THE OLDER STAGES 207 



5 or 6 days/ and for all we know in the sub-zero conditions in which it seems the krill eggs are laid 

 the corresponding development might take longer. We have to consider, too, the period hatching to 

 Metanauplius or First Calyptopis. We do not know this in E. superba, but in M. norvegica it seems 

 (Heegaard, 1948) it might take up to 15 days or even more. Again, in the krill a longer period might 

 elapse. Thus an unsegmented egg, liberated say in the slope waters of the south-western Weddell 



Fig. 31. Supposed influence of the sinking shelf water on the Hberated eggs. The Nauplii, Metanauplii and First Calyptopis 



are partly diagrammatic (see pp. 205-8). 



Sea, might spend a minimum of 21 days in the bottom water before, as a Metanauplius or First 

 Calyptopis, it came within reach of our searching nets which in Weddell West and Weddell Middle 

 did not go deeper than 1000 m. If the speed of the deep current were 2 knots^ the eggs and resultant 

 larvae could, therefore, it seems, be carried at least 500 miles to the east. Although we do not know 

 the speed of the bottom water there is some historical evidence (pp . 2 1 2- 1 5 ) that it may be higher than 

 is generally supposed. Bruce, for instance, was particularly impressed by the strength of the deep 

 current that on occasion seemed to be sweeping his trawls, already heavily overweighted, off the 

 bottom below the Weddell Sea, and he calls attention, too, to the famous occasion when Ross failed 

 to strike bottom in this region, his sounding line evidently having been swept away by a current of 

 similar force. It is possible in fact, as Deacon (1959) has recently said, that the bottom water has 

 a transport comparable with that of the Gulf Stream. 



With fishes, as Simpson (1956) observes, an important source of error in positioning spawning from 

 captures of eggs or larvae lies in the drift of the eggs and larvae away from the places where the eggs are 

 laid. He notes that where currents are strong the error will be particularly large for many winter- 



1 In the eggs of Thysanoessa inermis and T. raschii the same period is said to last from 6 to 7 days (Ponomareva, 1959). 



^ Such a speed is by no means impossible. The recently discovered Cromwell Current, the great underwater stream that 

 flows below the equatorial Pacific, is said to have a maximum velocity of 3 knots at a depth of 500 ft. (Knauss and King, 1958 ; 

 Boehm, 1959; Deacon, 1960(2; Charnock, 1960). 



