260 POPULATIONS OF THE SEA 



different maximal depths for the perception of sunlight or moon- 

 light. Obviously, the spectral composition of the light must be 

 considered in such work. 



The problems of lunar periodicity are intimately connected with 

 the perception of very weak light intensities or small variations in 

 the light intensity. There are many periodic processes, such as 

 moulting, migrations, for example in the silver eel (Lowe, 1952), 

 and cycles of reproduction, the rhythms of which coincide with the 

 phases of the moon. The disco\ery of these phenomena is a typical 

 example of methods based on statistical comparisons. But it still 

 remains to be explained whether biological rhythms are merely 

 induced by variations in the intensity of the moonlight or whether 

 there are other factors involved. In intensive laboratory experi- 

 ments, Hauenschild (1955, 1956) investigated the lunar swarming 

 in the polychaete Platynereis dumerilii. He was able to induce 

 experimentally a new thirty-day rhythm with artificial light. 

 Factors other than light were excluded and the dominance of light 

 became evident during the course of his experiments. The minimal 

 threshold for variations in the intensity of the light is 0.02 to 0.1 

 lux. This means that animals in their native benthic habitat must 

 receive at least 0.02 to 0.1 lux more on nights of full moon than 

 at the time of the new moon. The intensity of bright full moonlight 

 at the surface is said to be about 0.5 lux. Certainly moonlight 

 undergoes reflection from the sea surface and extinction with 

 increasing depth below the surface in the same way as sunlight. 

 Therefore one must postulate that intensities sufficiently high to 

 exceed the threshold must reach benthic habitats. Consequently, 

 we need exact data on the penetration of moonlight into sea water 

 and we need physiological research on the sensitivity to very weak 

 intensities of animals exhibiting various responses to lunar 

 periodicity. There are many other questions concerning the special 

 endogenous mechanisms for these rhythms, their hereditary 

 character, and the external factors which determine them. 



Many instances are knowm of animals having been damaged not 

 only by ultraviolet light but also by visible light. Merker (1926) 

 observed a strong interference with respiration in Mysis under 

 strong illumination. Harvey (1929) recorded a decrease of about 



