392 JOHNSON AND BKINTON [CHAP. 18 



Atlantic Oceans as well. N. megalops, rather than N. difficilis, occupies the 

 transitional belt in the North Atlantic. 



The bisubtropical euphausiid Thysanoessa gregaria (Fig. 4) lives in the 

 transition zones of both hemispheres in the Pacific. In mid-ocean its low-latitude 

 boundary is near the 30° parallel in both hemispheres. In the eastern 

 boundary currents the range reaches toward the tropics, terminating in equa- 

 torial water. This is near 15°S in the Peru Current and 18°-20°N off Baja 

 California. 



The temperature-salinity envelopes for T. gregaria (Fig. 4b, c) show that 

 both the northern and southern habitats are intermediate in character between 

 equatorial and subarctic (or subantarctic) waters. In both cases there is over- 

 lapping with the central water-masses. In the South Pacific the T-S habitat 

 of this species also extends southward into subantarctic water and northward 

 into modified equatorial water. Distributions of the southern transition zone 

 species are closely related to the east-west belt of the Subtropical Convergence, 

 which curves northward as the South American continent is approached and 

 becomes poorly defined in offshore waters of the Peru Current. Northward- 

 reaching tongues in the distributions of T. gregaria and N. megalops are main- 

 tained, in part, by a northward-flowing subantarctic current component and 

 a deeper remnant of southward-flowing equatorial water. T-S curves indicate 

 the similarity of the equatorial water to both the South Pacific central water 

 and the subantarctic water. 



The term "transition zone" is particularly descriptive of the North Pacific 

 habitat of T. gregaria where the T-S envelope of the species falls in part 

 between the envelopes of typical subarctic and central water and in part 

 within the central envelopes. In the South Pacific it almost completely overlaps 

 both envelopes. It is to be noted that the T-S habitat of T. gregaria in the 

 North Pacific differs somewhat from that in the South Pacific. The differences 

 lie mainly in the salinity. The temperatures in the two zones are nearly the 

 same. Apparently a broader range of salinity is found in the northern zone 

 within the range of temperature tolerance. It will be seen that the disjunct 

 habitats of central species, when described in terms of the relationship of 

 temperature to salinity, differ from each other in exactly the same way as 

 those of the transition zone. Salinity per se probably has little direct effect in 

 controlling the geographical distributions of oceanic species. Rather, the 

 essential features of the oceanic habitat are (1) continuity in the system of 

 circulation (water-mass) implying a permanent or semi-permanent place of 

 origin of the water, (2) a range of temperature to which the species can adapt, 

 (3) an adequate food supply, and (4) an area sufficiently large to enable re- 

 stocking despite loss by dispersal. 



In an alternative explanation there may be genetic differences, not yet 

 discernible, between the two populations of T. gregaria. Such differences are 

 already suggested in the Nematoscelis difficilis-N . megalops pair. Geographically 

 separate populations of a species will, in time, diverge genetically as a con- 

 sequence of selective pressures that differ between the two environments. 



