3S 6 DISCOVERY REPORTS 



and to show that according to whether an animal has a marked vertical migration or not, 

 so its distribution is related in general to the production of the phytoplankton in dif- 

 ferent areas — whether by the grazing activities as shown by Harvey (1934), by the 

 vertical migration itself being modified by pre-existing differences in the concentration 

 of the phytoplankton, or by both factors working together. 



Planktonic " navigation " 



We must note that zooplankton organisms which usually have a range of vertical 

 migration over only some 200 metres will on occasion make much greater vertical move- 

 ments. Particularly interesting are the deep-water concentrations of a number of 

 Copepoda in the region of the Antarctic Convergence on the line between South Georgia 

 and the Falkland Islands. There is a most marked concentration of C alarms propinquus 

 at 750-1000 m. shown in Fig. 62 and lesser deep-water concentrations of Calanus 

 rimillimus (Fig. 59), Ctenocalanus vanus (Fig. 72), and Scolecithricella minor (Fig. 78), on 

 this line, suggesting that these animals may use vertical migration as a form of "naviga- 

 tion" on a grand scale. It appears that these animals, on being carried by the surface 

 layer to the limit of the Antarctic Zone, may descend, as a balloonist would from one air 

 current to another, and make use of the great oceanic current system to return into the 

 Antarctic Zone again in the intermediate layer flowing back towards the Pole. Fig. 62 

 suggests that a separate race of Calanus propinquus inhabits the sub-Antarctic Zone 

 and is separated from the former race by the Antarctic Convergence. 



Reassortment of the plankton 



The consideration of vertical migration leads one to recognize that there must be a 

 continual reassortment of animals within the plankton community. If organisms A, 

 B, C and D are together at one night they may be considerable distances apart on the next 

 night, if their vertical migrational range is different, and the speeds of the different water 

 layers are not the same. Whilst the places of those passed on may be taken by others of 

 the same species which have followed, often, owing to the patchiness of their distribu- 

 tion, the numbers of each different species brought together within a given volume of 

 water on consecutive nights must vary considerably. 



SOME DIFFERENCES BETWEEN THIS AND OTHER 



MARINE AREAS 



Whilst this report is not the place in which to enter into elaborate speculations, it 

 may not be altogether unprofitable to reconsider briefly some of the principal differences 

 between this and other marine areas in the light of the discussion on the ecological 

 relationship between the zoo- and phytoplankton. Temperature is usually thought to 

 be the most important factor governing the character of the polar animal plankton, and 

 this appears to be very reasonable ; the alternative suggestions made in this section are 



