AMPHIPOD GENUS PARATHEMISTO BOWMAN 367 



The findings of Yamada (1933) are more instructive. In his report on 

 plankton collected from a series of stations off both coasts of Korea, he 

 found P. japonica present off the east coast south to nearly 38°, but 

 absent from the Tusima Straits and the Yellow Sea. This distribution 

 correlates well with the hydrography of the Sea of Japan. As Uda 

 (1934) has shown, the water on the Asiatic mainland side is much 

 colder than that on the Japanese side. The cold water flows south 

 along the mainland coast (Liman Current) to the latitude of about 

 36°-38° N. and forms several counterclockwise eddies as it flows. Only 

 a small amount of mixing occurs between this cold water, presumably 

 formed by excessive cooling in winter, and the warm, north-flowing 

 water of the Tusima Current, a branch of the Kuroshio. 



Yamada's findings suggest that P. japonica is distributed in the Sea 

 of Japan according to the extent of the cold water, as Tokioka (1940) 

 found for the chaetognath, Sagitta elegans. The occurrence of P. 

 japonica at Albatross stations along the west coast of Honshu makes 

 this hypothesis improbable. Uda's (1934) isotherm charts show that 

 in June most of the Sea of Japan is characterized by temperatures at 

 least as low as those of the southern boundary of the Ctyashio. The 

 Oyashio boundary temperatures in Uda's charts are 17°-19° C. at the 

 surface and at 25 m., about 17° at 50 m., and about 15° at 100 m. 

 Most of the water of the Sea of Japan at these depths is as cold or 

 colder, while the water of the southward-flowing current in the west is 

 considerably colder. Hence a wide distribution of P. japonica in the 

 Sea of Japan is to be expected from our knowledge of its distribution 

 off the east coast of Japan. 



It is reasonable to believe that P. japonica evolved from P. pacifica 

 or from a common ancestor of both species as a result of geographical 

 isolation. The current patterns in the Japan Sea provide little oppor- 

 tunity for P. pacifica to enter from the Pacific Ocean. Almost all the 

 water entering the Sea of Japan is warm water derived from the Kuro- 

 shio; only a very small quantit}' of cold water enters through Tartary 

 Strait between the Asiatic coast and Sakhalin Island. Water leaves 

 the Japan Sea by way of the Straits of Soya (La Perouse) and Tsugaru, 

 north and south of Hokkaido, respectively. Information concerning 

 water movements in the Okhotsk Sea is still scanty. According to 

 Brims (1958) there is some exchange of water with the Pacific Ocean 

 through the Kuriles. The very limited collections available to me 

 indicate that this exchange of water has been insufficient to allow P. 

 pacifica to become established in the Okhotsk Sea. 



Actually, therefore, the P. japonica population in the Okhotsk Sea 

 and especially in the Sea of Japan is effectively isolated from extensive 

 gene flow from the P. pacifica population. The three northwestern 

 Pacific occurrences of P. japonica reported in this paper may be ex- 



