3 i2 DISCOVERY REPORTS 



either ballast or gas. The "navigation" of the marine plankton organisms would be no 

 doubt an automatic one. The data were now re-examined for any evidence for or against 

 this hypothesis. 



Fortunately we made a sufficient number of observations to determine broadly the 

 relative powers of vertical migration of the more important plankton forms within the 

 area, and in order to follow the discussion frequent reference should be made to the 

 special section devoted to the subject (pp. 231 etseq.), and details of the vertical migration 

 studied in the graphs which illustrate it. We see that each of the macroplankton species 

 which showed in its horizontal distribution smaller numbers in areas of greater phyto- 

 plankton production showed also a marked vertical migration: i.e. all the Euphausians 

 and their larvae, the amphipods Paralhemisto, Primno and Vibilia, the pteropod Limacina 

 helicina and the tunicate Salpa fusiformis. The behaviour of the different copepods is of 

 special interest. We saw that in their horizontal distribution in relation to the phyto- 

 plankton (indicated by phosphate values) we could divide the Copepoda into three 

 categories, Groups A, B and C. Those in Group A, Calamis prophiqmts, Clmisocalanus 

 laticeps, Ctenocalamis vamis, Parenchaeta antarctica, Scohcithricella minor, Pleuromamma 

 robusta, Metridia gerlachei and M. Ulceus, were all fewer in the zones of greater phyto- 

 plankton production, as were the macroplankton organisms just mentioned, and we 

 see that all these forms have also a marked vertical migration. In Group B we placed 

 Calamis acittus, Rhincalamis gigas and Oithona frigida, copepods which showed little 

 or no correlation with phytoplankton concentrations. They are forms which are less 

 frequently found in the upper 50 m., and they shozv little or no vertical migration (see 

 Figs. 108 to 113). 1 Not only does this mean that they may be almost indifferent to the 

 phytoplankton concentrations above them, but that if the hypothesis is correct they 

 will have usually no means of "navigation", and it is noteworthy that it is just these 

 three forms which we find to have the most uniform distribution of any of the 

 Copepoda (see Figs. 63, 67 and 84). We have said "almost indifferent to the phyto- 

 plankton" and "usually no means of navigation", because presently we shall see that 

 in cases of extreme phytoplankton production they may be influenced. 2 In the third 

 group of Copepoda we placed Calamis siniillinuis, which " prefers" a high concentration 

 of phytoplankton but avoids extreme quantities, and Drepanopus pectinatus, which so 

 far as our investigations go "prefers" and flourishes in the highest phytoplankton 

 concentration met with. Whilst these two species show a marked vertical migration, 

 they differ from the other Copepoda in coming to the surface a long time before any of 

 the others do and remaining at the surface longer (see Figs. 99 and 100). Just as they 

 occur in higher concentrations of phytoplankton than do the other forms, so do they 



1 Mackintosh (1934) confirms this lack of vertical migration in Calamis acutus and Rhincalamis gigas; 

 he does not, however, deal with Oithona frigida. 



2 With these three copepods we might place the chaetognath Eukrohnia hamata, for this species, whilst 

 showing a moderate correlation with medium phosphate value in the N 100 H samples, showed no such 

 correlation in the N 70 V nets; it also shows very little vertical migration, is only rarely taken at the surface 

 and is very evenly distributed. The numbers however in the N 70 V nets were small; more information is 

 required regarding this species. 



