356 DISCOVERY REPORTS 



Now in the Falkland Sector of the Antarctic, as already seen (p. 292), there was, in 

 the season 1932-3, a southward movement of the isotherms towards the end of the 

 summer. This was more marked for the lower temperatures below the 3-0° C. isotherm 

 than for higher temperatures, and it was especially pronounced in the Weddell Sea area, 

 south and south-east of South Georgia (Figs. 6, 7). This, in all probability, is due to 

 the break-up of the pack-ice as the season advances. The region of optimum temperature 

 ( I •0-4-0° C.) for the spawning of Rhincalanus thus extends itself southwards towards 

 the end of the summer, and there is every reason to believe that this is a phenomenon 

 of yearly occurrence, accompanied by the replacement of a "cold-water" plankton by 

 a "warm-water" one in higher latitudes towards the end of the season (Mackintosh, 

 1934). This would then provide an explanation of the late summer spawning of 

 Rhincalanus in the higher latitudes and of the general southward movement of the 

 spawning area, for the stock in water which is below the optimum temperature limit 

 will not spawn until either it has drifted into warmer water within the spawning range 

 or the water itself has been raised to within the limits by the seasonal increase in 

 temperature, consequent upon the southward movement of the isotherms. In this 

 manner, therefore, the spawning time may be related to the break-up of the pack-ice. 

 The correlation between the time of spawning of R. gigas and the position of the 

 pack-ice, as already mentioned, further suggests that ice conditions, through their 

 influence upon the temperature of Antarctic seas, are the governing factor in deter- 

 mining the spawning of this species.^ 



We have suggested, in connection with the delay of the spawning time of Rhincalanus 

 in colder waters, that the relationship of the temperature to growth, that is to develop- 

 ment, is a factor of the greatest importance, and that in colder waters the spawning time 

 is delayed by the retardation of the attainment of maturity in the parent generation. 



We have seen that in Weddell Sea water having an average temperature for o-ioo m. 

 between o and i-o° C. the Rhincalanus population in January 1932 had only reached 

 copepodite stage v in development (p. 330), while at stations in South Atlantic water 

 where the temperature of the surface 100 m. was above i-o°C. the population had 

 spawned and hatched eggs. Thus over-wintered forms which drift into Weddell Sea 

 water having a temperature below i-o° C. fail to reach the adult stage by midsummer. 



1 Since this report went to press a paper by Ottestad (1936) has appeared dealing with the biology of 

 four species of Antarctic copepod (Calaniis acutus, C. propinquus, Rhincalanus gigas and Aletridia gerlachci) 

 from the collections of the Norvegia Expedition. The author found a relationship between the com- 

 position of the stock in stages and the temperature of the surface layers in C. acutus, C. propinquus and, 

 to a lesser extent, M. gerlachei. He related the spawning of these species to the break up of the 

 pack-ice, and concluded that as the older individuals drift northwards into water uncovered by pack-ice 

 they "mature and commence to spawn" (p. 23). He found no such relationship, however, in R. gigas 

 and concluded that "there must be a fundamental difference in the life histories of the two species" 

 (C. acutus and R. gigas, p. 28), and that R. gigas, must be a native of sub-Antarctic waters. The 

 conclusions of Ottestad for C. acutus and C. propinquus are similar to those which we have reached, 

 independently and with more abundant material, for R. gigas. The two former species evidently 

 spawn when the temperature of the water in which they are carried reaches a certain optimum, which lies 

 within lower limits for these species than for R. gigas. 



