RHINCALANUS GIGAS 357 



We also noted that in November 193 1 in the Drake Passage (p. 324) the over-wintered 

 population in warm sub-Antarctic water consisted mostly of adults, while that in 

 Antarctic water, south of the convergence, consisted mostly of stages iv and v. In the 

 coldest Antarctic water (— 1-5° C.) comparatively large numbers of stage iii were taken. 

 Evidently, young stages which developed during the winter in the coldest Antarctic 

 water only reached stages iii and iv by the spring, while those which developed in warmer 

 Antarctic water reached stages iv and v. The same age difference, although less marked, 

 was also found in October 1932 in the Drake Passage (p. 341). All the above facts can be 

 explained on the assumption that, as there is an optimum temperature range for spawn- 

 ing, so also there is an optimum temperature range for growth and development, and 

 that temperatures which approach the lower limit of that range slow up the develop- 

 ment. The limits of this range cannot be exactly fixed in the present work, but they do 

 not seem to be widely different from those of the optimum spawning range, and it would 

 appear that an appreciable slowing up of the rate of development of the population is to 

 be found in water with an average temperature for the o-ioo-m. layer below i-o° C. 



That reduction of temperature does have a retarding effect upon the development of 

 some marine animals is well known. Murray and Hjort (1912, p. 555) quote the lobster, 

 whose eggs and larvae are only developed during the warmest part of the year, "and 

 it has been found that a fall of a few degrees is sufficient to retard the development for 

 several weeks". Gran (1902) explained the different ages of the stocks of Calanus 

 finmarchicus in the Norwegian Seas (p. 62) by assuming that "the yearly developmental 

 cycle is disarranged through the influence of outside factors, especially temperature", 

 and, again, that "the length of life will differ in different regions. External factors can 

 act upon the rate of development and thus on the length of life" (p. 64). 



In this connection it is perhaps permissible to mention the results of Coker (1933), 

 who experimented with two species of Cyclops. One of these, when kept at room tem- 

 perature, spent a disproportionately long time in a state of arrested development at 

 stage iv. It could be made to develop normally, without any arrested stage iv, by being 

 placed in a refrigerator. Moreover, eggs hatched at room temperature mostly failed to 

 develop, and those which did so developed slowly and remained at stage iv for 120-140 

 days, a period corresponding to the theoretical duration of several cycles of develop- 

 ment. Another species, on the other hand, developed exactly seven times more rapidly 

 at room temperature than in the refrigerator. Thus while one species passed through its 

 development normally at a low temperature (about 8° C.) a higher temperature (about 

 22° C.) slowed down development. The other species behaved in an exactly opposite 

 way and developed normally at the higher temperatures but was retarded at the 

 lower. Runnstrom (1929) determined the optimum range for the development of 

 littoral Mediterranean-boreal forms, but his tabulated results show in every case that 

 temperatures as little as half a degree above or below the optimum range for the species 

 had the effect not of retarding development but of preventing it altogether. The eggs 

 failed to segment, or segmented irregularly and died, or reached early embryonic 

 stages and then died. 



