396 JOHNSON AND BRINTON [CHAV. 18 



E. The Equatorial Species 



The euphausiids Euphausia distinguenda and Nematoscelis gracilis provide 

 examples of two patterns of distribution (Fig. 6a, b), both related to the region 

 of the Pacific equatorial water-mass. These extend into the Peru and California 

 Currents at the eastern boundary of the ocean. Euphausia distinguenda is 

 carried westward by the North and South Equatorial Currents, until the range 

 becomes attenuated in mid-ocean. The influence of the Equatorial Counter- 

 current, 2°-8°N, is evidently a disadvantage to this species. (The distributions 

 of other eastern equatorial species, E. eximia and E. lamelligera, are com- 

 pletely split along the east-west axis of the counter current, according to 

 sampling carried out in 1952 and 1955.) E. sibogae, very similar to E. dis- 

 tinguenda, is found in the warmer waters of the Indo-Australian Archipelago. 



The equatorial water-mass is narrowed in the western Pacific. The range of 

 Nematoscelis gracilis narrows in the same way, though this species is carried 

 northward to Japan by the warm Kuroshio. 



Four Pacific equatorial euphausiids, including the two just discussed, are 

 found also in equatorial waters of the Indian Ocean but not in the Atlantic. 

 This is in conspicuous agreement with the distribution of equatorial water- 

 masses, recognized in the Pacific and Indian Oceans, but not in the Atlantic. 



Each of the planktonic groups thus far studied with respect to distribution 

 contains tropical species which occupy both the equatorial region and the 

 central regions of the western Pacific. The limits of these distributions generally 

 follow the 24°-26°C summer surface isotherm. This assemblage was called 

 the equatorial-west-central fauna by Bieri (1959) and Bradshaw (1959), and is 

 made up of numerous epipelagic species including Sagitta robusta, Euphausia 

 tenera, the foraminiferan Pulleniatina obiquiloculata and the pteropod Limacina 

 inflata. 



F. Species of the Antarctic Ocean 



The niches in the Antarctic Ocean appear to have evolved in relation to four 

 boundaries : the Antarctic continent, the edge of the pack ice, the Antarctic 

 Convergence and the Subtropical Convergence. The meridional component of 

 Antarctic circulation, together with sinking at the Antarctic and Subtropical 

 Convergences, partially separates each of the epipelagic zones from its neigh- 

 bors. Mackintosh (1934) noted that, here, the limits of distribution of species 

 may be controlled by vertical migrations between surface water which has a 

 northward-moving component and southward-moving deep water. 



John (1936) showed that Euphausia crystallorophias is associated with the 

 coast of the Antarctic Continent, E. superba with the zone between the continent 

 and the Antarctic Convergence, and E. frigida with the region between the ice 

 edge and the Antarctic Convergence. These may be regarded as species of the 

 antarctic water-mass. Three species of polychaetes, Rhynchonerella bongraini, 

 Vanadis antarctica and Tomopteris carpenteri, are believed to be endemic to 

 waters south of the Antarctic convergence (Tebble, 1960). Euphausia vallentini, 



