SECT. 4] 



LIGHT AND ANIMAL LIFK 



459 



photographs of animals larger than 1 cm were obtained from more than 1300 

 exposures, it appears that most of the flashing was produced by very small 

 organisms, perhaps chiefly by unicellular forms. Up to the time of writing, 

 relatively few studies of bioluminescence have been made in the deep sea, but 



LIGHT INTENSITY (^W/m^) 

 10"' 10"' 



Fig. 1. Schematic diagram to show the penetration of sunhght into the clearest ocean 

 water (A; = 0.033) and into clear coastal water (A; = 0.1 5) in relation to minimum in- 

 tensity values for the vision of man and of certain deop-soa fishes. The approximate 

 minimum values for the attraction of Crustacea (Nicol, 1959), colour vision in man, 

 and for phytoplankton growth arc indicated as well as the range of intensity of 

 bioluininescence in the sea. The penetration of moonlight and the approximate 

 intensities of upward scattered (?/) sunlight and moonlight in the clearest ocean water 

 are also shown. 



Since light penetrating deep into the sea is confined to a narrow band of wave- 

 lengths, the values given for colour vision represent the approximate inaximuin 

 depths at which a blue hue would be observed. 



present evidence is that in many situations the light produced locally by living 

 organisms provides a luminous flux as strong as, or stronger than, the daylight 

 penetrating from the surface. Bioluminescence providing continuous light or 

 individual flashes of intensities above the threshold for animal perception has 

 been recorded at all depths investigated to 3750 m. The light intensity, fre- 

 quency and duration of the flashes have been shown to vary greatly and 

 indicate the presence of very different populations at various depths. The 

 strength and varied pattern of the light from oceanic organisms, coupled with 



