460 CLABKE AND DENTON fCHAP. 10 



the high transparency of the water, suggest that bioluminescence is probably 

 of great importance in the ecological relations of the deep sea, 



Bioluminescence is not always independent of light and we may give as an 

 interesting example the behaviour of the dinoflagellate, Gonyaulax polyedra. 

 This organism has an endogenous diurnal rhythm in which phases of bright 

 and feeble luminescence alternate. This rhythm is normally synchronized with 

 the natural cycle of night and day. During a phase in which its capacity for 

 luminescence is high (normally night time) illumination diminishes this capacity. 

 During a phase in which its capacity for luminescence is low (normally daytime) 

 the effect of illumination is to enhance the luminescence during the subsequent 

 "bright" phase. The relationships between luminescence, photosynthesis and 

 light in Gonyaulax polyhedra are discussed by Sweeney, Haxo and Hastings 

 (1959). 



Animals produce luminescence either intracellularly or extracellularly and 

 sometimes by controlling the activity of luminous bacteria. Sometimes the 

 flashes emitted are given by small photophores, sometimes they cover the whole 

 animal, whilst sometimes a luminous secretion is discharged into the sea. But 

 whatever the source of the flash, it is usually blue or bluish green in colour, 

 other colours of light being rarely emitted. Furthermore, although this light 

 extends over a considerable part of the spectrum, it is confined to much 

 narrower bands of wave-lengths than either daylight (at the surface of the sea) 

 or the light given by artificial sources of light such as a tungsten lamp or a 

 common fluorescent tube. In, for example, Myctophum punctatum (a lantern 

 fish) more than 80% of the luminescent energy lies between 450-550 my.. Since 

 the known visual pigments of deep-sea animals absorb blue and green light very 

 efficiently, we can usually get an approximate idea of how eff'ective a lumines- 

 cent flash will be for vision by assuming that all its energy is concentrated at 

 the wave-length of maximum emission. Nicol (1958, 1960), in the course of a 

 most extensive study of the physiology of bioluminescence, has compared the 

 energies emitted by a number of animals, and, for long-lasting flashes, he gives 

 values ranging from about 1 x 10"^ to 2 x 10"^ jxW per cm^ of a receptor 

 surface placed at 1 m from the luminescent source and normal to it. Such 

 energies are trivial when compared with daylight (the solar constant is 0.135 

 W/cm2) but they would generally be well above the visual threshold for the 

 dark-adapted human eye looking at a small source of light. 



The combined luminescence of countless organisms near the surface, or 

 concentrated at some other depth, might sometimes act as an extended source 

 of light. However, luminescent light sources, unlike penetrating daylight, are 

 usually small sources ; the luminous flux which they give on a receptive surface 

 will decrease, not only because of the absorption and scattering of light by sea- 

 water, but also as the inverse square of distance as the light spreads out on 

 leaving the source. 



The visual problems of deep-sea animals are, then, to detect both penetrating 

 daylight and the flashing lights of other animals and to use both daylight and 

 luminescent light to see the objects around them. 



