I94 DISCOVERY REPORTS 



PART IV. THE ZOOPLANKTON 



SECTION II 



By A. C. Hardy, m.a. and E. R. Gunther, m.a. 



DISTRIBUTION (cont.) 

 CRUSTACEA (cont.) 



Amphipoda 



Examples of the different species of Amphipod taken in our samples were submitted 

 to Dr K. H. Barnard, who kindly identified them for us. 



Parathemisto (Euthemisto) gandichaudi (Guer.). This species was by far the most 

 prominent planktonic Amphipod within the area, occurring in particularly large num- 

 bers off the north-east coast of South Georgia in March 1926. Here it was found to be 

 feeding largely upon young Euphausians and no doubt, considered ecologically, it is a 

 very important element in the plankton community. 



Out of the fifty-three stations in the survey round the island in December and 

 January 1926-7, it was taken at all but four, and at some stations, again off the north- 

 east coast, it was present in very large numbers but not equalling those of the preceding 

 March. From the results of this survey it would seem that it is more abundant in water 

 of Weddell Sea origin than in that of Bellingshausen Sea origin ; but before coming to a 

 conclusion on this, the hypothesis of animal exclusion, discussed in Part V, should be 

 considered, for we have seen that to the south-west there is a particularly heavy 

 production of phytoplankton. 1 The two series of consecutive net hauls taken off 

 the north-east coast in January 1926, and described and illustrated on p. 254 and 

 Fig. 133, showed that this species was very patchy in its distribution. At first sight this 

 might seem to have a very serious effect upon our conception of its distribution. But the 

 length of tow of the nets in the consecutive net series was half a mile only, whereas at 

 each station in the December-January survey the nets were towed for a whole mile, and 

 in the previous March for 3 miles. A tow of 1 mile is likely to cut through a patch, and at 

 each station there was a series of three nets one below the other approximately 50 m . apart . 

 Again, the species exhibits a marked vertical migration, rising towards the surface in 

 large numbers in the hours of darkness; this is described and illustrated on p. 237 

 and Fig. 121. Whilst the chart of distribution in Fig. 87 is based upon three net hauls 

 taken at different levels at each station, the massing of these Amphipods at the surface 

 during the night should be kept in mind, and the chart of distribution should be com- 

 pared with Fig. 89, which shows the stations taken in the hours of darkness. 



1 The collection of Amphipods sent to Dr Barnard for treatment in his systematic report did not include 

 all the specimens of the commoner species, but only samples, and by being sent only specimens from the 

 north-east side of the island it is most unfortunate that he should have been misled into believing that " not a 

 single pelagic Amphipod was captured on the south-west side of South Georgia" (see p. 25 of his Report). 

 Reference to Fig. 87 will show that this is not so. 



