CIRCULATION OF THE MACROPLANKTON 



Calanus acutus 



409 



o m. 

 100 



250 



500 



750 



1000 



Euphansia triacantha, E. vallentini and E. frigida. The rise and fall of Pleuromamma is 

 clearly shown in Table VII. The three species of Euphausia were too scarce for the effect 

 to be seen well in 8o° W, but reference can be made to the previous papers mentioned 

 above. 



It is evident that if a species rises and sinks sufficiently far every day it may spend an 

 equal amount of time in the northward moving surface water and the southward 

 moving deep water, and would thus have the means of preserving the limits of its distri- 

 bution without needing to sink to a lower average level at any one time of year. The 

 fact that these four species do not appear to make any marked annual migration lends 

 support to the theory that such species as Calanus acutus make their annual migration 

 for the special purpose of compensating for their northward drift in the surface layers. 

 It is true that Pleuromamma did not seem to reach the surface layers in December as it 

 did in other months, but this phenomenon is hardly comparable to the annual migration 

 of some other species. 



The distribution of some of the less abundant species is not so easily explained, for 

 some live in the warm deep water and rarely, if ever, enter the surface layers and others 

 seem to keep to the surface at all times. All the species which have been picked out and 

 counted from the N 70 samples in 8o° W may be grouped as follows : 



(1) No effective daily vertical migration but a marked descent from the surface layers 

 in summer to the warm deep water in winter: Rhincalanus gigas, Eukrohnia hamata, 

 Calanus acutus. 



