202 Light 



as those which are abroad during the dayhght hours when Hght would 

 appear to be much more important. 



It is a mistake to assume that the pigment developed by an organism 

 necessarily plays a critical role in its present relationships. In some 

 instances the pigment may be primarily a by-product of metabolism 

 without any ecological significance. The red color of the deep-sea 

 shrimp is a case in point. Since there is no red light deep in the 

 water, the shrimp must appear as black as the deep-sea fish which 

 share the same habitat. Similarly, it is unnecessary to assume that 

 the coloration of the scarlet tanager, the oriole, and other birds with 

 brilliant plumage provides any protective resemblance. Certain 

 writers have practically turned themselves inside out in attempts to 

 find a protective function for all bright colors. No one in his right 

 mind would try to claim that CJiromodoris, a bright blue nudibranch 

 with orange and gold spots, was protectively colored as it creeps over 

 the gray rocks and brown seaweeds of the tidal zone of southern 

 California. In such instances either the bright colors do not attract 

 enemies, or the organism survives in spite of being conspicuous be- 

 cause of the possession of other, sufficiently advantageous attributes. 



Activity and Vision 



Photokinesis. Light controls the locomotory activity of many of the 

 lower organisms by a direct action upon their speed of locomotion— 

 a phenomenon known as photokinesis. The magnitude of this reac- 

 tion among animals without eyes was well illustrated by a laboratory 

 test on the larvae of the mussel crab. Pinnotheres maculatus (Welsh, 

 1932). These animals swam the length of a 29-cm trough toward a 

 light of 0.5 meter-candle in 34 sec. As the light intensity was in- 

 creased, their swimming rate was accelerated regularly until at 46 

 meter-candles they made the trip in 17 sec. Many other animals 

 show a similar increased activity under increased illumination whether 

 or not they are oriented to the light. This simple direct relationship 

 is of profound importance in the lives of many of the aquatic inverte- 

 brates and of the smaller terrestrial forms including insects. 



Vision. When activity is controlled by light among higher forms 

 it is usually through vision. We should inquire as to the circum- 

 stances of illumination under which vision is possible. On land day- 

 light is everywhere sufficiently strong at some period for the vision of 

 those animals that possess eyes. In the water because of the rapid 

 rate at which light is reduced in intensity we may suspect that depths 

 would soon be reached at which vision is no longer possible. Let us 



