Bioluminescence

Bioluminescence is the capacity of living organisms to emit visible light. In doing so they utilize a variety of chemiluminescent reaction systems. It has historically been confused with phosphorescence and the latter term is still frequently (and erroneously) used to describe marine bioluminescence. Some terrestrial species (e.g., fireflies) have the same ability, but this adaptation has been most extensively developed in the oceans. Bioluminescent species occur in only five terrestrial phyla, and only in one of these (Arthropoda, which includes the insects) are there many examples. In contrast, bioluminescence occurs in 14 marine phyla, many of which include numerous luminescent species. All oceanic habitats, shallow and deep, pelagic and benthic, include bioluminescent species, but the phenomenon is commonest in the upper 1000m of the pelagic environment.

                Bioluminescence involves the oxidation of a substrate (luciferin) in the presence of an enzyme (luciferase). The distinctive feature of the reaction is that most of the energy generated is emitted as light rather than as heat.  here are many different, and unrelated, kinds of luciferin, and biochemical and taxonomic criteria indicate that bioluminescence has been independently evolved many times. Marine animals are unusual, however, in that many species in at least seven phyla use the same luciferin. This compound is known as coelenterazine because it was first identified in jellyfish (coelenterates) and its molecular structure is derived from a ring of three amino acids (two tyrosines, and a phenylalanine). Nevertheless, many other marine organisms use different luciferins. In some animals (e.g., jellyfish) the luciferin/luciferase system can be extracted in the form of a stable ‘photoprotein’ that will emit light when treated with calcium.

                Other common planktonic luminous organisms are copepod and ostracod crustaceans, cnidarians (jellyfish and siphonophores) and comb jellies.

Copepods are in effect the insects of the sea and are the commonest planktonic animals. Many species are luminous. Most of them do not flash but have glands on their limbs or bodies from which they squirt gobbets of luminous secretion into the water as a defensive distraction. Ostracods, though less abundant, also produce luminous droplets from groups of gland cells. Usually this is a defense, but the males of some shallow-water species of Vargula swim up off the bottom to signal to the females. They encode a luminous message in the combination of the frequency of their light puffs, their swimming trajectory, and the timing of their displays. The displays are equivalent to complex smoke signals, or skywriting, using light. Occasionally both copepods and ostracods may swarm in such numbers that their secretions light up the wave crests or the entire ocean surface. The luciferin of Vargula (previously named Cypridina) was the first to be identified and is a tripeptide similar to coelenterazine, but made up of three different amino acids. Certain other ostracods use coelenterazine instead.

                Copepods and ostracods, like bacteria, dinoflagellates, and most other marine organisms, produce blue or blue-green luminescence. These wavelengths penetrate oceanic water best, so they are visible at the greatest range. Many cnidarians and comb jellies also produce blue light, but in a few the luminescence is a vivid green. These animals have incorporated a green fluorescent protein into the luminous cells, or photocytes. The energy from the luciferin–luciferase reaction is transferred to the fluor and is therefore made visible as green light. Some species of jellyfish, siphonophores, and comb jellies can not only flash but also pour out a luminous secretion. The secretion may include scintillating particles, which flash independently in the water. In other species of cnidarians the light-emitting cells (photocytes) are situated all over the surface of the body and a stimulus can set off one or more waves of light that may circle over the surface for several seconds. None of these animals has image-forming eyes, so their bioluminescent displays must be aimed at other animals, probably as a defense against predators or simply to protect their very fragile tissues from accidental damage by a blundering contact.

                There are many luminous worms, though most of them spend their time on the sea floor. Syllid worms (fireworms) come to the surface in shallow waters for a luminous mating display, whose timing is linked to the phase of the moon. They have a greenish light, while the pelagic worm Tomopteris is very unusual in producing yellow light. Scale worms when attacked can shed their scales, which then flash independently. A similar tactic is used by luminous brittlestars; when grasped they shed their arm tips,

leaving them to flash and writhe in the predator’s grip, like the lizard that sheds its tail. Many other echinoderms (relatives of brittlestars) are bioluminescent, including sea cucumbers, sea stars and sea lilies. Most of these live on the deep-sea floor and, like the jellies, lack image-forming eyes. Other bottom-living luminous animals include species of seaspiders, acorn worms, snails and clams, as well as

cnidarians such as sea pens and gorgonians.