HYDROGRAPHY 163 



sary food matters only in yellow light, which does not pene- 

 trate far, while Red Algse, which live in deeper water, 

 assimilate better in the blue light, which reaches lower 

 depths. 



The colours of some animals seem to be related to the 

 amount of Hght at the depths in which they Uve. On Sir 

 John Murray's cruise in the " Michael Sars," in 1910, 

 Dr. Hjort made a detailed investigation of the colours of the 

 Atlantic pelagic fishes in relation to their distribution in 

 depth, and his results show that the surface fishes down to 

 about 150 metres are colourless ; from 300 to 500 metres the 

 fishes are silvery or grey ; and at depths of 1,000 to 2,000 

 metres they are black or dark-coloured, and are associated 

 with red-coloured Crustaceans, which at that depth would 

 lose their colour and appear black. These prawn-like 

 Crustacea, found in various parts of the oceans at depths of 

 300 fathoms and more, only look red when red hght-rays fall 

 upon them, and as no red rays penetrate so far through the 

 water, these and other brightly coloured deep-sea animals 

 in their natural habitat must appear dark, and probably 

 quite inconspicuous, and only show up in their bright colours 

 when brought to the surface. 



Many of the surface animals, apart from fishes, are blue or 

 violet, and tone in with the sea around them, while others 

 down to 50 fathoms or so are gelatinous and quite transparent. 

 Some of the surface animals — fish, crabs, and others — of the 

 Sargasso Sea are coloured, and even shaped, so as to resemble 

 parts of the " Gulf -weed " on which they Hve, and so become 

 inconspicuous in their natural surroundings. 



Many pelagic animals respond to different degrees of inten- 

 sity of sunHght. Some Radiolaria in tropical seas flom-ish 

 on the surface, others at varying depths below in what may 

 be regarded as a subdued twihght, and one section of the 

 group, the Phaeodaria (Challengerida) Uve only at a consider- 

 able depth, over 400 fathoms, probably for the most part 

 below the photic zone. Other members of the plankton 



