DISTRIBUTION OF YOUNG STAGES OF EUPHAUSIA SUPERBA 151 



station is situated between Elepiiant and Clarence Islands and over 7000 young E. superba were 

 taken. Much smaller numbers were taken at Sts. 538-541, but some were present at each of the 

 stations.^ On the Cape Melville-Trinity Land line krill was in very small quantity at Sts. 543 and 547, 

 two individuals being obtained at each, and was absent from the remaining stations. On the Snow 

 Island-Trinity Island line there were ten at St. 549 and one each at Sts. 552 and 553, the southern- 

 most stations in that line. 



VERTICAL DISTRIBUTION 



In this section the vertical distribution and migrations of E. superba from Calyptopis i 

 to adolescent will be described. It was shown previously (p. 116) that, whereas the 

 Metanauplius tends to remain in the same position whatever the time of day, there was 

 evidence in the later larval stages of a pronounced diurnal migration. Before going on 

 to deal with this in detail, it may be well to mention the difference in our results from 

 those which Ruud obtained from his material (Ruud, 1932). He states in the summary 

 of his paper (p. 93) that E. superba is a surface species, and that both the adults and the 

 young in his catches were taken, almost without exception, above 200 m. "In other 

 words", he remarks, "it belongs to the Antarctic surface layer." That this statement 

 does not apply to all stages of the life history is made evident by the examination of the 

 present material. The distribution of the adults is not within the scope of the present 

 paper except to the extent that they are associated with the distribution of eggs, but there 

 is some reason for believing that the adults seek the deeper water for the deposition of 

 eggs. So far as larval stages from Calyptopis to adolescent are concerned we can demon- 

 strate that they are not by any means restricted to the first 200 m. 



The routine 70-cm. vertical net hauls made at plankton stations covered a range of 

 1000 m. from surface downwards or from surface to within a few metres of the sea- 

 bottom if the depth were less than 1000 m. Six hauls were made with closing nets, as 

 follows: 50-0, 100-50, 250-100, 500-250, 750-500 and 1000-750 m. The nets were 

 hauled at a constant speed of i m. per second. 



The depth through which the two uppermost nets were hauled is one-third that of the 

 next lower net and one-fifth that of the three remaining nets. In order, therefore, to 

 make the actual number of individuals in any one net haul comparable with the re- 

 mainder, it is necessary to employ the lowest common multiple, which is 15, to obtain 

 corrected totals. In the diagrams used to demonstrate vertical migration (Figs. 72 and 

 73) the width in each column represents the numbers in a 50-m. vertical haul between 

 the indicated levels expressed as a percentage of the total. The day is divided up into 

 six periods of four hours each, and each station has been placed in the four-hour period 

 during which the greater part or the whole of the vertical net series was fished. 



The larvae have been arranged in six groups of increasing development, the first three 

 of which comprise the three Calyptopis stages; the fourth includes Furcilia i, i.e. larvae 

 with non-setose pleopods; the fifth includes Furcilia 2-4 and the intermediate forms 

 with six and four terminal spines ; the sixth group comprises Furcilia 5 and 6, including 

 what were formerly recognized as Cyrtopia stages. 



1 For some reason not recorded these larvae were not measured. The numbers were: St. 538 twenty- 

 three, St. 539 twenty-seven, St. 540 three and St. 541 nine. 



