Marine Biology

Bioluminescence

Bioluminescence of ‘living light’ is of wide occurrence in the marine environment, especially in the deeper zones of the sea. Marine organisms like dinoflagellates, radiolarians, hydroids, jellyfishes, alcyonarians, ctenophores, bryozoans, polychaetes, brittle stars, many crustaceans, gastropds, bivalves, cephalopods, prochordates,gastropods,bivalves, cephalopods, prochordates, fishes, etc., are able to produce light. Light –producing animals are of wide occurrence and represented in all the marine communities from the surface of the sea to abyssal depths. In the tropical waters, it is more common than in higher latitudes.

The light produced by living organisms is cold light, since only a very negligible quantity is lost as heat. This light is different from sunlight, as it is free from ultra-violet and infrared rays. Besides the predominant blue colour, colours such as yellow and green are also produced. The light is produced by an oxidizing reaction involving substances like luciferin, possibly a poly petide and an enzyme, luciferase. This may not be the case in all instances of light production. In many organisms, the reagents involved are not clearly known. Some animals are known to discharge luminous secretions into the water. In certain others, the reaction is truly intracellular the group of lunminous cells being backed by reflecting layer and sometimes covered by a lens. In some other animals, the presence of luminous bacteria in the tissues is responsible for the light production. In cases of extracellular luminescence, the luminous secretion is discharged into the ambient water. The light-producing glands in such cases are eith unicellular or multilcellyular structures distributed throughout the body or restricted to definite areas. In the nemertean worm , Emplectonema kandai, the photogenic cells are distributed throughout the body. In Chaetopterus variopedatus, the luminous secretion is liberated from glands situated in the aliform notopodia. In Odontosyllis, the glands lie at the base of the parapodia. In yhr ostraced Cypridina, the secretory cells of the photophore contain two kinds of inclusions; yellow granules of luciferin and small colourless granules of luciferase, When discharged, they dissolve in sea-water, producing a blue light. Many mollusks have luminescent organs. In the pelagic gasttopod phyllirrhoe bucephala, the luminescent organs made of single cells or group of cells are scattered all over the body. The luminous material is in granular form. In the bivalve photas dactylus, the luminous secretion is liberated from three different areas, namely, a narrow band on the anterior edge of the mantle, a pair of bands in the inhalant siphon and two triangular spots near the retractor and two triangular spots near the retractor muscle. In the deep-water squid Heteroteuthis dispar, the luminous gland has a reservoir having two openings to the mantle cavity.

Intracellular luminescence is also widespred among the marine animals. In Nocitluca, light production is achieved by granules present in the periphery of the cell. In hydromedusae, the luminous cells are grouped beneath the endoderm of marginal canals and in sea-pems in the endoderm of tentacles. Ctenophores have glandular structures within the rladial canals. Intracellular lluminescence is of wide occurrence among crustaceans such as shrimps and euphausids. The photophores of certain pelagic caridean and penaeid shrimps are distributed over the appendages. In Sergestes, a biconvex cuticular lens alnd an underlying layer of cells constitute the light organ. In the euphausid Meganyctiphanes, the photophores contain lenses and a thickened corneal layer, besides the gland cells. In the deep-sea squids such as Lealchia the ocular photophores contain lenses and a thickened corneal layer, besides the gland cells. In the deejp-sea squids such as Leachia, the ocular photophores have a central glandular mass, lying within a reflector of connective tissue. Many deep-sea telecosts such as myctophum, Argyropelecus, stomias, Photostomias and Astromesthes have numerous photophores, arranged in groups along the body.

The pattern of light production and the intensity of light produced etc are ubjeted to individual variations. In many cases , a luminous slime is secreted over parts of the body or released as a glowing cloud as in the case of the squid lHeteroteuthis. In some animals, the intensity of light produced is high. Thus, Macdonald (1927) observed that five specimens of the euphausid Meganyctiphanes norvegica placed in a two-litre jar could emit sufficient light to read a newspaper.

There is still much uncertainty about the possible function and significance of luminescence in marine animals. Even though bioluminescence does not apparently serve any useful purpose to the organisms which produce it in the surface water, in those forms living in the dark abyss of the ocean. It may help at least to some extent. This is true in the case of certain higher forms with specialized luminous organ, having a nervous control and arranged in definite patterns. Many deep-sea forms living in complete darkness use this as the only source of light. This is clearly indicated by the fact that eyes are present in deep-sea forms , whereas in the fresh water fauna of dark caves where there is no light producing organisms, the fishes are virtually blind. The light produced helps animals with eyes to recognize or locate individuals of their own species or their prey. Luminescence is thus an important source of light in the dark depths of the ocean. It is helpful to attract a passing prey or to meet the opposite sex and to keep together in large shoals as in the case of fishes. To some organisms, these may serve as recognijtion signs, a means of communication or courtship display. Thus both sexes of the polychaete Odontosyllis produce light during spawning. The luminescent organs on the tips of barbels and anterior fin rays of stomiatoid fishes such as Eustomias and Chirostomias and Ceratisa and other ceratioid angler-fishes, fuction as allure for the prey. In some, such as the deep sea shrimp systellapsis the squid Heteroteuthis, the mysid Gnathophausia and the teleost Malacocephalus which discharge a luminous cloud into the surrounding water when irriated, light production serves as an aid to frighten or confuse their attackers. The wide occurrence of bioluminescence in the animal kingdom, is an indication that is serves some useful function to the organisms possessing it.