GOPALAKRISHNAN: ZOOGEOGRAPHY OF NEMATOSCEUS 



is distinguished from the typical form on the basis 

 of the difference in dimension of the proximal pro- 

 cess of the petasma. In the old form the proximal 

 process does not reach the distal end of the median 

 lobe, whereas in the new form it is extremely long, 

 reaching far beyond the distal end of the median 

 lobe. The proximal process of the old form is ser- 

 rated at the distal end and that of the new form is 

 without any serration. There is also an apparent 

 size difference between the two forms; the body 

 length of the old form is found to be significantly 

 larger than that of the new form. Moreover, the 

 upper lobe of the eye of the new form is narrower 

 than that of the old form. Along the equatorial 

 zone, where the distributions of the two forms 

 overlap, another form, which is recognized to be 

 "intermediate" of the old and new forms with re- 

 gard to the length of the proximal process, is also 

 encountered. All three forms are distinguishable 

 only in mature adults: males on the basis of struc- 

 tural differences of the petasma and females on 

 the basis of differences in the body lengths and eye 

 shape. Further discussion of them will appear in a 

 forthcoming taxonomic paper (manuscript). 



Nematoscelis gracilis is by far the most abun- 

 dant species of Nematoscelis in the tropical re- 

 gions of the Indian Ocean, including the Arabian 

 Sea and the Bay of Bengal ( Figures 4, 5). Along the 

 full extent of the ocean at the Equator this species 

 was found to be distributed between lat. 20°N and 

 20°S. The southern limit extended farther south 

 along the regions of the western and eastern 

 boundary currents, the Mozambique Current, and 

 the West Australian Current, respectively. Lar- 

 vae and juveniles were caught year-round from all 

 geographical areas within the distributional 

 range of the species. Therefore, spawning appar- 

 ently occurred everywhere within that range. 

 Larvae consisted of metanauplii, calyptopes, and 

 furcilias; metanauplii were undoubtedly under- 

 estimated since they (body length < 1 mm) could 

 have been washed away through mesh apertures 

 of the net. 



During the NE monsoon maximum numbers of 

 larvae and juveniles per sample were caught off 

 the coast of southeast India (Figure 4a). For ex- 

 ample, during Kistna cruise 26, 840 individuals 

 were caught at station 704 (lat. 13°N, long. 81°E); 

 and 764 individuals from station 705. Four sta- 

 tions had numbers exceeding 300 per sample from 

 the same area. However, during the SW monsoon 

 maximum abundance was in the western Indian 

 Ocean north of the Equator (Figure 4b). Both lar- 



vae and juveniles were absent from the northern 

 and northeastern Arabian Sea. There the oxygen 

 concentration in the subsurface waters is as low as 

 0.1 ml/1 (see Figure 2B). They were also absent to 

 the north of lat. 10°N along the west coast of India; 

 but during the NE monsoon they were caught as 

 far north as lat. 15°N in the coastal waters of 

 southwest India. The North Equatorial Current, 

 flowing westward, has a northwestward compo- 

 nent after passing the southern tip of India during 

 the early part of the NE monsoon, bringing a 

 spawning population of this species as far north as 

 lat. 15°N. In the area of the Mozambique Current, 

 larvae and juveniles were caught only as far south 

 as lat. 30°S. 



Adults of the old form are confined mostly to the 

 Arabian Sea and the Bay of Bengal and the new 

 form to areas of the Southwest Monsoon Current 

 and the South Equatorial Current which is 

 situated south of the Equator. During the NE 

 monsoon old forms are frequently caught in the 

 areas of the North Equatorial Current, whereas 

 new forms occur only in the Equatorial Counter- 

 current and south of it. The two forms overlap in 

 distribution along the northern boundary of the 

 Countercurrent from east of long. 75°E. In the 

 eastern Indian Ocean the distribution of old forms 

 extends farther south (as far as lat. 14°S) (Figure 

 5a). "Intermediate forms" were encountered all 

 along the overlapping zone. They were present 

 along the western and eastern boundaries of the 

 ocean. Most transport of the intermediate forms to 

 the west is through the North Equatorial Current. 

 In the South Equatorial Current it was found only 

 at one station (Koyo Maru cruise 14, station 19: 

 lat. 13°49'S, long. 94°16'E). In the western Indian 

 Ocean there was no overlap in distribution of the 

 forms, and the new forms did not occur north of the 

 Equator. Moreover, during this period the Somali 

 Current flows southwestward so that it brings the 

 old form as far south as lat. 3°S. Therefore, the 

 pattern of distribution of the two forms in the 

 upper layer may be influenced by the direction of 

 water flow in the equatorial current system which 

 is subject to seasonal changes. During the NE 

 monsoon no samples were taken from the Mozam- 

 bique Current area north of lat. 30°S. However, 

 mature adults of the new form were caught in one 

 sample taken from the southeast coast of Africa 

 (Natal cruise 6, day station 161: lat. 34°21 'S, long. 

 26°21'E). Thus it appears likely that the Mozam- 

 bique Current, which is stronger during the NE 

 monsoon than in the opposite season (as judged 



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