374 ADAPTATIONS TO MEDIA AND SUBSTRATES 



invented by pro-fishes, and partly for the purpose of allowing them to go 

 down from the surface to the less brightly lighted depths. A group of 

 English workers recently found that the absorption maxima of a num- 

 ber of marine fish rhodopsins range from A,505m[X to A,545m[l. They 

 hoped to find a relation between the particular maximum of a given 

 species and the depth preferred by that species. This was not established; 

 but the investigators failed to take the broader viewpoint, from which 

 one can see that since the A,510-540m|X band penetrates deepest, it 

 will be most conspicuous in the spectrum at any lesser depth. And since 

 the sun's rays within this range have equal facility of penetration through 

 water, a rhodopsin would be maximally efficient with its absorption peak 

 located anywhere in this band. 



There may be a close adjustment of a particular fish to a particular 

 quality of light available at his preferred depth — it may be that the in- 

 vestigators mentioned were simply unable to obtain sufficiently accurate 

 information as to just what depth a given species does prefer. More prob- 

 ably however there is a weight of other factors which usually make it im- 

 possible for a fish to be at all precise in this matter. A sandy-bottom 

 species like a flounder, for instance, has to be content with bottoms which 

 vary considerably in distance from the surface. Even more upsetting are 

 the barrier-effect of the thermocline and the seasonal turnover of lakes, 

 for a fish which responds to critical temperatures may swim at one depth 

 for a part of the year and at a very different depth, or at no particular 

 depth, for another period. Still other fishes may be free of any control by 

 the thermocline and still show no close restrictions as to depth. The wall- 

 eyed pike, for example, remains in deep water by day and comes into the 

 shallows to feed at night. Many marine fishes also show such rhythms. 

 Astronesthes, for example, lives in the gloom at 200 meters by day and 

 follows the twilight upward to spend the night at the surface. Such fishes 

 are responding to a particular quantity of light regardless of the time of 

 day, or the depth, at which they find it, and not to a quality of light 

 which is characteristic of a particular depth when the sun is high. 



Since we are thus led to expect only a very general correspondence 

 between differential sensitivity and depth of swimming, the A,505-545m[X 

 range of rhodopsin maxima seems close enough to the A,510-540m|l band 

 of best-penetrating wavelengths to give us the right to say that the very 

 color of rhodopsin itself, like the ruddy color of some photosynthetic pig- 

 ments of the deepest-living seaweeds (Rhodophyceas) is an adaptation to 

 water. That same red color, in human rhodopsin, is still another heritage 



