1 84 THE SEAS 



anemones, many worms, starfish, or sea squirts, appears 

 to have no value to the owner from the point of view of 

 protection at any rate. 



There are quite definite differences between the colours 

 of animals inhabiting different depths in the ocean as is 

 shown clearly in Plate 70. This has long been known to 

 sailors and fishermen ; they know that the fish from the 

 surface of the sea in tropical regions are usually sky-blue, 

 whereas the deep-sea fish are always darkly coloured, 

 black, brown, violet or red. During the cruise of the 

 Michael Sars, hauls made in the Sargasso Sea revealed 

 the presence near the surface of blue flying fish, others of 

 a silvery colour and the transparent young stages of such 

 fish as the eel ; at depths of 300 metres fish with silvery 

 sides and brownish backs were captured, while below 500 

 metres only black fishes and red prawns were found (Plate 



7i). 



It is noteworthy that these black fishes and red prawns 



only live in the regions where no or very little light can 



penetrate, living nearer to the surface in the Norwegian seas 



than in the tropics owing to the slighter extent to which 



light penetrates in the former. Another point of interest 



is that different species of the same genus, or even different 



varieties of the same species, which live at different depths 



are differently coloured, those from the greatest depths 



having the greatest and darkest pigmentation. The fishes 



from the abyssal region where there is no trace of light 



whatever appear to be usually brown, blue or violet, 



though what part pigmentation can play in the lives of 



animals always surrounded by impenetrable darkness it 



is impossible to say. 



The connection between light intensity and colour in 



marine animals is difficult to explain ; in some cases the 



colour produced may be protective but this is only the 



