SECT. 4] LIGHT AND ANIMAL LIFE 463 



the ej^e will be exposed to some near-ultra- violet light the lens usually 

 contains pigments absorbing the far blue and the near-ultra-violet light ; in the 

 deep-sea forms the lens is transparent to wave-lengths down to about 310 mji, 

 (Denton, 1956; Kennedy and Milkman, 1956; and Motais, 1957). 



In vertebrates, two kinds of receptors, the rods and cones, are found in the 

 retina. The rods are used for night vision and the cones for day vision. As we 

 might expect, the eyes of deep-sea fish contain only rods (Brauer, 1908). These 

 rods have been shown moreover to contain golden coloured photosensitive 

 pigments, absorbing maximally at about 480 my. and especially suitable for 

 absorbing the blue light which penetrates best into the ocean and is most 

 commonly emitted by luminescent animals (Denton and Warren, 1957 ; Munz, 

 1958 ; Wald, Brown and Brown 1957). The rods are very long and the amount 

 of pigment they contain is so high that often over 90% of the light striking 

 the eye is absorbed in one passage across the retina (Denton, 1959). These 

 golden retinal pigments differ from those of coastal fish, which are generally 

 red in colour, as well as from those of freshwater fish, which are generally 

 purple (Wald, 1945). 



In the spectral absorption curves of their retinal pigments the deep-sea fish 

 form a very homogeneous group. We find that the freshwater eel even before it 

 leaves fresh water to migrate to the deep sea changes its retinal pigment from 

 one with the purple colour, characteristic of a freshwater fish, to one with the 

 golden colour, characteristic of the deep-sea fish (Carlisle and Denton, 1959; 

 Brown and Brown cited by Wald, 1958). 



Such golden coloured photosensitive pigments are not found only in fish. 

 Kampa (1955) showed that the euphausid eye (this is much more difficult 

 material to work with than the vertebrate eye) contained such a pigment ; her 

 work has recently been extended by Fisher and Goldie (1958). 



As the spectral composition and distribution of light change with depth, so 

 the animals are found to change in colour. In the upper waters whilst fish are 

 often dark on their dorsal surface and silvery on their ventral surface, many 

 animals are transparent or tinted with blue. As we go below the photosynthetic 

 zone, silvery fish become numerous and there is an increase in the numbers of 

 reddish, dark and black animals whilst, at greater depths, dark colours in- 

 cluding red and black predominate. Since the light in the deej) sea is pre- 

 dominantly blue, pigments which absorb and so do not reflect blue light will 

 appear black no matter what other colours they reflect ; thus, the deep-sea 

 prawn Sergestes, which appears bright red in daylight, will be inconspicuous in 

 the deep sea. 



The behaviour of marine animals depends very much on stimulation by light 

 and some of the most striking examples of this are furnished by the diurnal 

 migrations which animals of many phyla undertake — migrations which, in 

 some oceanic animals, extend over several hundred metres. It is generally 

 thought that animals which would find the upper waters unsafe in the bright 

 light of day, hide in the depths of the sea and move into the food-rich upper 

 layers at night time. Hardy (1953) has suggested that vertical migrations into 



