AMPHIPOD GENUS PARATHEMISTO BOWMAN 359 



Bering Sea are needed to determine its northern boundary. The 

 investigations of Johnson (1934, 1953, 1956) and Stepanova (1937) 

 show that certain zooplanktonic species characteristic of the Bering 

 Sea and north Pacific Ocean may be carried into the Chukchi Sea by 

 the northward drift through the Bering Strait. P. pacifica does not 

 seem to undergo such transport. 



The boundaries of the area inhabited by P. pacifica, described 

 above and shown in figure 9, are in fairly close agreement with the 

 boundaries of north Pacific Subarctic Water as described and illus- 

 trated by Sverdrup et al (1946, p. 712, figures 202, 209a). It is 

 apparent that P. pacifica, like Sagitta elegans and Eukrohnia hamata, 

 is hy virtue of its temperature requirements an inhabitant of sub- 

 Arctic Water, and like these chaetognaths can serve as a biological 

 indicator of this cold water of low salinity. However, its distribution 

 differs from that Sagitta elegans, recently charted by Bieri (1959), in 

 that it penetrates farther to the south and does not enter the Arctic 

 Ocean, where S. elegans is common. The euphausiid, Euphausia 

 pacifica Hansen, judging from the details given by Boden, Johnson, 

 and Brinton (1955), has a distribution more nearly like that of P. 

 pacifica than does S. elegans. 



Temperature and body size: It is well known that the size of 

 marine poikilotherms is related to the temperature of the water in 

 which they develop (Sverdrup et al., 1946, pp. 855-857; Marshall and 

 Orr, 1955, pp. 81-89). This relationship has been investigated for 

 P. pacifica, and is summarized in figure 10. The mean length of the 

 adult females was determined for each station and plotted against 

 the temperature at a depth of 30 meters. 



The results show a slight trend toward greater size at lower tem- 

 peratures. Coefficients of correlation for cruises 1, 5, and 9 are 

 respectively —0.526, —0.571 and —0.582. For the numbers of pairs 

 of observations, 14, 20, and 19, the values of r required at the 5 

 percent level of significance are 0.532, 0.444, and 0.456. On this 

 basis the correlation between Parathemisto body length and 30-meter 

 temperature is significant for cruises 5 and 9, and nearly significant 

 for cruise 1. 



It can be seen from figure 10 that in July (cruise 5) Parathemisto 

 averaged somewhat larger than in March or November. The mean 

 length of adult female Parathemisto from all stations of a cruise was 

 5.07 mm. in March, 6.22 mm. in July, and 5.25 mm. in November. 

 Several explanations are possible for this seasonable variation in 

 length : 



1. The Parathemisto populations sampled during the three cruises 

 may belong to successive generations, the July generation having 

 developed under conditions which resulted in greater size at maturity 



540580—60 2 



