24 o DISCOVERY REPORTS 



In Figs. 125 and 126 we see a remarkable phenomenon of the almost complete 

 absence of E.frigida and of E. triacaniha, during the hours of daylight in the upper 

 200 m. of water. As soon as it becomes dark we find them, particularly E. frigida, 

 abundant in the upper layers coming to the very surface. It must mean that a vertical 

 migration of some 600 ft. is accomplished in a very short space of time. It may be 

 suggested that these forms definitely avoid the net during the hours of daylight, but are 

 caught in the night when they cannot see the net, but such considerations can hardly 

 apply to other forms which we see making vertical migrations, such as Salps. On other 

 grounds, to be discussed later, this theory of the avoidance of the net is dismissed. We 

 are left in wonder as to how this remarkable migration of 600 ft., in at most an hour or 

 two, is carried out. One would think that it must involve more than power of loco- 

 motion, possibly some change in specific gravity. 



Thysajioessa, illustrated in Fig. 128, may at first sight show little migration, but it 

 was taken more often at the surface during the hours of darkness than in daylight, and 

 in the middle layers in greater numbers than are usual for daylight hauls, although there 

 are some marked exceptions to this. The two outstanding instances of large numbers 

 being taken at the surface during the daytime are at stations late in the season: St. 40 in 

 April and St. WS no in late May; cf. Euphausia superba. 



Other macroplankton 

 The chaetognath Eukrohnia hamata (Fig. 129) rarely appears in the upper 50 m. of 

 water, and little sign of any regular diurnal migration occurs, although it will be noted 

 that the only two occasions on which it was taken in any considerable numbers at the 

 surface were just before sunset and in darkness. 



The Pteropod Limacina helicina, illustrated in Fig. 130, shows a gradual tendency of 

 movement towards the surface from 1700 to 2200 o'clock, and a corresponding move- 

 ment downwards from 0200 to 0700 o'clock. 



We also see a marked general tendency for Salpa fusiformis (Fig. 123) to come to 

 the upper layers during darkness, particularly in the early hours of the morning from 

 midnight until 0400 o'clock. This somewhat delayed migration we see corresponds to 

 that of the Amphipod Vibilio, and, as we have already noted, we suspect there is an 

 association between the two. 



Recent confirmation 



Since this work was done Mackintosh (1934) has published a report on the distri- 

 bution of the macroplankton in the Atlantic sector of the Antarctic based on the examina- 

 tion of collections made over a wide area by 100 cm. diameter nets hauled obliquely 

 from 100 m. to the surface. He finds considerable diurnal variation in the numbers of 

 the majority of animals due to their vertical migration into the path of the nets at night 

 and away from them in the daytime. He was dealing with the macroplankton only, the 

 smaller species not being caught by these 100 cm. oblique nets. 



