390 JOHNSON AND BRINTON [CHAP. 18 



McGowan showed that, strictly speaking, the part of a T-S curve which 

 represents a precise depth of capture should be used to identify the water-mass 

 habitat. This procedure is valid where the population maximum is encountered 

 below the mixed layer. Divided hauls carried out at 150-m intervals by the 

 "Transpacific" expedition in 1953 made it possible to show a stratum-of- 

 capture. In the case of Poeobius, the segments of T-S curves plotted for 

 these layers generally fell within the envelope of the subarctic water-mass 

 (Fig. 3). 



Surface plankton has been related to the surface T-S property (Bary, 1959). 

 Distributions included in the present discussion are based on oblique open-net 

 hauls. These may sample (1) only the upper limits of an extensive vertical 

 range, or (2) a stratum thicker than the local vertical range of the species. In 

 these cases it can be useful to identify the water-mass habitat either by means 

 of the full T-S curve at the locality or by the part relating to the usual vertical 

 range of the species, if this is known. 



Recurrent species groups (in the sense of Fager, 1957) were distinguished 

 among certain North Pacific chaetognaths, euphausiids and pteropods (Fager 

 and McGowan, in press) on the basis of the "Transpacific" data of Alvarino, 

 Bieri, Brinton and McGowan. A subarctic group concorded best with 

 shape-of-T-AS-curve (50-1000 m), supporting the concept of the water-mass 

 habitat. 



C. The Transition Zone Species 



The region of transition between subarctic and central populations in the 

 mid-Pacific and between subarctic and equatorial populations in the California 

 Current is regarded as a biogeographical zone because it harbors (1) endemic 

 species, (2) bisubtropical species, limited in the North Pacific to this belt, and 

 (3) maximum densities or regions of dominance of certain species having 

 broader total ranges. A zone between the subantarctic and central regions in 

 the Southern Hemisphere is distinguished in the same way. 



In the western Pacific, where the cold subarctic region is in contact with the 

 warm Kuroshio and the Kuroshio Extension, a narrow transition zone was 

 recognized in which the copepod Calanus pacificus is particularly abundant 

 (Bogorov and Vinogradov, 1955; Brodskii, 1955). In mid-ocean the transition 

 zone was identified by Hida (1957) as an area of mixed fauna and variable 

 biomass, usually characterized by large numbers of the pteropod Limacina 

 inflata. A chaetognath identified by Hida as Sagitta lyra was abundant in the 

 transition zone. This form, now distinguished from the more widespread S. lyra 

 as S. scrippsae (Alvarino, 1962), appears to be endemic to the transition 

 zone. 



The euphausiid Nematoscelis difficilis lives in this narrow transition belt. A 

 relict population of this species is also maintained in the northern part of the 

 Gulf of California. N. megalops, a sibling species close to N. difficilis, occupies 

 the transition zone of the Southern Hemisphere, occurring in the Indian and 



