THE LARVAL STAGES 105 



While then it seems clear that eastward dispersal of the larvae in the Weddell region is brought 

 about by both deep and shallow transport, it is equally clear, at least in the East Wind zone and 

 (p. 123) in the eastern part of the Weddell drift, that there must be a period when they are moving 

 towards the south. That is, when in the course of their long upward passage they traverse, developing 

 from First Nauplii to First Calyptopes as they go, the warm mainly south-flowing deep current 

 sandwiched between the bottom and surface streams. The full significance of this movement is 

 discussed on pp. 118-23 i^ ^^^ section dealing with the developmental ascent and its influence on 

 the distribution and conservation of the euphausian population. 



Diurnal vertical migration 

 It has been shown that the vertical movements of the First and Second Nauplii and of the Meta- 

 nauplius and its immediate successor, the First Calyptopis, must at the outset be upward. Let us 

 now consider the nature and extent of such subsequent vertical movements the larvae may undergo 

 once, as First Calyptopes, they arrive in the surface stream. 



In a diagram based on a series of vertical stations, widely separated in space and time but arranged 

 in a time sequence covering most of the 24 hr. of night and day, Fraser (1936, p. 116, Fig. 33) demon- 

 strates what at first sight appears to be a pronounced diurnal migration of the Calyptopis stages in 

 the course of which they become concentrated in the dark hours between 2200 and 0400 in the surface 

 ( 1 00-0 m.) layer, while for the rest of the time throughout the daylight hours they appear to be massed 

 at depths mainly between 500 and 250 m. At each station of this series deep-living Metanauplii 

 occurred in moderate to substantial numbers, the later stages, however, being represented for all 

 practical purposes by the First Calyptopis alone. Now since the latter is the stage which in the course 

 of the developmental ascent makes its earliest appearance between 1000 and 500 m., it would seem 

 that Fraser in the interpretation of his results had overlooked the possibility that the apparent massing 

 of the First Calyptopes in daj^time at depths below 250 m., instead of being as he supposed the result 

 of a migration from above, could equally well have sprung from the recent deep moulting of the 

 Metanauplii along with which they were captured. In other words, in a species such as this in which 

 climbing larvae are constantly feeding the surface population from below the establishment of the 

 reality of diurnal vertical migration, in so far at least as the First Calyptopis is concerned, presents 

 a more complex problem than it would seem to do in species that do not have their early existence 

 at such enormous depths as those at which the first young krill appear. In fact, as will be seen presently, 

 if we fail to reckon with the developmental ascent, and all it implies, the diurnal movements of the 

 First Calyptopes cannot be worked out without sometimes involving a major misunderstanding of 

 what is often taking place at levels below 250 m. 



The basic aspects of the problem are illustrated in Fig. 6 which presents the vertical distribution of 

 the First Calyptopis at oceanic stations where deep Metanauplii were moulting and the developmental 

 ascent was in progress. Stations where only one Metanauplius occurred or where Metanauplii only 

 occurred are not included. In its construction Fig. 6 follows the conventional lines that Fraser used, 

 the day being divided into six periods of 4 hr. each, the short-term gatherings of the 50-0 m., 100-50 m. 

 and 250-100 m. nets multiplied by 5, 5 and 5/3 respectively to bring them into line with the deep 

 gatherings from the 500-250 m., 750-500 m. and 1000-750 m. layers, the resultant catch-figures being 

 finally expressed graphically as a percentage of the total catch between 1000 m. and the surface. A 

 check on the significance of the resulting graphs is provided by tables showing the actual catch-figures 

 upon which each is based. Figs. 8-1 1 , which follow presently, have been constructed in the same way. 



In his original figure portraying the vertical movements of the First Calyptopis Fraser (1936, 

 p. 152, Fig. 72) shows it massed conspicuously in the 100-50 m. layer between 1800 and 2200 hr., 



