Bioluminescence

The marine fauna includes a wide variety of bioluminescent species, with numerous examples known in almost all the major groups (Boden and Kampa, 1964; Herring, 1978; Tett and Kelly, 1973). Although the most numerous bioluminescent organisms are very small, chiefly dinoflagellates, the phenomenon is also commonly exhibited by many larger animals, especially fish, crustacea and cephalopods living in the mesopelagic zone. The common occurrence of bioluminescence in the sea, in contrast to its rarity on land, is probably the result of the much lower intensities of light in the sea. On land, even on the darkest

Stomiatoid Fish

Figure 4.13 Transverse vertical section through a subocular light organ of a stomiatoid fish, Astronesthes elucens.

(From J.A.C. Nicol (1960). Studies on luminescence. On the subocular light organs of Stomiatoid fish. J.M.B.A, UK, 39, 529.)

Figure 4.13 Transverse vertical section through a subocular light organ of a stomiatoid fish, Astronesthes elucens.

(From J.A.C. Nicol (1960). Studies on luminescence. On the subocular light organs of Stomiatoid fish. J.M.B.A, UK, 39, 529.)

nights, there is ample illumination for good vision by animals adapted for nocturnal life, but darkness in the sea is virtually total except for bioluminescence. It is especially common in warm surface waters, but evidently the deep levels are by no means uniformly dark because so large a part of the population carries light-producing organs (photophores).

The overall reaction involved in bioluminescence is the oxidation of a substrate, known generally as luciferin, catalysed by an enzyme, luciferase. The light energy released is transferred into another fluorescent compound which then emits its own light (of a different and characteristic wavelength). Almost all organisms that bioluminesce use a similar process with slight variants in the structure of the luciferin and luciferase between species. Only a few of the chemicals involved have been identified. Some animals discharge luminous secretions into the water. In others the reaction is intracellular, the mass of photogenic cells often being backed by a reflecting layer (Figure 4.13) and sometimes covered by a lens. Masses of luminous bacteria within the tissues are responsible for the light production in a few cases.

Bioluminescence is employed by marine animals in myriad ways some of which are described below (Robison, 1995). These include improving vision, attracting prey, recognition, for defence, attracting mates and territorial defence and for camouflage. However, why it should be advantageous to certain bacteria to luminesce is not obvious. It is possible that in some cases biochemical reactions that produce light are fulfilling some important function in connection with intracellular oxidations, the emission of light being merely incidental. However, although light may be of no direct use to bacteria, the value of this light to other organisms is so great that it has led to the evolution of symbiotic relationships of remarkable refinement in the elaboration of various types of photophores containing luminous bacteria.

In some species, photophores evidently emit beams of light which illuminate the field of vision. The fish Pachystomias has light organs close to the eyes which emit flashes of red light (Denton, 1971). In contrast to most deep-level fishes, which are sensitive mainly to blue light, the eyes of Pachystomias respond to red light enabling it to see without being seen. Light may also serve to attract prey, and a number of deep-water fish have remarkable luminous lures. These may take several forms, such as the luminous barbels in Linophyrne (Figure 4.12b) or structures resembling a fishing line with luminous bait (Gigantactis, Figure 4.12f) in some cases complete with hooks (Lasiognathus, Figure 4.12e). In Galatheathauma there is a luminous lure within the capacious mouth.

Photophores are often arranged in highly distinctive patterns which differ between closely related species and between the sexes, probably providing a means of recognition and facilitating shoaling. Some creatures can flash their lights on and off, and this may serve to confuse, alarm or dazzle attackers, or in some cases may even attract the attackers' own enemies. Luminous clouds squirted into the water, such as the luminous ink of the squid Heteroteuthis, and some worms, are presumably a means of defence, perhaps by dazzling or possibly by making attacking creatures themselves visible to their own predators. In some cases the attacker becomes coated in sticky luminous material.

The luminescence of surface water usually associated with blooms of dinoflagellates may possibly have some protective function by discouraging the upward migration of grazing copepods. They produce their light when stimulated mechanically. Such 'contact flashing' also occurs in mid-water amongst the larger gelationous animals and, once started, can spread as previously quiescent fish and other animals move away causing a 'storm' of light.

