groups, these organisms glow in the wake of swimming fish or dolphins (see later) or passing ships.
III. Bioluminescence and Marine Mammals
There is no known example of marine mammals emitting true bioluminescence. Nonetheless, it is undoubtedly a common part of the marine mammal ecosvstem and it may be used by some species to locate prey. Much of the information about marine mammals and bioluminescence is anecdotal.
As noted earlier, many squid species bioluminesce. Inasmuch as squid are the basis of the diet for many marine mammals, bioluminescing prey may be an important cue for locating prey. Sperm whales (Phijseter macrocephalus) dive to great depths and are known to feed on a variety of species, including many deep-water species and giant squid. Thus, sperm whales may be aided
in locating prey that are bioluminescent, although the species’ echolocation capabilities are almost certainly the primary means of locating its prey. A number of deep-diving marine mammals [e.g., elephant seals (Mirounga spp.), beaked whales, and other odontocete cetaceans] probably key on bioluminescing organisms when searching for prey. This raises the question, however, of bioluminesce being maladaptive if it increases the chances that an organism will fall victim to predation. The social and ecological significance of bioluminescence must outweigh the cost of announcing an organism’s presence to a predator.
Some copepod species are among those that bioluminesce, including some that are preyed upon by skim-feeding large whale species, such as right whales (Eubalaena spp.). Some scientists believe that this attribute is an important visual cue to prey location for foraging right whales.
Vast swarms of dinoflagellates that bioluminesce when agitated are well known to all mariners and are occasionally seen by divers and beachgoers. These organisms bioluminesce when stirred by the mechanical action of a boat hull passing through them or by wave action. The same phenomenon has been seen when dolphins, riding on the pressure wave in front of a boat, for example, swim through the dinoflagellate swarms.
Vision, and therefore light, is vital to prey capture in many marine mammal species. While not true bioluminescence, cetaceans may use reflected ambient light in the capture of prey. For example, the inside surfaces of the mouth and the lower jaw of some sperm whales often have light-colored pigment. Some scientists believe that the reflective surface may be attractive to some prey, e.g., squid, and that prey actually move toward the reflective surface. This reflectivity may be enhanced by bioluminescence occurring on the white lower jaws of sperm whales who have been feeding on bioluminescent squid, a suggestion made because sperm whale jaws on whaling ships have been seen to “glow.” Also, several authors have suggested that the asymmetric (i.e., present on the left side, but absent on the right) coloration patterns on the jaw and head of fin whales (Balaenoptera physalus) may serve as a light-colored backdrop by which small schooling fish are corralled. These are intriguing hypotheses that have not been tested. Of course, epidermal pigmentation patterns also play important social signaling roles for marine mammals that live in groups.
