128 DISCOVERY REPORTS 



To obtain a complete growth curve, monthly averages of the larvae must be included. Using 

 Eraser's measurements, I recalculated these averages, which had been originally worked out on a half- 

 monthly basis, and I also made monthly frequency tables of his measurements of young adolescents. 



These larval averages show the rate of growth during the first six or seven months, but when the 

 adolescents make their appearance in August, it is not sufficient to work out the average length of all 

 adolescents per month. Some selection is necessary, because early in the southern spring, that is, in 

 August, September and October, the overlap of generations brings about the co-existence in the catch 

 of young adolescents of stage 1 with late adolescents of stages 3, 4 and 5 of earlier generations, and the 

 inclusion of these larger adolescents in the calculations gives a wrong idea of the growth rate in these 

 particular months. Later in the season, the population becomes more sharply divided into adolescents 

 and adults, and the question of selection does not arise. I have, therefore, included in the calculations 

 for August, September and October, only the measurements of Eraser's adolescents and of my own 

 specimens at stage 1 . In calculating the average lengths of the adults, I have used all specimens which 

 could be expected to mature within the southern summer, that is stage 6 as well as stage 7 in the males, 

 and stages 4-7 in the females. 



In Eraser's original graph of larval growth, there is a marked decrease during the winter months, 

 June, July and August. This tends to disappear when his results are combined with mine (Fig. 3), 

 and may have been due, in part, to scarcity of material. I think, too, that the apparent slowing-up of 

 growth during the second winter, in the transition period between adolescence and maturity, can also 

 be partly explained on these grounds, although the colder temperatures and less abundant food almost 

 certainly have some retarding effect upon the growth rate. 



Before one year's growth is over, that is, as soon as the adolescents appear in August, it becomes 

 possible to distinguish between males and females, and the curve can therefore be divided into two 

 parts (Fig. 3). The rate of growth in the two sexes is very similar. Although the females are con- 

 sistently smaller than the males, the two curves follow approximately the same course. The period of 

 adolescence occupies, at a minimum, a whole year and is shorter in the males than in the females. In 

 the males, true adults, carrying fully formed spermatophores, appear for the first time in September. 

 In the females, true adults, fully gravid, appear three months later in December. The total period of 

 growth from the egg to the adult occupies a minimum of twenty-two months in the male, and twenty- 

 five months in the female. 



FACTORS INFLUENCING GROWTH RATE 



Obviously, the main factor which influences the growth rate of E. superba is the supply of food. Hart 

 (1934) writes that this "consists very largely, if not entirely of diatoms and other phytoplankton 

 organisms". He found that the most strongly silicified diatoms could be identified with certainty in 

 the stomach contents, but that those with thinner cell walls were too rapidly digested to be easily 

 recognizable. 



In a later paper (1942), Hart discusses the factors which control the production of phytoplankton 

 in the Antarctic zone as a whole. He states that chief among them are the physical influences of 

 "light, the degree of stability of the surface layers and the (interrelated) effects of the pack-ice", and 

 that these three agents are "certainly the prime causes in determining the time of the onset of the 

 main increase" in the abundance of phytoplankton. This time, " falls later in the year as one proceeds 

 southwards ", as much as two months elapsing between its occurrence in the northern and the southern 

 regions of the Antarctic zone. However, Hart considers that none of these factors " adequately accounts 

 for the vastly greater richness of the neritic areas as compared with the oceanic regions". Recent 



