GOPALAKRISHNAN: ZOOGEOGRAPHY OF NEMATOSCELIS 



20- 30- 



20- ISO- 140" 150* 



Figure 13. — Locality records and nighttime abundance oi Nematoscelis microps adults in the Indian Ocean: b - SW Monsoon period. 



is subtropical. Because the boundaries between 

 the full breadth of tropical and subtropical regions 

 are not well defined hydrographically, it is usually 

 difficult to correlate species' distributional pat- 

 tern to the hydrographical zones of these two 

 areas. 



DISCUSSION 



The distributions of Nematoscelis species are 

 associated with general hydrographical features 

 in each ocean. In the Indian Ocean the seasonality 

 in abundance of this genus is more pronounced in 

 the Northern Hemisphere than in the south; and it 

 is probably related to changes in monsoonal re- 

 gimes. The hydrographical features of the Ara- 

 bian Sea and Bay of Bengal limit the northern 

 boundary of distribution of all species of 

 Nematoscelis except A^. gracilis old form. There 



appears to be a general break in the north-south 

 midocean distribution of N. microps and N. 

 tenella near the equatorial zone of the Indian 

 Ocean. Their northern boundary of distribution 

 corresponds to the approximate southern extent of 

 oxygen-poor waters ( < 1 ml/1) of the Arabian Sea 

 and the Bay of Bengal. 



Wyrtki (1973) proposed a division of the Indian 

 Ocean into three circulation systems: a seasonally 

 changing monsoon gyre, a southern subtropical 

 anticyclonic gyre, and the antarctic waters with 

 the Circumpolar Current. One unique feature of 

 the Indian Ocean is the persistence of a hydro- 

 chemical front at about lat. 10°S, separating the 

 high-nutrient, low-oxygen content waters of the 

 monsoon gyre from the low-nutrient, high-oxygen 

 content waters of the subtropical gyre. The exis- 

 tence of such a front is very well reflected by the 

 chemical characteristics of the subsurface water 



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