254 DISCOVERY REPORTS 



follows, however, again appears to be accompanied by a marked slowing up of the growth-rate, the 

 vast majority of the second year swarms showing little upward trend in the scattered values of their 

 modes from April to October. Thereafter, when the phytoplankton blooms again, there is a final 

 and abrupt acceleration to the full adult stature. 



It will be seen that there are seven adolescent swarms, enclosed in Fig. 55 by a circle, that fall 

 outside the natural trend of the modal values exhibited by the swarms as a whole. They were recorded 

 at stations made in February and March at the beginning of the second year of growth, and although 

 their modal values lie above the general trend for this period, the developmental condition of the 

 sexes in them, the males dominantly in stage 3 and the females dominantly in stage 2,^ suggest 

 that their position in the plot is correct. For if, as Bargmann (1945, p. 124) has shown, each develop- 

 mental stage probably lasts 2 months in the male and 2^ months in the female, it is obvious that not 

 one of these seven swarms would reach full maturity for at least another 10 months or more. Possibly, 

 however, they are exceptional, representing perhaps the maximum possible growth attainable during 

 the first 15 or 16 months of larval and adolescent life. It is perhaps significant, too, that all seven 

 come from the South Georgia whaling grounds where in summer (p. 79, Table 5) the krill 

 encounter, and perhaps grow most rapidly in, the warmest conditions of their temperature range. 

 There may, however, be another explanation of this apparent anomaly. Having regard to the large 

 size of the individuals of them it is possible that all seven swarms may after all be at the beginning 

 of their third year of life, not, as plotted in Fig. 55, at the beginning of their second, 26-27 

 month old swarms in fact that, having failed to ripen to maturity, will not spawn perhaps for another 

 12 months. In other words, it is suggested that the life-span of this species, normally 27-28 months, 

 may occasionally, if rarely, be protracted for yet another year.^ In E. triacantha Baker (1959) 

 finds a similar possibility. This is in fact an interesting point, because it provides a loophole for 

 there being interbreeding between broods that would otherwise appear to have been completely iso- 

 lated in time. Without some such interbreeding one might have expected that such isolation, over 

 a vast period of time, might have produced two distinct subspecific races. Although as yet no one has 

 examined our material minutely enough to say if there are in fact any differences in the characters 

 of the krill spawned in alternate years, such a study, it is possible, might throw some interesting 

 light on the processes of speciation. One would not, of course, expect there to be much difference 

 between the two races because selection by the environment would be the same for both. 



The figures in brackets at the bottom of this diagram show the prevailing monthly condition of the 

 sexes in the northern or Weddell swarms.^ They are based throughout on the sex stage determinations 

 of Bargmann (1945, Appendix, Table 19) and have been worked out by taking the dominant stage of 

 male and female sexual development in the samples she examined and expressing it as a monthly 

 average. Little further comment need be made on these stage figures here, the developmental condi- 

 tion of the sexes in the swarms having already been discussed on p. 250. It may be noted, however, 

 that the dominance of the male, in so far as sexual development goes, which is detectable from 

 December of the first year onwards, gives way in March-April of the third year to a dominance in 

 the female. Occurring as it does at the very end of the life-span of the swarms this reversal of 

 dominance can probably as already mentioned (p. 247) be ascribed to the fact that the fully adult 

 males, having transferred their spermatophores, have already begun to die off, leaving the swarms 

 consisting largely of spent or gravid females. 



1 Following Bargmann (1945, Appendix, Table 19). - See also p. 402. 



^ Although our field observations contain much general information as to the developmental condition of the swarms in the 

 East Wind drift, noting, for instance, the presence or absence of spermatophores in the males and females, and whether the latter 

 were gravid or spent, there are little precise developmental data, based on dissection, from these high latitudes. It has accordingly 

 not been possible to work out exact figures for the prevailing monthly condition of the sexes for the swarms in the East Wind zone. 



