2S0 DISCOVERY REPORTS 



The dominance of the adult female is particularly pronounced, for instance, at Station WS 915 

 where a full grown swarm consisting of stage 7 males and gravid females was encountered in the East 

 Wind zone near Enderby Land in February 1936. The sample obtained consisted of 215 males and 

 531 females all of which were measured to the nearest millimetre. Their length frequencies in 2-mm. 

 groups, plotted on a somewhat finer scale than has been used in Fig. 52, are shown in Fig. 53. 



SO-i 



40 



;30- 



'20 



10 



■ 100 



SC^ 



32 34 36 38 4042 44 46 48 SO 52 54 5658 60 62 64 66 

 LENGTH RANGE 



I 2 3 4 5 6 7 



STAGE d* 



I 2 3 4 5 6 7 



STAGE $ 



Fig. 53. Sexual dimorphism and developmental condition of the sexes in a fully adult swarm of E. superba, showing 



dominance of the female in a sample from St. WS 915. 



Developmental condition of the sexes 



In Table 19 of Bargmann's appendix it is repeatedly shown that, except in the very young mixed 

 larval and adolescent swarms encountered from August to November, the dominant state of sexual 

 development in the male is normally one or two stages, sometimes more, ahead of the corresponding 

 state in the female. The advanced state of the male, as Figs. 50-2 show, persists throughout the 

 greater part of the life of the swarm up to the point of pairing, when as might be expected swarms in 

 which both males and females are dominantly at stage 7 are not uncommonly encountered. 



An ingenious device has recently been constructed by Vittorio and Livia TonoUi (1958) and 

 used to study the form and internal structure of plankton patches in the Italian lakes. It could well, 

 it seems, be employed with advantage in future studies of the biometrics of krill swarms, with especial 

 reference perhaps to the larval swarms. 



A rather long plankton net ends in a polysthyrol tube that reaches the boat. The tube is here connected to a turbidi- 

 meter, from which another tube runs into a metallic box, where a controlled aspiration can be exerted. The water, 

 containing plankton filtered by the net, ascends through the polysthyrol tube, and, flowing through the turbidimeter, 

 enters the vacuum box. In this way, by regulating both the speed of the boat and the aspiration to a constant value, 

 we can obtain, through the variation of the turbidity values, an indication of the density of the particles suspended 

 in the layer where the net is fishing. In the vacuum box we have twenty vials, each provided with a filtering window. 

 When we are warned that the net has reached a patch of plankton, i.e., when turbidity increases, we can defiect the 

 current in which the plankton is suspended successively into each vial. In this way we can collect successive 

 uninterrupted subsamples along the same patch. 



