Vision 203 



inquire what is the minimum ilkimination for aquatic animals to see 

 sufficiently to feed, to find their mates, or to avoid dangers. Interest 

 also centers on the maximum depths in the aquatic environment at 

 which responses to day and night still exist. 



A laboratory test can be set up in which the intensity of light is 

 varied over a considerable range with relative ease. But it is not so 

 easy to ask a fish whether or not he can see small objects— at least 

 it is not so easy to get him to reply. An answer was obtained, how- 

 ever, from the fresh-water sunfish, Lcpomis, by using the response of 

 the fish to background motion. If this type of fish is placed in a 

 glass cylinder with a surrounding screen made of bars and spaces, the 

 fish responds by a turning motion if it sees the screen rotate. It is 

 through this kind of reaction that many fish maintain their position 

 in a stream. Reactions of this sort are involved in rheotaxis, dis- 

 cussed in an earlier chapter. By reducing the light intensity in an 

 experiment of this type until the sunfish no longer responded, the 

 minimum illumination under which the fish could see small objects 

 similar in size to the bars of the screen, was found to be one ten- 

 bilhonth (or 10"") of the value of full sunlight (Grundfest, 1932). 

 The threshold sensitivity of the human eye is similar, and the value 

 for other vertebrates is probably generally of the same order of 

 magnitude. 



Assuming that the threshold illumination for other species of fish 

 is similar to Lepomis, determinations were made from transparency 

 measurements of the approximate depths at which this minimum 

 intensity of light would occur under different circumstances, and 

 hence the maximum depths at which vision would probably be pos- 

 sible, although other optical conditions such as color and contrast with 

 background would have to be taken into account (Clarke, 1936). 

 In most lakes except the most turbid and in typical coastal areas 

 vision would appear to be possible for fish similar to Lepomis at all 

 levels right down to the bottom. In the open ocean beyond the mar- 

 gins of the continental shelf the water is far deeper than this maxi- 

 mum depth for vision. If deep-sea fishes can see as well in blue 

 light as the sunfish tested could see in green light, then vision would 

 be possible for them at depths of more than 700 m in the clear 

 tropical ocean. 



Many aquatic animals may show some activity response to the 

 increase or decrease of light at still lower intensities. Studies of the 

 diurnal vertical migration of zooplankton indicate that certain species 

 may react to light from the surface at 800 m and possibly at 1000 m, 

 or more than half a mile down. This depth probably represents the 



