i6o DISCOVERY REPORTS 



It may be that at St. 461 the animals which undergo a diurnal vertical migration are 

 still influenced by light intensity to a greater extent than those remaining at the surface, 

 or it may be that their movements indicate the last vestiges of a physiological rhythm 

 acquired in earlier larval history and persisting when the stimulus which originally 

 promoted it is no longer active. This may be so, but the important fact is that with 

 adolescence there is an abandonment of diurnal migration, and whereas the majority 

 of the larvae from Calyptopis i to Furcilia 5 undergo a rhythmic alteration in vertical 

 position the adolescents are persistently at the surface. 



The significance of this distribution is that it afi'ects the distribution of krill in the 

 Antarctic, but before dealing with this the general water movements within the Antarctic 

 should be recalled. Deacon (1933, p. 226 et seq.) states: 



The movement of Antarctic surface water away from the Antarctic regions towards the north is 

 known to take place all round the Pole and it is also known that there is a similar movement of 

 Antarctic bottom water towards the north near the sea-bottom. To make up for this transport of 

 water away from the Antarctic regions in the surface and bottom layers there must be a movement 

 towards the Pole in the intermediate layer. This is supplied by a movement southwards in the warm 

 deep layer.. . .There can be no doubt that the water at the level of maximum temperature in the 

 Antarctic Zone has a component of movement southwards. 



Again on p. 173: 



In the Antarctic Zone the surface layer is composed of cold poorly saline water which lies in a 

 shallow well-defined layer above warmer deep water. It has a depth of 100-250 m. 



The discontinuity is shallower in the south than in the north. The observed occurrence 

 of E. stiperba in this system of currents having components of direction opposed to one 

 another is worth considering. It has been shown that eggs, Nauplii and Metanauplii 

 are taken in greatest abundance in the deeper water, their component of movement will 

 therefore be southwards. Calyptopis and early Furcilia have a diurnal vertical migration 

 which brings them into the north-flowing surface water during the hours of darkness 

 but which returns them to the south-flowing deep water in the daytime. Their com- 

 ponent of movement will therefore be dependent on the time spent in each water layer 

 each day and on the rates of movement of the two water-masses. The accumulation of 

 late Furcilia and adolescents at the ice-edge would suggest that the resultant direction of 

 movement of the larvae is generally in a southerly direction. The late Furcilia and 

 adolescents, as we have just shown, are surface-loving in their habit, and they will be 

 borne northwards in the surface water of the Antarctic streaming away from the Pole. 

 The distribution of adults does not come within the scope of this paper, but it may be 

 anticipated that they would be found more generally distributed between the Antarctic 

 convergence and the ice-edge than are the adolescents. For instance, at St. 461 in the 

 A flight of nets there were 9220 Furcilia and adolescents, but only 2 adults; in the B 

 flight 3682 Furcilia and adolescents, with 4 adults; for C the figures are 6534 and 10, for 

 D 1437 and 9, for E 42 and none, for F 13 and none, and for G 3584 and 122. There is 

 no indication at this station of adult krill in quantity in any way indicative of the vast 

 shoals which occur elsewhere. Again at the stations succeeding 461 along the ice-edge, 



