DEVELOPMENT OF EUPHAUSIA SUPERB A loi 



observed in Eiiphausia superba, that sexual maturity may be reached before the adult 

 form is attained. It is proposed therefore in the present paper to use the term "adoles- 

 cent" to describe that portion of development between the 6th Furcilia stage and the 

 part of the life history where changes take place associated with sexual maturity. The 

 scope of the paper, so far as adolescents are concerned, is limited to the younger indi- 

 viduals in this phase and does not include reference to those that from their size require 

 special examination to decide whether they are adolescent or adult. 



By the time the euphausiid reaches the 6th Furcilia stage the major developmental 

 changes have been effected and in appearance it is characteristically a euphausian. Such 

 alterations as take place subsequently chiefly involve the elaboration of existing struc- 

 tures, for instance, increase in the number of setae on the antennal scale, in the number 

 of segments in the antennular flagellum, in the number of lobes on the branchiae, and 

 so on. These changes are gradual and show great individual variation. Attempts which 

 were made to arrange the adolescents in groups or stages dependent on the number of 

 setae on the antennal scale did not yield any satisfactory results, and it was made obvious 

 by inspection of the animals that in other appendages as well no hard and fast pattern 

 of development exists. 



It should be emphasized again that this generalized development is not an exclusive 

 feature of the adolescents, but that it is incipient, in some characters at any rate, in early 

 Furcilia stages and becomes more and more evident as development goes on. Thus 

 although Furcilia 6 is recognized in the main by having one terminal and three postero- 

 lateral spines on the telson, as opposed to two postero-lateral spines in the adolescent, 

 yet the distinction between the two stages is diffuse and ill-defined. 



In Appendix I the length frequencies of late Furcilia and early adolescents taken in 

 the i-m. nets have been set out. The time of year when the larvae were obtained, the 

 total number and the number examined, the average length for each phase of develop- 

 ment, the percentage of the total in each phase and the general average have been 

 stated. 



The first point to be noted from this analysis is that Furcilia 5, representing larvae 

 about 8 mm. in length, although occurring in August and September is in such very 

 small numbers that it may fairly be concluded that the vast majority of the larvae reach 

 the Furcilia 6 stage before the end of the southern winter. 



Inspection of the table shows that the total number of E. superba at different stations 

 varies very greatly, and that the small numbers at some of the stations tend to give 

 anomalous results. It is possible, however, to get an approximate indication of the re- 

 lative abundance of the two phases of krill principally represented, namely Furcilia 6 

 and adolescent. 



In Table XXXVIII the average percentage occurrence of Furcilia 6 is stated. The 

 percentages from which these averages are derived are of catches where the number of 

 euphausians examined was considered sufficiently high to give a trustworthy idea of the 

 proportions in which Furcilia 6 and adolescents were present.^ 



1 The arbitrary number of 48 was selected. 



