SECT. 4] BIOLOGICAL SPECIES, WATER-MASSES AND CURRENTS 385 



temperature gradient of 10°-12°C, it is not yet possible through the study of 

 distribution to establish that a boundary isotherm can limit a horizontal range 

 of such species. 



A. The General Applicability of Zooplankton-Wate.r-Mass 

 Relationships 



In each of the several systematic groups thus far studied in the zooplankton 

 of the Pacific (based mainly on samples collected by the California Cooperative 

 Oceanic Fisheries Investigations, The Pacific Oceanic Fisheries Investigations 

 and the Scripps Institution of Oceanography) certain species are recognized as 

 having geographical distributions that are in close agreement with the major 

 water-mass provinces. Pelagic Foraminifera (Bradshaw, 1959), Chaetognatha 

 (Bieri, 1959 ; Alvarifio, 1962), pteropod molluscs (McGowan, MS) and euphausiid 

 crustaceans (Brinton, 1962b) each include species limited to subarctic, sub- 

 tropical and tropical zones. These three ecological zones are designated subarctic, 

 central and equatorial in this discussion, following the clear role played by water- 

 masses in the zonation of the high seas. Studies of the distribution of poly- 

 chaetous annelids (Dales, 1957; Tebble, 1962) and certain pelagic tunicates 

 (Berner, 1960) and copepods (Johnson, 1938) in northern and northeastern 

 Pacific waters are in general agreement with the recurring patterns. 



A fourth Pacific faunal zone lies in temperate waters at the northern limit of 

 the central region. In the western and mid-Pacific this is a zone of transition 

 between central and subarctic faunas, and occupies water of the North Pacific 

 Drift, 38°-45°N. Toward its eastern limit the zone diverges: a northern branch 

 enters the eastern Gulf of Alaska while a southern branch occupies the Cali- 

 fornia Current, terminating off Baja California. The California Current and the 

 deep and coastal countercurrents which aid in maintaining plankton of the 

 Current lie mainly within this zoogeographic province. However, the character- 

 istic species of the zone are abundant only to the north of about 30°N. A "transi- 

 tion region", distinguished by Sverdrup, Johnson and Fleming (1942) on the 

 basis of physical properties, lies in the California Current where subarctic, 

 central and equatorial waters converge. Certain widely distributed species 

 maintain their highest concentrations in this transition zone. Other species are 

 endemic to it. 



An analogous zone of transition lies in the Southern Hemisphere, 35°-45°S, 

 reaching northward in offshore waters of the Peru Current. This belt includes 

 the region of the Subtropical Convergence and the northern part of the West 

 Wind Drift. It is the habitat of endemic species and of certain bisubtropical 

 species (i.e. species occurring on each side of the tropics but not within them) 

 found also in the transition zone of the North Pacific. 



The quantitative aspects of many of the distributions included in this 

 discussion are based on samples obtained above a depth of 300 m. These 

 generally contained epipelagic populations, including the developmental stages 

 of many species having adults concentrated in the stratum below 200-300 m. 



