FISHERY BULLETIN; VOL. 72, NO. 4 



Nematoscelis microps 



24% OF THE SAMPLE 



• 25-49% 



• >50% 



a 



Figure 14. — Distribution of Nematoscelis in the Atlantic Ocean based on plankton samples: a - N. 



microps. 



(Wyrtki, 1973). The boundaries of distribution of 

 many zooplankton species appear to fall within 

 this zonal band. This is also the area of the north- 

 ern boundary of the subtropical species N. atlan- 

 tica. Evidently the southern boundary of A^. 

 gracilis new form is not confined to this zone; since 

 the South Equatorial Current carries it as far 

 south as lat. 20°S. The subtropical convergence 

 located at about lat. 40°-41°S separates the south- 

 ern subtropical anticyclonic gyre and the antarc- 

 tic circumpolar water. This is the region of the 

 southern boundary of distribution of both the sub- 

 tropical species N. atlantica and the warmwater 

 species A^. microps and A^. tenella. 



Brinton and Gopalakrishnan (1973) recognized 

 different euphausiid faunal assemblages in the 

 Indian Ocean, each of which is bounded mainly 

 around lat. 10°N, 0°, 10°S, 25°-30°S. Latitude 10°N 



delimits the northern distribution of not only A^. 

 microps and N. tenella but also other euphausiid 

 species such as Euphausia tenera, Thysanopoda 

 monacantha, T. tricuspidata, Nematobrachion 

 flexipes, Stylocheiron abbreviatum, and S. lon- 

 gicorne (Brinton and Gopalakrishnan, 1973). 

 Therefore, the Arabian Sea and Bay of Bengal 

 north of lat. 10°N contains large numbers of only 

 A'^. gracilis old form along with Stylocheiron indi- 

 cum, S. carinatum, S. affine, Pseudeuphausia 

 latifrons, Euphausia diomediae, and E. distin- 

 guenda. In both the Arabian Sea and the Bay of 

 Bengal a low level of oxygen (as low as 0.1 ml/1) 

 persists year-round in the upper oxygen minimum 

 layer (Wyrtki, 1971). However, temperature and 

 salinity vary seasonally in these areas. The sur- 

 face salinity ranges are 30-33%o for the Bay of 

 Bengal and 34-37%o for the Arabian Sea. Biologi- 



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