LIGHT IN MARINE ECOSYSTEMS 259 



regularly by day and also by season. These variations, depending 

 on the time of day and on the season, are much more precise than 

 temperature variations. Thus, some twenty years ago experimental 

 work showed that the reproductive cycle of birds may be con- 

 trolled by artificial illumination independent of temperature. 

 Hence, the precision in the timing of bird migrations is apparently 

 due to the regularity of the seasonal rhythm of light in the environ- 

 ment (Rowan, 1938). It should be noted that light is not a homo- 

 geneous factor, but that it includes several components: the total 

 intensity, the various wavelengths of the spectrum, and polariza- 

 tion. Therefore it is interesting to outline the physiological- 

 ecological effect of this factor in marine biology. In doing this, I 

 shall restrict myself to certain problems of animal physiology 

 since the relationship between plants and light will be discussed 

 by other members of the Congress in connection with primary 

 production. 



It is a fascinating fact that the honeybee is dependent on 

 polarized light in the sky in orienting itself when flying out to and 

 returning from its food supply. It is also known (Waterman, 1950; 

 Kerz, 1950; Burdon-Jones and Charles, 1958) that some littoral 

 animals, such as Limulns, Eupagurus, and Littorina, can perceive 

 polarized light. It is still not yet known whether polarized light is 

 merely a directional factor. Indeed, it would be interesting to 

 know to what depths littoral and oceanic animals can distinguish 

 this sort of light. Furthermore, it is important to learn whether 

 fishes can perceive polarized light and, if so, whether they orient 

 themselves by it during their migrations. 



The depths to which different animals can perceive light at all 

 is an open question. Clarke (1936) took 750 m as a rough limit for 

 a deep-sea fish, the eyes of which may be considered comparable 

 to Lepomis. Waterman et al., (1939) set 1500 m as the approximate 

 depth for crustaceans. It is necessary to solve this problem not 

 only by analogy but also by modern methods, for instance, by 

 measuring the absorption of eye pigments under limited artificial 

 conditions or by electro-physical measurements of the sensory 

 cells. In different animals excitation of sensory cells are stimulated 

 by very different light intensities. Hence, there will be very 



