THE LARVAL STAGES 97 



mentioned above. At both stations, however, hydrological conditions (p. 102) seem to have been 

 unusual. It would appear, therefore, that its normal habitat is below 1000 m. It may well in fact be 

 far below, for at forty-four stations with still deeper net hauls in the Weddell and East Wind zones we 

 find it only twice in the 1500-1000 m. layer (Stations 2594 and 2603) and in neither instance it seems 

 could we rightly claim to have struck its normal habitat in deep oceanic water, Station 2603 having 

 been made on the continental slope (Table 14) over a depth of only 1450 m., Station 2594, although 

 truly oceanic, in the abnormal hydrological conditions referred to above. It could well be, therefore, 

 that its true locus of optimum abundance in the deep ocean is below 1500 m. Only in slope waters, 

 as for instance at Station 2603, is it likely that it might be somewhat higher. 



(4) Although Ruud with his shallow nets was unable to visualise the relatively short deep-living 

 larval phase our much deeper observations reveal, his statement that the larvae belong to the Antarctic 

 surface layer remains essentially correct, for as Table 15 shows it is for all practical purposes at this 

 high level that by far the greater part of the larval existence is spent and where the development 

 from Calyptopis One to Furcilia Six takes place. 



The developmental ascent 



It is evident from these analyses that wherever the three stages, the Second Nauplius, the 

 Metanauplius and the First Calyptopis, or wherever the two stages, the Metanauplius and the 

 First Calyptopis, occur together in substantial numbers the pattern of their bathymetric range 

 relative to one another is always precisely the same, the Second Nauplius at its so far observed 

 optimum^ ranging from 1500 to 1000 m., the Metanauplius at its optimum from 1000 to 500 m. and 

 the First Calyptopis at its optimum from 750 m. to the surface. Clearly this constantly repeated 

 pattern represents what may be described as a ' developmental ascent ' involving first the moulting 

 of the Second Nauplius, followed at a somewhat higher level by that of the Metanauplius, the product 

 of this second ecdysis, the First Calyptopis, becoming the earliest developmental stage of the new-born 

 krill to reach the Antarctic surface layer. From the relative positions of the three stages in the bathy- 

 metric scale one can judge with considerable accuracy the several levels at which these successive 

 moultings take place. For instance, it is clear that as the larvae rise the Second Nauplius must 

 begin, and, except for very rare instances, complete, its moulting to become a Metanauplius below 

 1000 m. and that the latter in turn must begin its moulting to become a First Calyptopis between 

 1000 and 750 m. and complete the process in the 750-500 m. layer. The final moulting of the Meta- 

 nauplius must in fact be accomplished long before it reaches the 500 m. level, for if it were otherwise 

 far larger numbers of this stage would undoubtedly have been found in the 500-250 m. layer than 

 the few rare stragglers (which in any case probably came from a deeper level) that have so far been 

 recorded there. That the early larvae undertake this remarkable climb, involving them in a journey 

 of a mile or more to the surface, is confirmed by the obvious signs of moulting and growth (see foot- 

 notes to Tables 13 and 14) that have been recorded in the deep Metanauplius and Calyptopis stages. 

 For example, at Stations 854 and 855 (Table 14) many of the Metanauplii taken at the 1000-750 and 

 750-500 m. levels were seen to be in active moult, a characteristic half-way condition in which the 

 incipient First Calyptopis form is often seen emerging, or about to emerge, from the Metanaupliar 

 husk, while the small and stunted appearance of the First Calyptopes taken in the deep 1000-750 and 

 750-500 m. nets at Station 855, compared with the robust and obviously well nourished condition of 

 the higher level First Calyptopes recorded at the same station, clearly points to the former having 

 been the very recent product of the actively moulting Metanauplii along with which they were 

 1 Further observation, however, may show that its real position in the bathymetric scale in oceanic water is below 

 1500 m. 



