264 



POPULATIONS OF THE SEA 



is one of the most important factors in primary productivity. 



(2) Light aids many animals by means of a visual orientation. 

 Polarized light may have special significance in this connection. 

 The importance of weak light intensities, especially in connection 

 with lunar periodicity must also be taken into consideration. 



(3) Light often damages animals. A hypothesis is proposed that 

 certain sensitizing substances may play a part in this. (4) Light 

 and temperature seem to be correlated or interfere with one 

 another. (5) Obviously tolerance to a decreased salt content is 

 lessened in the light. 



We should try to find a physiological interpretation for the last 

 three points, but we do not have sufficient facts and must depend 

 on mere suggestions of a rather theoretical nature. Provisionally I 

 find this in the following considerations: Light as well as tempera- 

 ture and salt content affect an animal's respiration. In its normal 

 habitat, the respiration rate is in part conditioned by temperature 

 and in part by light. Respiration has two chief components: 

 (1) internal respiration within the cells and (2) the exchange of 

 gas between an animal and its environment. These must be in 

 balance. It will shift in the same direction with a rise in tempera- 

 ture and with increasing light. Therefore, in order for respiration 

 to continue within its optimal range, an animal must avoid light 

 with a rise in temperature and A'ice versa. In Fig. 1. the crosses 



Day 



B 



Night 



+ + + 



+ + 

 + + 



Fig. 1. The combined effect of light and temperature on the vertical distribution 

 of zooplankton (schematically). 



