766 LIGHT AND LIFE 



individual of the same species. Assuming a visual threshold equal to 

 that of dark-adapted man, and water of 90% transmission, Nicol's 

 figure is 5 to 16 meters. ; 



It will be seen that all the functions of bioluminescence that seem 

 possibilities for bathypelagic squids and shrimps involve an ocular re- 

 sponse in the same species, or, usually, in the same individual. It 

 thus seems mandatory to assume that photosensitivity evolved before 

 bioluminescence, and that it thereby provided the opportunity for 

 uses which could in turn act as selective pressures for the retention 

 and elaboration of the light-producing capacity, once it appeared. 

 This deduction is quite in keeping with the known ancient history 

 of vision, at least in the arthropods (trilobites and eurypterids) , and 

 would seem to be an unusually safe anchorage in proverbially shifty 

 waters. 



That is, it seems safe unless one also asks the question, "Why should 

 abyssal squids and Crustacea have developed eyes in the first place, 

 if there was nothing down there to see?" Actually, of course, we are 

 rescued from the egg-hen dilemma by several considerations. For ex- 

 ample, practically all the free-swimming macrofauna are by necessity 

 predators. Hence food-chain requirements suggest that abyssal forms 

 must have evolved from surface or shallow-water forms that were 

 tied originally to filter-feeding prey living on surface phytoplankton. 

 In this environment, sunlight would have provided, we assume, an 

 eye-evoking selective pressure. 



The postulation of an eye-photophore functional association is sup- 

 ported by the absence of photophores in the blind deepwater shrimp 

 Bentheuphausia (4) and by the finding of Welsh and Chase (32) 

 that, of the sergestid shrimps they studied, the four species with photo- 

 phores had the largest eyes, in accord with their suspicion (31) that 

 deepwater crustacean eyes degenerate in the absence of photophores. 

 An eye-photophore association is supported also by the existence 

 of cephalopod molluscs, and of shrimps, with excellent eyes but no 

 photogenic organs, but the complete absence of representatives with 

 the opposite complement— photophores but no eyes.« However, not 

 enough is known about crustacean distribution to permit the con- 



• Though this statement may he correct, strictly speaking, the inevitable excep- 

 tion to the law is indicated by Nicol's statement (23) that the majority of lumi- 

 nescent copepods are blind. Though copepod photophores are not eye-like, and 

 the animals are not bathypelagic, and we have already conceded that biolumi- 

 nescence has no perceptible function in many animals, it is disconcerting to have 

 an instance turn up in the Crustacea. 



