AMPHIPOD GENUS PARATHEMISTO — BOWMAN 373 



interchange of land mammals strongly suggests that the bridge was 

 submerged during middle Eocene time (Simpson, 1947a, 1947b), 

 although physical evidence in support of this submergence has not yet 

 been found in Alaska or the islands of the Bering Sea. During most 

 of the remainder of the Tertiary Period the bridge was above sea level. 

 After middle Pleiocene time but before the beginning of the Pleisto- 

 cene epoch, the bridge was again submerged. The bridge emerged 

 during each glacial interval and submerged during each interglacial 

 interval of the Pleistocene; it sank below sea level for the last time 

 between 11,000 and 12,000 years ago. There has thus been ample 

 opportunity for the evolution of species in the north Pacific differing 

 from their arctic relatives. If the northward flow of the current in the 

 Bering Strait severely limits the numbers of P. abyssorum entering the 

 Bering Sea, it is evident that this species would be markedly handi- 

 capped in establishing itself in competition with the abundant P. 

 pacijica. 



3. P. abyssorum avoids shallow water, hence does not enter the 

 shallow Bering Strait. As suggested b}^ the specific name, P. abys- 

 sorum, according to Stephensen (1923, p. 23), is found, as a rule, where 

 the depth of the sea is considerable, 400-3,127 meters. It occurs in 

 shallower water, as in the Gulf of St. Lawrence, where Bousfield 

 (1951) reports it from stations where the water was less than 100 m. 

 deep, though it was rare in the shallow water inside the Belle Isle 

 Strait. Curiously, Bousfield found that P. abyssorum was an indica- 

 tor of the relatively warm Gulf of St. Lawrence water, rather than, 

 as might be expected of an Arctic species, of the cold water of the 

 Labrador Current, which (Miters the gulf on the north side of the 

 Belle Isle Strait. It is most abundant at the deep water stations of 

 the gulf, which do not happen to be in the region influenced by the 

 Labrador Current. It probably enters the gulf through ( 1 al>ot Strait, 

 rather than through the shallow Belle Isle Strait; this entrance also 

 accounts for the larger numbers at the stations in the southern part of 

 the gulf. 



The depths of the sea at the stations in the Beaufort and Chukchi 

 Seas occupied by the U.S.S. Burton Island ranged from 18 to 4,050 m. 

 The depths at which P. abyssorum occurred are summarized in table 2. 



The shallowest station at which P. abyssorum was taken was 53 m. 

 The entire Bering Strait is less than 50 m. deep, much of it considerably 

 less, and this shallowness may be an important factor in preventing 

 P. abyssorum from gaining entrance to the Pacific Ocean. The com- 

 parative scarcity of P. abyssorum in the Prince of Wales Strait may be 

 related to the shallowness of the water there; the deepest station was 

 150 m., and only five stations were more than 100 m. 



