io8 



DISCOVERY REPORTS 



ADOLESCENT KRILL i 



Of the 8029 specimens of E. superba which I have examined, 6006 were adolescent and of these 3073 

 were males and 2933 females. The youngest adolescents first make their appearance in any number 

 in August ; they show no trace of external sexual characters, but internally the reproductive system is 

 recognizable, and by dissection under a low-power binocular microscope the sex of each individual 

 can be determined. I have described the development of the reproductive system in the short paper 

 forming an introduction to this one, which has been published in vol. XiV of the Discovery Reports. 

 It will therefore be sufficient to summarize this development here, before discussing how growth 

 proceeds during adolescence. 



Ruud (1932) has drawn attention to the fact that, in E. superba, investigation of the testis and ovary 

 is the only reliable method of determining maturity, and that the reproductive system of each in- 

 dividual must therefore be examined before the composition of any specific population can be estimated. 

 After examining the euphausian material obtained during the cruise of the S.S. ' Vikingen', he dis- 

 tinguished four stages of maturity in both males and females. These are listed in Table 2. 



Table 2 (after Ruud). Stages of maturity 



Ruud states that he does not know of " any practical method by which the degree of maturity of the 

 testicle can be ascertained". Consequently, he included within stage 1 all those male euphausiids 

 which were not fully adult (i.e. all those with no visible spermatophores in the ejaculatory ducts), and 

 he found that the specimens showed a very wide range in length: i6-6-44-4 mm. 



As a criterion of development in the females, he used the diameter of the egg, and again found great 

 variation in length in the specimens included within stage 1. 



It is clear that this first group of Ruud's, comprising as it does males and females of such difi^erent 

 size, covers the whole period of adolescence, during which time the reproductive system becomes 

 mature. A closer investigation of these adolescent forms has thrown more light on the development 

 of E. superba. 



ADOLESCENT MALES 



Although during adolescence the individual growth rate varies very considerably, five stages can be 

 distinguished in the development of the male sexual organs, both internal and external. These stages 



Table 3. Growth stages in the male 



Stage 



2 

 3 

 4 

 5 



Internal structures 



Primitive condition. Small testis, simple uncoiled 



vas deferens 

 Small posterior flexure appears on vas deferens 

 Lateral pocket appears on posterior fle.xure 

 Anterior flexure appears on vas deferens 

 Coiling on vas deferens near posterior flexure 



Stage 



B 

 C 

 D 

 E 



External structures 



Undifferentiated ist pleopod 



Petasma appears as an undivided lobe 

 Petasma becomes divided into two lobes 

 Wing develops above petasma 

 Wing grows and curves over petasma 



