GOPALAKRISHNAN: ZOOGEOGRAPHY OF NEMATOSCELIS 



I4<r IKT 



Figure 17. — Locality records and nighttime abundance of Nematoscelis tenella adults in the Indian Ocean: b - SW Monsoon period. 



lantic counterpart at the Drake Passage (An- 

 tezana, manuscript). The morphological differ- 

 ences between A'', megalops and A^. difficilis 

 already reflect presumed genetic differences. 



Atlantic populations of A^. atlantica, N. microps, 

 and A^. tenella are not in direct communication 

 with their respective counterparts in the Pacific, 

 but the North and South Atlantic populations are 

 in communication at the Equator. It appears that 

 the Atlantic and Indian Ocean populations of 

 these species are in at least seasonal communica- 

 tion around the tip of South Africa. 



A further aspect of the zoogeography of 

 Nefnatoscelis in the Atlantic lies in the fact that 

 A^. atlantica, not A^. gracilis, occupies the equator- 

 ial belt, permitting north-south continuity of the 

 species. Atlantic expatriates of A'^. gracilis new 

 form are found only off southwest Africa, probably 

 transported by the Mozambique Current. The cool 



Benguela Current region may then limit northern 

 transport of this tropical form, or the low oxygen 

 (0.5 ml/1) in the minimum layer of the current, lat. 

 10°S and 15°S (Bubnov, 1966), might be a barrier 

 to the new forms as it appears to be in the northern 

 Indian Ocean. 



There are similarities between distributions of 

 species of Nematoscelis and those of other zoo- 

 plankters. For example, the distribution of N. 

 megalops is like that of a copepod, Clausocalanus 

 ingens (Frost, 1969) and Thysanoessa gregaria 

 (Brinton, 1962); horizontal boundaries of A'^. mi- 

 crops and A^. tenella are like those of other 

 tropical-subtropical species, C. mastigophorus 

 and Stylocheiron carinatum . It is likely that both 

 biological (species interaction) and physical 

 [water mass, Sverdrup et al. (1942)] reasons are 

 responsible for the numerous similarities. 



McGowan (1971) classified the patterns of dis- 



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