VERTICAL MIGRATION 237 



and considerable migration into the upper layers during the hours of darkness. The 

 rapidity with which these forms must rise and sink is clearly shown in the figures for 

 the N 70 H and N 100 H results. Very few specimens are taken during the hours of 

 daylight, but as soon as darkness sets in the nets begin to capture them in large numbers. 

 The possibility that the animals are avoiding the nets during the daytime, so that such 

 apparent migrations are not real, is discussed on pp. 244 and 250, where the conclusion 

 is reached that these extensive migrations are actually taking place. Fig. 1 1 5 shows a 

 vertical migration of Scolecithricella minor extending over some 500-700 m. It seems 

 scarcely possible that so small a copepod could rise some 2000 feet in a few hours by 

 locomotive power alone. It seems likely that some change in specific gravity must occur. 



Microcalanus pygmaeus (Fig. 115) shows little or no migration. The slight difference 

 between the night and day results may not signify a regular diurnal movement. 



Oithona frigida (Figs, no and 1 13) only rarely occurs in any considerable numbers in 

 the upper 50 m. of water, and like Calanus acutus and Rhincalanns gigas it shows no 

 sign of a regular diurnal migration. The results for all three methods of investigation 

 confirm this conclusion. 



Amphipoda 



The Amphipod Parathemisto gaudichaudi (Fig. 121) shows a gradual rise towards 

 the surface beginning some 3 hours before darkness, and a corresponding fall in the 

 early hours of the morning after sunrise. Occa^onally during the daytime moderate 

 numbers are to be found at the surface, as many as 464 being recorded soon after 12 

 noon on one occasioq^fcgm: the most part in the daytime the numbers taken in the 

 upper 50 m. are small^H Kg the night the majority of hauls taken in the surface waters 

 record large numbers — o9»me occasions over 2000. 



In the case of Vibilia antarctica (Fig. 124), although the number of observations is 

 small, we see a definite rise to the surface waters from just before midnight till about 

 4 o'clock in the morning, a behaviour which is similar to that shown by Salpa fusiformis 

 (Fig. 123). We have already suggested on p. 199 that there is an association between 

 these two forms. 



We have very few observations regarding Cyllopus, but Fig. 122 shows that most 

 of the specimens taken in the upper layers have been taken during the hours of darkness. 

 On very few occasions has this form been met with in the surface net during the 

 daytime. 



Euphausiacea 



The behaviour of Euphansia supcrba appears to be very erratic, but we see from Fig. 



127 that far more specimens are taken at the surface during the hourf^f darkness 



than during daylight. Three prominently high catches were taken at the surface during 



daylight at Sts. 40, 41 and 43, and it is perhaps r^eworthy that all these three stations 



were taken in the same week at the end of Marcrrand the beginning of April ; this is 



late in the season, when the light is weaker than in the middle of the Antarctic summer, 



and at this time the migration may be less marked. 1 



1 This phenomenon is reconsidered on p. 326 in Part V and a relation with phytoplankton conditions 

 tentatively suggested. 



DXI 3I 



