DISTRIBUTION OF YOUNG STAGES OF EUPHAUSIA SUPERBA 159 



water, before the coming of the cold conditions of winter, the larval krill is more or less 

 evenly distributed, or even if it is in small swarms of individuals which have developed 

 from the same brood, in its migrations upwards to the surface it will be fairly evenly 

 distributed in the surface water. But when pancake-ice is formed and, later, pack-ice, 

 the cutting off of the light rays necessary for photosynthetic processes will diminish the 

 production of diatoms underneath the ice and there will be a movement of the krill to 

 areas of greater abundance of food at the periphery of the ice-field. That this happens 

 is proved by observations on the circumpolar cruise and the voyage which the ' William 

 Scoresby' made into the Weddell Sea (see pp. 137, 148). The latter instance is most 

 striking because the ship penetrated far into an area which is normally ice-covered for 

 the greater part of the year, and the stations where krill was taken are all concentrated 

 on the northern part of the line between South Georgia and the point east of the South 

 Sandwich Group, where the ship turned southwards into the water of the eastern 

 Weddell Sea. No krill was taken in the part of the Weddell Sea which is normally ice- 

 covered, although the observations included a station at the ice-edge with hauls 

 throughout a twenty-four-hour period (St. WS 552). It cannot be doubted that krill 

 does not penetrate far beyond the edge of the ice-field, and that to some extent at any 

 rate the concentrations met with in the vicinity of the pack are caused by the movement, 

 towards the periphery of the pack, of krill which was more uniformly distributed before 

 the formation of the ice-field. 



An explanation is required however to account for the persistence of some of the krill 

 in a vertical migration towards the surface at night. Russell (1927, p. 235), quoting 

 Michael's results (191 1, p. 144) of investigations into the vertical distribution of certain 

 Chaetognatha, states: 



All individuals do not react towards light, temperature and salinity in the same way. While the 

 majority migrate towards the surface during twilight hours and toward deeper water during intense 

 light and darkness, a few almost always remain in deeper water during twilight and on the surface 

 during intense light and darkness. Similar individual differences occur with respect to temperature 

 and salinity. This means that those optimum conditions favourable to the species as a whole are not 

 favourable to each individual or in other words the characteristic organization, constitution or physio- 

 logical state of each individual modifies the effect of light, temperature and salinity on its behaviour. 



The principle involved in the foregoing quotation can be applied to E. superba, although 

 there is a modification in behaviour. At the time of year we are considering the optimum 

 conditions for the greater part of the krill are at the surface, but for the rest they are 

 at a variable distance from the surface dependent on the time of day. Now it has 

 already been pointed out that, even in the Calyptopis stages where there is a pronounced 

 diurnal migration, there are still appreciable numbers of larvae to be found at the surface 

 at midday. In the later larvae with which we are dealing the predilection for the surface 

 has become emphasized. To this extent the diflFerence in distribution may be onto- 

 genetic in origin, simply because the time of year when FurciHa 6 and adolescents are 

 predominant coincides with the period of winter conditions and concomitant alteration 

 in light intensity. 



