218 Light 



morning, as well as the depth to which the animals migrate, is corre- 

 lated with the differences in time of sunset and sunrise and in the in- 

 tensity of the noon sun during January and July. The complexity of 

 the response to light is revealed by the fact that in this same area, the 

 males and the various young stages of the same species exhibit quite 

 different patterns of behavior. 



Meters 

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 40 

 60 

 80 

 100 

 120 



100 

 120 



25-26th January 

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20 



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60 



80 



100 



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Fig. 6.17. Diurnal vertical migration of female Calanus finmarchicus in the Clyde 

 Sea area during 24-hour periods in January and in July. The abundance of the 

 population is indicated by the width of the figures. The times of sunset and 

 sunrise are indicated by arrows. (Nicholls, 1933, Copyright, Council of Marine 

 Biol. Assoc, of United Kingdom. ) 



Reactions to gravity and to temperature change often modify the 

 course or the extent of the vertical migration (Clarke, 1933). The 

 various theories dealing with the reactions of the zooplankton to the 

 factors believed to control vertical migration have been summarized 

 by Gushing (1951). The magnitude of the migration is indicated by 

 the fact that the copepod population may often move more than 100 

 m in a vertical direction. This represents a journey of more than 

 15,000 times the animal's own length twice each day. Even more ex- 

 tensive diurnal vertical migrations are carried out by euphausiids, 

 fish, and perhaps other large active forms. The widespread occur- 

 rence of changes in level of huge populations in the open ocean, dur- 

 ing the course of the day, has been revealed by midwater echoes re- 

 ceived by the underwater acoustical equipment of ships. A discus- 

 sion of the kinds of animals probably responsible for this "deep scat- 

 tering layer" and its vertical oscillation is presented by Moore (1950). 



Similar vertical movements occur in terrestrial environments al- 



