I30 DISCOVERY REPORTS 



are able to benefit fully from the spring maximum, reach greater average lengths than those developing 

 later, when grazing down has brought about a decrease in the food supply. This variation in size has 

 already been mentioned (p. no), and Fig. 3 shows that it is maintained throughout the life-cycle, 

 these larger adolescents giving rise to the very big adults, which occur at the beginning of the breeding 

 season. 



The influence of the spring diatom maximum on the average lengths of the stages is also apparent, 

 though it is perhaps more consistently marked in the males than in the females. If plotted graphically, 

 the average lengths show a fairly rapid rise from October or November to a peak in February or 

 March (Figs, i and 3), after which they generally tend to decrease slightly or to remain almost sta- 

 tionary. The period of increase in length corresponds roughly with the time of the phytoplankton 

 maximum, and the succeeding period of slackened growth corresponds with the time of the post- 

 maximal decrease. A similar rise and fall at the same time of the year is seen in the maximum lengths 

 of the stages. The onset of the southern winter is doubtless also a factor which comes into play at this 

 time and influences the rate of growth, for Deacon (1933) has shown that the difference between the 

 summer and winter temperatures of the Antarctic surface water is as much as four degrees. 



The other factors mentioned by Hart, light, surface conditions and pack-ice, except in so far as 

 their broad seasonal variations will certainly influence the growth rate, more properly affect the dis- 

 tribution of E. superba, and are outside the scope of this paper. This distribution was being worked 

 out by my colleague, J. W. S. Marr, but unfortunately its completion has been interrupted for the 

 time being by the war. 



CONCLUSIONS 



This investigation extends Fraser's work on the growth of E. superba from the egg to the beginning of 

 adolescence, and amplifies Ruud's sketch of a two year life-cycle. 



In order to estimate accurately the composition of the euphausian population, a method was devised, 

 by intensive study of the reproductive system, for determining the degree of maturity of each in- 

 dividual. It was found possible to distinguish between males and females immediately the larval state 

 was left behind, and to divide the period of their growth to maturity into 7 stages. This method gives 

 a convenient way of checking deductions based on measurements of length alone, and has shown 

 clearly that individual length is not necessarily a reliable criterion of development, since there is 

 evidence to show that length may be the first thing to be influenced by variations in the factors 

 affecting the rate of growth. Division of the population into growth stages, combined with estimations 

 of the average length of these stages, however, gives a good idea of the life history. 



The spawning season, which extends over 5I months, begins in November or December. Eggs 

 spawned then are probably adolescent by August, and mature about thirteen months later in Sep- 

 tember and October. The males grow more rapidly than the females, attaining slightly greater average 

 lengths on the whole, and requiring a probable minimum of 22 months to reach maturity, as against 

 25 months in the female. It seems likely that each state lasts 2 months in the male and 2\ months in 

 the female, though these times are not definitely established as yet, lack of material at certain periods 

 of the year making the evidence too scanty. 



Pairing was first found to take place in October, before the females were fully adult. The sper- 

 matophores are therefore carried for some time before fertilization can occur, the evidence showing 

 that this is effected externally, while the eggs are being laid. Gravid females are present in surprisingly 

 small numbers. This fact, coupled with Fraser's records of eggs and early larval stages in the deeper 

 water layers, seems to indicate that the females go down deep to spawn. More evidence on this point 

 is needed, before this can be definitely established. 



