SECT. 4] BIOLOGICAL SPECIES, WATER-MASSES AND CURRENTS 393 



D. The Pacific Central Species 



Plankton species of the central waters may be grouped according to the 

 extent to which their ranges conform with the distribution of the most typical 

 central water. 



The euphausiids Nematoscelis atlantica, Euphausia brevis (Fig. 5) and E. 

 hemigibba are most numerous in the warm, barren mid-parts of the North 

 Pacific gyrals. N. atlantica and E. brevis are bisubtropical, occurring also in 

 the South Pacific central zone, while E. hemigibba is replaced in the 

 Southern Hemisphere by E. gibba. T-S envelopes for these species, as shown for 

 E. brevis (Fig. 5b, c), are in agreement with the T-S characteristics of the central 

 water-masses. 



The distributions of E. brevis and N. atlantica, unlike those of many other 

 central species, do not extend into the region south and east of Japan, where 

 central water is affected by the warm Kuroshio system and the submerging 

 subarctic water. 



A second central group may be represented by Euphausia mutica, Sagitta 

 pseudoserratodentata, and the foraminiferan Globorotalia truncatulinoides. These 

 species are found in low concentration toward the central parts of the oceanic 

 gyrals, while areas of abundance are associated with the margins of the ranges. 

 These margins are places where the central environment impinges on more 

 fertile waters of the subarctic region, the eastern boundary currents, and the 

 equatorial water-mass. 



Ranges of the species of this second central group frequently enter near-shore 

 waters in the southern part of the California Current at 20°-34°N. This is a 

 region south of Pt. Conception, California, where the climate of the northern 

 epipelagic region is modified by the effect of the subtropical latitude and by 

 admixture of southern and offshore water. 



The species of a third central group have North and South Pacific zones of 

 distribution, joined in the western Pacific but separated in the eastern Pacific. 

 The eastern region from which these species are excluded lies in the equatorial 

 water-mass, characterized by low sub-surface values for temperature and dis- 

 solved oxygen. The chaetognath Sagitta californica and the euphausiid Stylo- 

 cheiron abbreviatum are examples of this group that may be showing either 

 incipient bisubtropicality or a coalescence of northern and southern zones of 

 distribution. 



The composite range of the species-pair Thysanopoda aequalis-T . sub- 

 aequalis (Boden and Brinton, 1957) falls in the third group. In the North 

 Pacific, T . subaequalis is found in the western part of the ocean while T. 

 aequalis lives in the eastern part. This is the only known case in which the 

 eastern and western central water-masses of the North Pacific support distinct 

 populations of closely related zooplankton species. (The eastern and western 

 North Pacific central water-masses were distinguished by Sverdrup, Johnson 

 and Fleming (1942), but are not now generally believed to be distinct gyrals.) 

 In the South Pacific the ranges of the two forms overlap ; T. aequalis lives in 

 cooler water than T. subaequalis and is the more widespread form. 