A notable feature of the photophores of many migratory species of the upper mesopelagic zone between about 200 and 700 m is that they are positioned so that their light is apparently directed mainly downwards, even in animals whose eyes are directed upwards, for example the hatchet fish Argyropelecus, and

Marine Hatchetfish

Figure 4.14 Diagram to illustrate how downwardly directed photophores may reduce the conspicuousness of animals at deep levels when seen out of focus. The two objects represent the silhouettes offish, one with and one without photophores, seen from below against a background of weak illumination from above. If the diagram is viewed in a dim light or with the eyes sufficiently closed to blur the outlines, the right-hand object becomes less visible than the left.

Figure 4.14 Diagram to illustrate how downwardly directed photophores may reduce the conspicuousness of animals at deep levels when seen out of focus. The two objects represent the silhouettes offish, one with and one without photophores, seen from below against a background of weak illumination from above. If the diagram is viewed in a dim light or with the eyes sufficiently closed to blur the outlines, the right-hand object becomes less visible than the left.

Opisthoproctus. These photophores can hardly play any part in illuminating the field of vision, and the most likely explanation of their function is that they camouflage the shadow of the animal when viewed from below against the faint background of light from above. If these photophores emit downwards a light equal in intensity to the background illumination, the silhouette of the creature must virtually disappear (Figure 4.14). Many mesopelagic euphausids, decapods and fish of the upper 700 m have bodies which are mainly transparent or silvery, though with some pigmentation of the dorsal surfaces, and have downwardly-directed photophores close to the most opaque parts of the body, and in many cases internally situated. Some of these animals are able to adjust both reflectivity and light emission to suit changing illumination, and their vertical migrations may well serve to keep them within the light conditions under which they are least visible. Detailed examination of the reflectors associated with downwardly directed photophores in the hatchet fish Argyropelecus aculeatus (Denton, 1971) reveals that these reflectors direct part of the light laterally, the light intensity varying with direction in a way similar to the reflection of light from above by the scales of silvery fishes, whereby they are made inconspicuous when seen from the side.

The mesopelagic macrofauna can be broadly divided into two main groups, one living above about 700 m and the other below this depth. The two groups have rather different adaptations with respect to bioluminescence and pigmentation. Above 700 m there are many species, both fish and crustacea, which are transparent but possess a few relatively large chromatophores. Many of the fish have silvery scales covered by dark chromatophores which can be expanded or contracted fairly rapidly to regulate the reflectivity of the scales. The crustacea have semi-transparent cuticles but are termed 'half-red' species because they become scarlet if their chromatophores are fully expanded. In both the fish and crustacea of this group, ventrally-placed, downwardly-directed photophores are common, usually internally positioned beneath the densest parts of the body, and the eyes are well developed. Many of the fish have swimbladders and make diel vertical migrations.

Below 700 m the animals are more opaque; the fish are usually dark in colour and the crustacea, the 'all-red' species, have heavily pigmented cuticles of orange chitin with numerous small chromatophores covering the surface. In this group bioluminescence is less common, and photophores, if present, are superficially placed, not directed downwards, and evidently have functions other than camouflage by counter-illumination. Fewer of these species make vertical migrations, the eyes are generally smaller and many of the fish lack swimbladders.

There can be little doubt that these differences between the two groups relate mainly to different light conditions in the two zones. The upper group lives where solar light fluctuates appreciably, and can adapt to these changes in several ways. These animals can vary from transparent to fully opaque, and from reflective to non-reflective, by expansion of their chromatophores. The fish can become darkly pigmented and the crustacea scarlet - a colour non-reflective in the blue light which is the only part of the solar spectrum to penetrate to these depths. Many of these creatures use counter-illumination for camouflage, matching the background illumination as they make changes of level. The lower group lives where the light is, at most, very dim. These are not translucent, nor do they need to make use of counter-illumination. They have slight control of reflectivity by changes in their chromatophores, the crustacea changing colour between orange and scarlet.

Bioluminescence is by no means limited to the pelagic species. There are many luminescent benthic animals, notably polychaetes, echinoderms and the common piddock (Pholas dactylus). Quite why piddocks should be luminescent since the adults are permanently trapped in the holes they bore into rock and wood, remains a mystery. Recently the chemicals responsible for the piddock's luminescence have been successfully extracted and used as a medicinal 'tracer' in the human body in place of radioactive tracers.

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  • Amanda
    Does global warming encourage bioluminescence?
    4 years ago
  • Jennifer Koch
    What causes bioluminescence in a marina?
    1 year ago
  • kristian
    How does bioluminescence relate to climate change?
    27 days ago

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