576 THE BIOLOGY OF MARINE ANIMALS 



scales it seems clear that Na + at first stimulates the nervous system, then 

 the photocytes; K+ may act similarly, but always produces a protracted 

 response indicating maximal direct excitation of the photogenic cells. 

 Any gross alteration of the external medium is liable to produce depolari- 

 zation of the nerve membrane, causing excitation (e.g. fresh water, sugar 

 solutions, K+, Na + , NH 4 +, etc.). And the photocytes of different species 

 will themselves be excited directly by these abnormal media (28, 43). 



BIOLOGICAL SIGNIFICANCE OF LUMINESCENCE 



There is still much uncertainty about the possible function and significance 

 of luminescence in marine animals, although several explanations and 

 theories have been advanced. This is due in large part to the difficulty of 

 observing luminescent organisms under natural conditions and of noting 

 the possible effects of such displays on other animals. Neither is it easy to 

 devise experiments to put to test the various theories dealing with the func- 

 tion of luminescence, as has been done by Sumner, for example, in his 

 investigations of the survival value of colour responses. There are definite 

 indications, however, that not one but several explanations are involved, 

 and that luminescence will prove to have different functions in various 

 species. 



The luminescent bacteria and fungi usually shine continuously, but in 

 nearly all luminous animals light emission is under the control of the 

 animal. Harvey has suggested that the mechanism for producing light has 

 arisen in the course of evolution from some chemical process already 

 present in the cell, possibly one involved in cellular respiration. The wide- 

 spread and irregular occurrence of vital luminescence indicates that it has 

 appeared and evolved on many occasions. At first a fortuitous accompani- 

 ment of some intracellular reaction, without intrinsic significance, as in 

 bacteria, it has been elaborated for special purposes in higher organisms. 

 There is, of course, no reason why such transformations should not be 

 occurring today, and animals may indeed be encountered showing various 

 steps in the evolution of luminescence. In the protozoa, luminescence, it 

 has been suggested, may be a by-product of some reaction evoked directly 

 or indirectly by external stimulation and have no special biological role. 

 In Crustacea, molluscs and teleosts, however, the light-producing structures 

 are frequently so complex and highly organized that it is reasonable to 

 conclude they subserve special functions, and have considerable adaptive 

 and ethological significance. 



The main theories dealing with the function of luminescence may be 

 summarized under the headings of protection (specific and individual), 

 luring of food, illumination of surroundings for visual purposes and recog- 

 nition signals. These will be considered as far as the available evidence 

 permits. 



A general hypothesis dealing with the luminescence of planktonic and 

 other organisms has been outlined by Burkenroad (12). He suggests that 

 luminescence may act in the manner of a burglar-alarm in that the light 



