388 



JOHNSON AND BRINTON 



[CHAP. IS 



about by differences between the two optimal environments : the temperature 

 at 200 m is less than 4°C in the range of the "spined" form, but may be as high 

 as 7°C for the "unspined" form. 



Other northern species have wider environmental tolerances, permitting 

 them to range southward in the cool coastal waters off California. This group 

 includes Euphausia pacifica (Fig. lb), the copepods Calanus cristatus, Eucalanus 

 bungii bungii, and the pteropod Limacina helicina. Some of these species 

 persist until subarctic and upwelled components of the California Current 

 become thoroughly mixed with equatorial water toward the south, and with 

 central water in the offshore region. McGowan (MS) has shown that a northern 



Th/sonoes so longipes 

 "Spmed" form 

 NORTH PACIFIC 



35 33 



SALINITY (% ) 



la] 



Fig. 2. The shaded part represents temperature-salinity characteristics below about 

 150 m in the environment of Thysanoessa longipes. These observations were made 

 during the "Transpacific" expedition (1953) and the "Norpac" cruise (1955). Certain 

 of the curves that limit the T-S envelope for the species are shown as fine lines. The 

 more northern "spined" form (a) is seen to be more typically subarctic than the "un- 

 spined" form (b). (The water-mass envelopes are from Sverdrup, Johnson and 

 Fleming, 1942.) 



high-spired form of L. helicina may be distinguished from a low-spired form 

 occupying the transition zone and the California Current. He regards these as 

 geographical races or ecotypes. The limits of the distribution of Euphausia 

 pacifica off Baja California fall where subarctic and equatorial waters are mixed 

 in nearly equal proportions, according to the temperature-salinity charac- 

 teristics of that region (Tibby, 1943). Seasonal variability in the southward 

 extent of the distribution of E. pacifica in the California Current is small 

 (Brinton, 1960). In general, this species is absent from Baja California waters 

 during the winter and recolonizes that region at the onset of spring upwelling. 



