GOPALAKRISHNAN: ZOOGEOGRAPHY OF SEMATOSCELIS 



9Cr 90* IOC 110" 120* ISO* ^MT ISC 



sw 



Figure 16. — Locality records and da3rtime abundance of larvae and juveniles of Nematoscelis tenella in the Indian Ocean: b 



Monsoon period. 



Nematoscelis gracilis old forms in the northern 



much higher than in the area of the South 

 Equatorial Current. Correspondingly, N. gracilis 

 old forms are distributed in the northern part of 

 the equatorial current systems and new forms to 

 the south in less rich waters. The same relation- 

 ship exists in the Indian Ocean [cf. the Interna- 

 tional Indian Ocean Expedition Plankton Atlas. 

 Indian Ocean Biological Centre (1968)]. 



Each form ofN. gracilis is considered to be an 

 ecophenotype. Mayr (1971) recognized ecopheno- 

 type as a nongenetic modification of the pheno- 

 type in response to an environmental condi- 

 tion. However, the observed morphological differ- 

 ences, associated with reproductive structures, 

 suggest the possibility of genetic divergence of the 

 two demes. The intergradation (intermediate 

 forms) along the overlapping zones suggest in- 

 complete genetic isolation. 



part of the Indian Ocean differ from those of the 

 eastern tropical Pacific in that the Indian Ocean 

 forms are smaller in size. The degree of similarity 

 of these two populations is expressed quantita- 

 tively in a separate paper. 



Brinton (1962) pointed out that most of the 

 inter-ocean waterways in the Indo- Australian Ar- 

 chipelago, (e.g., the Strait of Malacca, Sunda 

 Strait, and Torres Strait) are too shallow (< 100 

 m) to allow interoceanic transport of the oceanic 

 euphausiid species. The deep pathway is through 

 the straits of Molucca and Halmahera, and the 

 Banda and Timor seas. Even central species like 

 Euphausia mutica are found along this route. 

 Nematoscelis gracilis new and old forms, A^. mi- 

 crops, and N. tenella show similar communication 

 between Pacific and Indian Ocean populations. 



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