THE DEVELOPMENT AND LIFE-HISTORY OF KRILL 



129 



work strongly indicates that this is due to minute traces of organic compounds, iron and manganese, 

 derived from the land, which exert a "strongly favourable influence on phytoplankton production . 

 The importance of the pack-ice in this connexion is strongly emphasized. Hart regards it as "giving 

 rise to what might be termed pseudo-coastal conditions at vast distances from land, where neritic 

 species maintained by the ice flourish for short periods when the latter disperses ". 



All these factors, since they influence the food supply, must have a bearing on the growth rate of 

 E superba but it is not a simple matter to produce evidence in support of this. Only an unbroken 

 series of observations extending over several seasons and made at short regular time intervals at the 

 same stations would provide reliable data. Unfortunately such a series is not available, the material 

 collected being too scattered and interrupted, so that there are many gaps in the chain of evidence 

 and attempts at correlation are always breaking down. 



1 



•so 



50 



40- 



"30 



20 



..^^.•^- \-':> 



\ U''^ 



V^o',.. 





Larvae -* A: — a — ^ — 



Males — ■ — ■ 



Adolescents ■ ■ 

 Adults IS 12 <-J 



Females 



Adolescents a a 

 Adults B a B 



50 



40 



■30 E 



2Q 



Notf 



First Year ot browth I 



Fig. 3. Growth curve showing average length per month of larvae, adolescents and adults. 



As the diatom maximum occurs earlier in the northern region of the Antarctic zone than in the 

 southern the average development of the northern E. mperba should be correspondingly more 

 advanced. But Fraser found no clear indication that larval development begins sooner in one area 

 than in another, though he obtained some suggestions that local variations in the abundance of food 

 may directly affect the average larval length. He did not feel justified however in concluding that 

 within a restricted area, food was the only factor involved, but decided rather that the effect was the 

 cumulative result of several factors acting locally. ^ ' ■ a . 



Comparison of the size of larvae, adolescents and adults fron, the different Anrarcc regions do 

 not give a satisfactory result either. This may be due to the fact that length alone ts not a rehable 

 criterion of development, but even if the developmental stages are taken mto account the evidence s 

 n"t more definite From the material available, it cannot be shown that older adolescents or fuHy 

 mature aduks occur any earlier in the northern Antarctic region than in the southern. But on Ae 

 other hand, there is evidence to show that those E. ^perbo, which are hatched early anywhere wthm 

 "arc ic zone, are directly intfuenced by the abundance of the food supply. Hart points out h 

 he summer decrease in phytoplankton may be due in part to a ^^-^^-^^ ^^^\°^XXX 

 this is probably brought about to some extent in the oceanic areas, anyhow, by intensive^ graz ng 

 down by the herbivorous zooplankton". This occurs during and ™™*ately after the period o he 

 spring maximum, in December, January and February. These months "made with he fi.t ha^ o^ 

 the spawning season in E. superba, and it would appear that those generations hatched early, which 



