258 POPULATIONS OF THE SEA 



to vary one or more factors in the environment while keeping 

 others constant cither under optimal conditions or under any other 

 controlled situation. Nevertheless, in determining ecological 

 behavior, the value of the results obtained in this manner is 

 limited for the following reasons: (1) biological factors, especially 

 those due to the presence of other members in a biocenosis are 

 lacking in an artificial environment; (2) it is impossible to repro- 

 duce the physical and chemical environments completely; (3) in 

 nature, environmental factors represent an indivisible system. 

 Alteration of any one factor, whether of biological origin or not, 

 modifies all other factors. 



Despite these reservations concerning this method, I believe 

 that a combination of statistical comparisons and experiment will 

 yield good results toward understanding biological correlations in 

 the sea (Kinne, 1956). 



Experimental physiological methods have been used for only a 

 short time in marine biology. Recently, several problems have been 

 solved experimentally. I shall give only a few examples : the work 

 of Hardy and Bainbridge (1951), Moore (1950-1959), and Lewis 

 (1959) on the vertical migrations of zooplankton; of Hauenschild 

 (1955, 1956) on lunar periodicity of the polychaete Platynereis 

 dumerilii; of Abbott and Ballantine (1957) ; and of Ray and Wilson 

 (1957) on the poisonous effect of Gymnodmiuni in causing mass 

 mortality of fishes; of Kinne (1959) on Gammarus duebeni; of 

 Schlieper (1956; Remane and Schlieper, 1958) on osmoregulation 

 in brackish-water animals. Many others should be mentioned also. 



I must restrict myself here to a few special topics. I want to 

 consider light especially because I believe it to be one of the most 

 important factors in various biological processes in the sea. It is 

 essential in plant assimilation for primary production in the oceans 

 and therefore for productivity in general. Also animals are directly 

 affected by light. Light permits them to orient themselves visually. 

 Weak or normal light intensities often have a stimulating effect, 

 as on muscle tonus, for example, but strong intensities may be 

 inhibitory or even damaging. There are extreme gradients in 

 illumination from the equator to the polar regions, from the 

 surface to the depths, from the coasts out to the open sea. It varies 



