222 



Light 



Fig. 6.19. Flashlight photograph of grunion {Leuresthes tenuis) spawning on 



Malibu Beach near Los Angeles, California. The females bury themselves tail 



first in the sand. The males then circle around them and fertilize the eggs. 



(Photo by Moody Institute of Science.) 



only during the proper hour. Just how this timing is controlled has 

 not yet been discovered despite a considerable amount of study. The 

 complexity of the interplay of the tidal and lunar influences and the 

 remarkable precision of the responses of the grunion are indicated in 

 Fig. 6.20. 



The foregoing are representative of the varied occurrences of lunar 

 periodicities in nature of which many more instances could be cited 

 (Korringa, 1947). A definite control of the timing of the reproduc- 

 tive cycle is usually indicated, and frequently other special activities 

 such as swarming or luminescent discharge are involved. The mech- 

 anism by which the timing is controlled is far from clear, however. 

 For some organisms the changing amount of moonlight may be of 

 chief importance either directly or as a degree of contrast with sun- 

 light or starlight. Since the intensity of full moonlight is more than 

 10" of noon sunlight, it is well above the threshold for the response 

 of many organisms. As an alternative the cyclic change in the rela- 

 tion of the moon's gravity has been suggested as possibly controlling 

 the activity of fish and other organisms. Many sportsmen strongly 

 believe that such lunar gravitational fluctuations influence the success 

 of their fishing. At the present time, however, we have no evidence 



