INTRODUCTORY 



21 



to changes in light intensity and are discussed in Chapter 8. Other phen- 

 omena of great interest concern differences in the pigmentation, eyes and 

 luminescent organs of pelagic animals from different depths. In the upper 

 waters many animals are transparent, or are tinted with blue. Below the 

 photosynthetic zone, where the light becomes weak, silvery fishes form a 

 conspicuous element of the fauna, and there is an increase in the number 

 of reddish or dark-coloured species. At greater depths, where daylight 

 fails, uniform dark colours, black, violet or red, prevail. Since the longer 

 wave-lengths of visible light are rapidly absorbed in the open sea, reddish- 



001 



0-05 



0-1 0-5 / 5 



Percentage of Surface Light 



100 



Fig. 1.7. Penetration of Daylight into Oceanic Water (Sargasso Sea) 



The numbers on the graphs refer to the median of the spectral ranges which are as 

 follows: red, 600-700; blue, 346-526; violet, 310-450; green, 490-620 m/<. (From 

 Clarke (9).) 



coloured crustaceans in the twilight zone will reflect little incident light 

 and in fact appear black at levels below about 25 metres. 



The eyes also exhibit great transformations, and those of mesopelagic 

 animals are sometimes large and specialized in shape and structure. 

 Degeneration of eyes is uncommon among abyssal cephalopods and fish, 

 but in some genera — e.g. Macrurus (Teleostei) — the species occurring 

 below the level of light penetration tend to have smaller eyes than meso- 

 pelagic forms inhabiting the twilight zone. Certain bathypelagic species of 

 crustaceans, squid and fish, however, have completely degenerate eyes and 

 are blind. Some groups also show a correlation between the size of the 

 eyes and the presence or absence of photophores. There are luminescent 

 animals at all levels in the sea, but it is estimated that luminescence is most 



