LIGHT IX MARINE ECOSYSTEMS 261 



one-half from the normal in the frequency of the heart beat in the 

 copepod Calanus, and Marshall et al. (1935) noticed a strong 

 increase in the respiratory rate in this same organism on exposure 

 to light. Plants are also known to be damaged by strong light: 

 most diatoms occur at a depth of several meters below the surface. 

 Many animals, especially in littoral habitats, prefer the dark or 

 are active only at night, and remain hidden during the day. This 

 too seems to indicate that they shun the light. 



The physiological basis for this phenomenon still needs to be 

 investigated. Probably strong light affects endocrine systems 

 (Rasquin and Rosenbloom, 1954). Damage resulting from exposure 

 to light can be increased markedly in the presence of fluorescent 

 substances. The visible rays of these substances may be harmful to 

 both animals and human beings. Such effects are widely known in 

 medicine. In marine biology, on the other hand, only Pereira (1925) 

 has reported such an effect. Eggs, sperm, and larvae of the sea 

 urchin Arhacia are not injured by exposure to light in pure sea 

 water or with the addition of eosin to cultures kept in the dark, but 

 they soon die when exposed to light and eosin at the same time. 

 Kalle (1949, 1951) demonstrated that sea water contains substances 

 with a light blue fluorescence. Johnston (1955) confirmed this, and 

 Koe et al. (1950) likewise isolated fluorescent substances from the 

 sea bottom. The origin and chemical nature of these substances is 

 not known, but it is supposed that they are derived from the 

 metabolism and decomposition of organisms. It seems to be proved 

 that the plant pigments phycoerythrin and fucoxanthin act as 

 photosensitizers. Therefore, we may suppose that some pigments of 

 pelagic diatoms or autotrophic flagellates are likewise fluorescent 

 and may have an effect as sensitizers if in solution. There are two 

 facts then : (1) the damaging effect of light on many animals in the 

 sea and (2) the presence of fluorescent substances in sea water 

 which in terrestrial habitats have a sensitizing effect. I am inclined 

 to combine these facts to form the hypothesis that within the sea 

 photodynamic effects are present which may play an important 

 role in ecological relations. Naturally this hypothesis needs further 

 confirmation. I think, however, that we can extend our under- 

 standing of such phenomena as, for example, under the heading 



