204 Light 



biological zero for the response to daylight penetrating from the sur- 

 face. Below this level day and night no longer exist, and no seasonal 

 change in illumination can be detected. Since the average depth 

 of the ocean is about 4000 m, it is clear that more than three-fourths 

 of the volume of this environment is devoid of any influence of day- 

 light. The whole of the deep sea is by no means completely dark, 

 however. Light of biological origin occurs irregularly through the 

 marine environment, and bioluminescent organisms are sometimes 

 very abundant in the sea. 



Bioluminescence. Our discussion up to this point has dealt pri- 

 marily with light emanating directly or indirectly from the sun, but 

 light of biological origin, known as bioluminescence, or popularly as 

 "phosphorescence," has ecological significance under certain circum- 

 stances. As mentioned above, bioluminescence is the only source 

 of light in the deep sea. Near the surface of the ocean and on land 

 it is frequently prominent during the night, but it rarely occurs in 

 fresh water. Luminescence is produced by members of various taxo- 

 nomic groups scattered through the animal kingdom including in the 

 sea certain fishes, crustaceans, coelenterates, and many other in- 

 vertebrates, and on land particularly the insects. Fungi and such 

 microorganisms as the dinoflagellates and many groups of bacteria 

 also are luminescent. For a complete summary of the occurrence 

 and physiology of bioluminescence the reader should refer to the 

 book on the subject by Harvey ( 1952 ) . 



The very considerable amount of illumination that can be provided 

 by luminescent organisms may be appreciated by anyone who catches 

 some fireflies and brings them into a darkened room. A few fireflies 

 in a bottle will provide sufficient light for reading newsprint. The 

 "phosphorescence" of the sea sometimes is almost dazzingly brilliant 

 on a dark night, at which time waves and the wake of a boat appear 

 like "burning water." The bodies of fish and other organisms swim- 

 ming through the water are outlined by millions of tiny lights. This 

 illumination is caused by the luminescent discharge of the plankton 

 organisms. Measurements have shown that the intensity of light 

 emitted by the surface waters of the sea may be one thousand times 

 greater than the threshold intensity for the vision of man. 



Bioluminescence is thus sufficiently strong to evoke reactions under 

 a variety of circumstances. Although this living light may serve no 

 useful purpose in some situations, in other instances it probably 

 fulfills one or more of the following functions : ( 1 ) illumination, ( 2 ) 

 recognition, (3) lure, and (4) warning. 



The luminescence of deep sea animals undoubtedly provides a use- 



