SUMMARY 337 



it does not reach that of the first summer, the average being 17 mm. per month, and after March 

 Httle or no further growth takes place. The two periods of rapid growth during the Ufe cycle 

 correspond with the periods of phytoplankton abundance described by Hart (1942) for his ' Northern 



Region'. 



A comparison of this growth curve with Bargmann's curve for E. superba (1945, p. 129), shows 

 that, whereas E. triacantha stops growing in the first half of the second year of life, E. superba is still 

 growing during the early part of the third year. Thus it appears that the larger size of E. superba is 

 reached by a longer period of growth rather than by a faster average rate. 



SUMMARY 



1. The material upon which this paper is based was contained in samples taken during oblique 

 hauls with the i m. net and vertical hauls with the 70 cm. closing net. 



2. The horizontal range of the adolescents and adults extends from Subantarctic water southwards 

 to the northern limits of the East Wind and Weddell drifts. The region of maximum abundance is 

 a circumpolar beh extending from 150 miles north to 200 miles south of the Antarctic convergence 

 and there is little or no seasonal variation in this distribution. The extreme range of surface temperature 

 is -I -80-1 1 -88° C, the greater part of the population living between 2 and 8° C. for most of the 



year. 



3. The distribution of E. triacantha is compared with that of E. superba, and it is shown that the 

 southern limit of the former roughly coincides with the northern limit of the latter. 



4. The region of maximum abundance of larvae is a beh extending for 200 miles south of the 

 Antarctic convergence. 



5. All stages from first calyptopis upwards perform a diurnal migration which becomes more 

 marked with increasing age. The lower limit of the vertical range does not normally extend below 



75° ™- 



6. Adolescent and adult E. triacantha do not form swarms and indeed are rather evenly distributed, 



but there is evidence of occasional small-scale swarming in the larvae. 



7. The mean lengths of the larval stages have been calculated and it is shown that the furcilia 

 stage with three terminal spines on the telson is usually omitted. 



8. Spawning takes place mainly in October and November and it is presumed to occur in the 

 region of maximum abundance of larvae. 



9. The development of both the male and female reproductive systems is similar to that of 



E. superba. 



10. Monthly length frequencies show distinct year groups. The normal life cycle takes two years 

 from hatching to breeding. Growth is slower in winter than in summer and ceases earUer than in 

 E. superba. 



REFERENCES 



Baker, A. de C, 1954. The circumpolar continuity of Antarctic plankton species. Discovery Rep. vol. xxvii, pp. 201-18. 



Bargmann H. E., 1937. The reproductive system of Euphausia superba. Discovery Rep. vol. xiv, pp. 325-5°- 



1945.' The development mid life-history of adolescent and adult krill, Euphausia superba. Discovery Rep. vol. xxiii, 



Boden^'b°^p!^'i955. Euphausiacea of the Benguela current, first survey, R.R.S. 'William Scoresby' , March 1950. Discovery 



Rep. vol. XXVII, pp. 337-76. 

 Gushing, D. H., 1951. The vertical migration of planktonic Crustacea. Biol. Rev. vol. xxvi, pp. 158-92. 

 David, P. M., 1955. The distribution of Sagitta gazellae Ritter-Zahony. Discovery Rep. vol. xxvii, pp. 235-78. 



