252 DISCOVERY REPORTS 



of growth beginning towards April of the following year with the onset of the winter phytoplankton mini- 

 mum. Surprisingly, however, neither show any marked acceleration at the end of the second year of life, 

 when the phytoplankton blooms again. ^ It will be seen that in December and January at the end of the 

 first year, and again in January and February at the end of the second, Ruud shows higher average 

 monthly length figures than Bargmann does either for her males or females. This discrepancy may well 

 have arisen through the curves having been based on the one hand on stomach samples, and on the 

 other on plankton samples, the whales (p. 1 43 , Fig. 1 4) being distinctly more efficient samplers of the larger 

 and more active krill than our stramin nets. Professor Hardy, with whom I have discussed this matter, 

 points out that the different effects of digestion and fixation upon the living krill could also account for 

 the difference, or contribute to it, digestion tending to relax the animals, fixation to contract them. 



Comparing his growth curve for Euphausia triacantha with Bargmann's curve for E. superba Baker 

 (1959) notes that whereas E. triacantha stops growing in the first half of the second year of life, E. superba is 

 still growing during the early part of the third. The greater size attained by the adult krill would, there- 

 fore, he concludes appear to be reached by a longer period of growth rather than by a faster average rate. 



The material on which the curves in Fig. 54 are based comes almost exclusively from the 

 Weddell drift, the Bransfield Strait and the South Georgia whaling grounds, the resultant curves, there- 

 fore, referring essentially to the relatively warm northern zone of euphausian abundance and not to the 

 colder slow-growing region of abundance in the East Wind zone. Bargmann's material however does in- 

 clude a few specimens from three samples obtained in the East Wind zone (Stations 575 and 602 in January 

 and Station 1359 in May) and it is interesting to note that the average length of the males and females 

 in all three falls below, in two instances well below, the natural trend of her male and female growth 

 curves for the northern zone.^ 



As SWARMS 



Since it now seems certain that E. superba from hatching onwards spends its whole existence as a member 

 of a swarm, itself (p. 230) virtually an individual organism, it has been considered worthwhile trying to 

 express the growth-rate in terms of the development of the swarms themselves. Without having recourse 

 to the massive labour of working out for every swarm the average length of the euphausians in it, 

 this has been done by plotting the sharply defined modes into which the length frequencies of the larval, 

 adolescent and adult swarms so consistently fall on a time-mode-value scatter diagram, the value of the 

 mode in each instance being taken as a simple and convenient measure of the developmental condition 

 of the swarm as a whole. In larval swarms, for instance, in which the mode is represented by a stage 

 and not by a length frequency, I have expressed the modal value in terms of the average length of the 

 dominant stage, a swarm for example in which the dominant stage is the First Calyptopis being given 

 a value of 2, the average length to the nearest millimetre of the First Calyptopes measured by Eraser 

 (1936, p. 25, Table viii). Thus, taking the larvae as a whole and still following Eraser, the dominance 

 of any particular stage in any individual swarm has been expressed to the nearest millimetre, or as for 

 the Fifth and Sixth Furcilias to the nearest two millimetres, in terms of modal value as follows : 



Modal value Modal value 



Dominant stage (mm.) Dominant stage (mm.) 



Nauplii or Metanauplii i Second Furcilia 6 



First Calyptopis 2 Third Furcilia 7 



Second Calyptopis 3 Fourth Furcilia 8 



Third Calyptopis 4 Fifth Furcilia 10 



First Furcilia 5 Sixth Furcilia 12 



* But see, however, p. 253, Fig. 55. 



^ Since this was written Nemoto (1959) has produced a growth curve based on material from the East Wind zone. This does 

 in fact show that throughout life both males and females in this far southerly coastal belt are distinctly smaller than their 

 contemporaries in lower latitudes. 



