RHINCALANUS GIGAS 355 



of the same age as the advancing summer generation sampled three weeks earUer in 

 the South Atlantic. 



In the southern Drake Passage in early February 1933 a half-grown summer genera- 

 tion was found, while in the more northerly Antarctic water in the Drake Passage, 

 near the convergence, there was a still older summer generation. Between the Falklands 

 and South Georgia in February the conditions were remarkably similar in the two 

 seasons (193 1-2 and 1932-3). In both seasons in Antarctic water near the convergence 

 (Sts. 830 and 1125) a summer generation was found approaching maturity, while near 

 South Georgia, in colder water, the summer generation was found to be much younger 

 (Sts. 831, 1127 and 1131). 



In Weddell Sea water in March 1933 two apparent age groups were found — a half- 

 grown summer generation in water flowing out of the Weddell Sea in a north-easterly 

 direction and a slightly older generation in water flowing into the Weddell Sea in a 

 westerly direction south of 66° S. 



Again in the Australian Sector during the winter great numbers of nauplii and very 

 young stages were taken near the ice, while at the same time of year a very much older 

 winter-spawned generation was found at the northern Antarctic stations between 

 Australia and the ice edge. When the over-wintered generation reappeared at the surface 

 in the spring of 1932-3 in the Drake Passage it was found to be in a more advanced con- 

 dition than the over-wintered generation which was found a month later in the same 

 region the year before, when the temperature of the water was lower. In November 

 193 1 there was a pronounced diff^erence between the age of the overwintered generation 

 in warm Antarctic water and its age in the cold water near the ice edge. 



In order to explain the large number of apparently diff'erent age groups in difi'erent 

 localities in the Falkland Sector and elsewhere, we have suggested, tentatively, that the 

 main spawning takes place within a certain sea-temperature range. This range appears 

 to lie between the temperatures i-o and 4-0° C. Temperatures below this range have 

 everywhere a similar efl^ect upon the Rhincalaniis population — growth and development 

 are retarded and spawning is thus delayed. Outside certain wider temperature limits 

 the spawning is stopped altogether. It is not possible to decide at present where these 

 wider limits lie, but appearances in the Weddell Sea suggest that the lower limit might 

 be between o and — i-o° C. The eflFect of temperatures higher than 4-0° C. also seems 

 to be to inhibit spawning, since very little spawning appears to take place in sub- 

 Antarctic water. 



The dependence of the breeding of marine animals upon constant optimum tem- 

 perature limits has been demonstrated and discussed by Orton (1920) and Runnstrom 

 (1927). Orton wrote (p. 362): "Temperature limits seem to influence breeding in various 

 ways which appear to be dependent upon the limitation of the breeding period by 

 apparently constant maximum and/or minimum temperatures. These temperatures 

 appear to be physiological constants for the species." Runnstrom showed that these 

 relationships between breeding and temperature have a direct bearing on the geo- 

 graphical distribution of the species. 



