GOPALAKRISHNAN: ZOOGEOGRAPHY OF NEMATOSCELIS 



Geographical Distribution of N. tenella 



Indian Ocean 



The geographical distribution of A^. tenella is 

 similar to that of A'^. microps. This species lives in 

 the equatorial and central water masses of all 

 oceans. In the Indian Ocean most records came 

 from between lat. 7°N and 38°S except in the east- 

 ern and western boundary current areas (Figures 

 16a, b; 17a, b). It was most frequently caught in 

 the equatorial zone and in the eastern ocean south 

 of the Equator. Larvae and juveniles occurred 

 throughout the range of distribution. There were 

 no seasonal differences in the distribution of lar- 

 vae and juveniles; however, in the western Indian 

 Ocean north of the Equator they were more abun- 

 dant during the SW monsoon than in the opposite 

 season. This species penetrates into the Arabian 

 Sea only in the western side. It is absent from the 

 Bay of Bengal. The pattern of distribution of 

 adults was like that of larvae and juveniles (Fig- 

 ure 17a, b). Mid- water trawls caught A'^. tenella 

 between lat. 8°N and 40°S (Figure 18). 



Atlantic Ocean 



Illig (1930) recorded A'', tenella from lat. 

 59°39'N, long. 8°49'W, but all other records are 

 from lat. 35°N to 35°S. Lewis (1954) reported this 

 species off southern Florida. In the present study 

 larvae and juveniles were found to be more abun- 

 dant in the western North Atlantic than on the 

 eastern side (Figure 14b). Around lat. 40°N, its 

 northern limit, it was caught in five plankton 

 samples. It has not been reported from the 

 Mediterranean Sea. In the South Atlantic this 

 species was found in the Lusiad collections off 

 South Africa. No samples were available from the 

 western South Atlantic nor has it been reported 

 from that region. It is also not known whether this 

 species occurs in the Benguela Current. 



Pacific Ocean 



Nematoscelis tenella lives in both hemispheres 

 of the Pacific, lat. 35°N-34°S. It does not occupy the 

 more coastal areas of the California Current or its 

 southward extension into the eastern equatorial 

 region (Figure 18). It is also scarce in the region of 

 the Peru Current (Brinton, 1962). In midocean, 

 Brinton recorded it as far south as lat. 34°S. This 

 species appears to be more abundant in the north- 



ern central gyre than in the south. It occurs 

 farther east in the tropical belt than N. microps. 

 The north-south range is continuous across the 

 Equator. It is a common species in the South and 

 East China seas and also in the Banda, Molucca, 

 and Timor seas. Apparent communication exists 

 between the Indian and Pacific populations. 



Faunal Zones and Biogeography 

 of Nematoscelis 



Faunal regions of the oceanic environment are 

 not as well defined as in the terrestrial habitats. 

 The oceanic environment, as distinguished by 

 Hedgpeth (1957), consists of a system of zonally 

 oriented hydrographic provinces arranged in 

 latitudinal succession within each ocean. In the 

 Atlantic Ocean, Dahl (1894) recognized four 

 epipelagic faunal regions: arctic, subarctic (tem- 

 perate), subtropical, and tropical. Steuer (1933) 

 described a similar system of classification but in- 

 cluded the Pacific and' Indian oceans. He recog- 

 nized circumpolar arctic, circumequatorial tropi- 

 cal, and circumpolar antarctic on a global scale. 

 The arctic region was subdivided into a circumpo- 

 lar subregion as well as Atlantic and Pacific sub- 

 arctic subregions. The antarctic region includes 

 the circumpolar antarctic and subantarctic sub- 

 regions. The tropical region was subdivided into 

 Atlantic and Indo-Pacific provinces. A system of 

 classification based on this and other subsequent 

 works [for example: Deacon (1933, 1937); Rad- 

 zikhovskaya (1965); Stepanov (1965); Frost 

 (1969); McGowan (1971)] is adapted here and 

 shown in Table 2. The division of the epipelagic 

 environment into faunal zones agrees with the 

 distributional patterns of many planktonic or- 

 ganisms [see McGowan (1971) for examples]. 

 Species oi Nematoscelis occupy one or more of the 

 subregions (Table 3). As mentioned before, A^. 

 difficilis is endemic to a zone of transition between 

 subarctic and central water in the North Pacific. 

 Even though this zone (Johnson and Brinton, 

 1963) does not have as well defined a tem- 

 perature-salinity envelope as other water 

 masses, it maintains endemic species, as well as 

 the densest part of the overall populations of some 

 subarctic and central species. Evidence is ac- 

 cumulating for the existence of a unique water 

 body in this zone with characteristic hydrographi- 

 cal and faunal properties (McGowan, 1971). The 

 extent and location of an analogous transition 

 zone in the North Atlantic, if such exists, is not 



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