AMPHIPOD GENUS PARATHEMISTO BOWMAN 371 



July 14 in waters west of Greenland. In the collections of the U.S. 

 National Museum is a lot consisting of three adult males, 13, 14, and 

 14.5 mm. in length, and a female 14.8 mm., with fully developed ooste- 

 gites, taken on May 18, 1902, between the Shetland and Faroe Islands. 



Several records of P. abyssorum or P. oblivia in the Pacific Ocean 

 are probably erroneous, since the authors almost certainly had P. 

 pacifica or P. japonica. These include the records of Shoemaker 

 (1930) and Thorsteinson (1941) from near Nanaimo, British Columbia, 

 and that of Holmes (1904) from Humboldt Bay, Popof Island, Alaska 

 (about 55° N., 161° W.). Possibly the specimens of "P. oblivia" 

 from stations 13 and 14 of the Canadian Arctic Expedition (Shoe- 

 maker, 1920) were P. japonica, and those from stations 21 and 27 

 were P. abyssorum. Irie's (1957a) record from the Tusima Straits 

 is probably P. japonica. 



The puzzling thing about the distribution of P. abyssorum is that 

 it has not been able to establish itself in the Pacific (fig. 13). This 

 absence cannot be simply a matter of temperature, since the Okhotsk 

 Sea and the western Bering Sea are regions of very low temperatures. 

 In fact P. libellula, an Arctic species which does not penetrate as far 

 to the south in the Atlantic as P. abyssorum, occurs in the Bering and 

 Okhotsk Seas. 



Three possible explanations may be given for the absence of P. 

 abyssorum from the Pacific: 



1 . The current flow through the Bering Strait has made it mechani- 

 cally impossible for P. abyssorum to enter the Pacific Ocean. The 

 hydrographic evidence for a northward flow of water from the Bering 

 Sea into the Chukchi Sea (Barnes and Thompson, 1938; LaFond and 

 Pritchard, 1952) is corroborated by Johnson's (1956) report of the 

 presence of characteristic Bering Sea copepods in the Chukchi Sea 

 (Eucalanus bungii bungii Johnson, Metridia lucens Boeck). While 

 the penetration of Arctic plankters into the Pacific would be retarded 

 by this northward current, it is unreasonable to suppose that it 

 would be completely inhibited. On the extreme west of Bering 

 Strait there appears to be some movement of water southward, and 

 Sewell (1948, pp. 399-401) believes that this movement has carried 

 certain Arctic planktonic copepods into the north Pacific. 



2. P. abyssorum is unable to compete successfully with the closely 

 related north Pacific species, P. japonica and P. pacifica. All three 

 species are very similar in structure and probably occupy similar if 

 not identical ecologic niches. Possibly the Pacific species evolved 

 during a time of isolation of the Pacific from the Arctic by emergence 

 of Bering Strait. Bering Strait has been above sea level during much 

 of geologic history. Hopkins (1959) has recently reviewed the 

 Cenozoic history of the Bering Land Bridge. Evidence of the 



