388 DISCOVERY REPORTS 



many ways very similar to that of Rhincalanus gigas and to the total macroplankton, 

 though the largest numbers occur perhaps a little farther north in December and March. 

 In December again there is a concentration in the surface layers as far south as 64 S, 

 and beyond this the majority are still in deep water. In March there is the same increase 

 in the surface layer in the higher latitudes, and the same tendency to increase, pre- 

 sumably by sinking, in deep water to the north of the convergence. In September the 

 species is mainly confined to the warm deep water, and the numbers on the whole are 

 rather less than in summer. In October and November it approaches the surface again, 

 and is highest and most abundant near the convergence. 



Rhincalanus gigas and Eukrohnia hamata are typical inhabitants of both the Antarctic 

 and sub-Antarctic zones. The Copepod, Calanus acutus (Table VI and Fig. 7), however is 

 a truly Antarctic species which does not extend much to the north of the convergence. 

 In December there is a surface concentration between about 61 ° and 64 S, and south 

 of 64° there are large numbers in deeper water. C. acutus is rather more patchy in its 

 distribution than the others we have so far considered, and the details in the arrangement 

 of the shaded contours in Fig. 7 are not to be relied on. However it is clear that in March 

 the largest numbers are in the Antarctic surface layer at least as far south as the line of 

 stations extends, and there is a strong suggestion of a sinking movement at St. 13 16 

 which is here the most northerly part of the range of distribution of this species. In 

 September the vertical distribution is very clearly defined. It has completely deserted 

 the surface layer and has sunk to a greater depth than either Rhincalanus gigas or 

 Eukrohnia hamata. The largest numbers occur between 750 and 1000 m., and there are 

 plenty below 1000 m. There is no obvious reduction in the total numbers for this month. 

 Increased numbers were taken in spring however. Like the other species Calanus acutus 

 first regains the surface immediately south of the Antarctic convergence in October, and 

 in November it appears to have regained the summer level. In the latter month indeed 

 it seems to have reached a more advanced stage than in the previous December. 



These three species then, which together probably constitute much more than half 

 the total mass of living matter in the plankton west of the Drake Strait, clearly spent the 

 summer near the surface and the winter in deep water, at least in the year 1934. 



Pleuromamma robusta is less common than the three species discussed above, and its 

 vertical distribution is quite different. It normally inhabits deeper water and unlike 

 the other three it undergoes extensive daily vertical migrations. For this reason I have 

 not attempted to represent its distribution by shaded contours. Table VII shows how- 

 ever that in December it was absent from the surface layer and occurred almost ex- 

 clusively between depths of 250 and 750 m. In December the even numbered stations 

 (1224, etc.) were those taken at night, and it is interesting to note that even in this deep 

 water, to which hardly a vestige of light can penetrate, there is an obvious upward move- 

 ment of this species at night and withdrawal in daytime. In March there is an astonish- 

 ing range of vertical migration. At night (again the even numbered stations) the largest 

 catches are at 100-50 m., but in daytime at 750-500 m. In September there is no 

 indication whatever of a general descent into deep water. The species approaches the 



