256 DISCOVERY REPORTS 



showing the remarkably backward developmental condition that persists in the East Wind larval 

 swarms from January to April, with the corresponding condition of the oldest contemporary larval 

 swarms from the Weddell zone added for comparison, the latter how the northern adolescent 

 swarms between November and April, that is on the threshold, and in the early part, of their second 

 year of life, tend persistently to be encountered in a more advanced condition (in so far at least as 

 length frequency may be said to indicate advancement), or at any rate to be composed of larger 

 animals, than their contemporaries in the southern zone. A corresponding disparity in size has been 

 found in Thysanoessa inermis which grows to 27 mm. in the Japanese Sea, but only to 22 mm. in the 

 colder Barents Sea (Ponomareva, 1954). 



% 



Fig. 58. Moulting. Monthly frequency of occurrence 

 of cast skins between the surface and 1000 m., the en- 

 circled figures showing the numbers of observations on 

 which the frequency in each instance was based. 



SEP lOCT |NOV I DEC I JAN | FEB |MAR | APR |MAY [ JUN | JUL | kijQ 



"j) (a) i,?g> (15) (47) (74 i g; (\BJ ijQ) [\2) ( 



® (g) (@) (^3; 64) ^ (18) ^ @ 



Fig. 59. Moulting. Monthly frequency of occurrence 

 of cast skins between the surface and 1000 m., the three 

 hauls from 250 m. to the surface being treated as 

 one. 



In the East Wind zone the sun's rays strike obliquely with the result that they are deficient in 

 intensity, much ultra-violet radiation is lost and the total available heat is low. All this, quite apart 

 from its adverse effect on photosynthesis, may contribute to the slow growth-rate we find among the 

 high southern krill, much as it affects the annual growth of many deciduous and evergreen plants 

 in the Arctic (Dansereau, 1955). 



MOULTING 

 In the vertical samples I examined I frequently noted complete casts of euphausian skins, or fragments 

 of euphausian skins, which from their large size could readily be referred to E. superba. Only casts 

 that could obviously be referred to this species were recorded, very small casts which, though possibly 

 those of young adolescent krill, might also have been confused with those of other and smaller species, 



