GOPALAKRISHNAN: ZOOGEOGRAPHY OF NEMATOSCELIS 



Institution. Most of the samples were from the 

 North Atlantic. Charts were prepared for each 

 species of Nematoscelis; but the abundance is ex- 

 pressed only as the percentage of the total 

 Nematoscelis present in the aliquot, because the 

 aliquot itself was an unknown quantitative frac- 

 tion of the whole sample. Atlantis II and Chain 

 cruises took samples using a 75-cm net from above 

 100 m; Delaware samples were also taken from 

 similar depths, but by using 1-m nets. Lusiad VII 

 cruise in the South Atlantic collected samples 

 from 200 m to surface. In all, 217 plankton sam- 

 ples were examined from the Atlantic Ocean. 



HYDROGRAPHY OF THE INDIAN 

 OCEAN 



In the upper strata of the sea, the general circu- 

 lation may be related to the prevailing wind sys- 

 tem. This wind-driven oceanic circulation affects 

 and modifies the distribution of water masses in 

 the upper strata; the wind-driven circulation 

 penetrates into deeper layers and can be recog- 

 nized at intermediate depths (Reid, 1965). The 

 thermohaline circulation, caused by density dif- 

 ferences in the ocean due to heat and water ex- 

 changes with the atmosphere, is thought to be 

 responsible for formations of deepwater masses 

 and the deep circulation (Neumann and Pierson, 

 1966). As a result of these types of circulations, 

 properties such as temperature, salinity, oxygen, 

 and nutrients show distinct features in their dis- 

 tributions in different oceanic areas, providing 

 different characteristic habitat conditions for in- 

 dividual species or communities of organisms. 



Since a major part of the present study deals 

 with the seasonal distributions of Indian Ocean 

 species of Nematoscelis, it is pertinent to provide a 

 general background of the available information 

 on the monsoonal seasons prevailing over the In- 

 dian Ocean. The surface currents of the Indian 

 Ocean during the monsoon periods are described 

 by Taft (1971) and for the present study I will 

 follow his discussion of the wind and current pat- 

 terns (see Neumann and Pierson, 1966, and Taft, 

 1971, for illustrations of surface currents of the 

 Indian Ocean). Hydrographical changes as- 

 sociated with the monsoon are well documented 

 for the Arabian Sea (Wooster et al., 1967 ). During 

 the NE monsoon the northeast winds of the 

 Northern Hemisphere cross the Equator into the 

 Southern Hemisphere, there becoming north- 

 westerly. This monsoon phase is established in 



November and persists through March; its max- 

 imum intensity is in January. During this phase a 

 broad doldrum belt develops at about lat. 5°S to 

 lat. 10°S between weak northerly winds and the 

 southeast trade winds. In the SW monsoon, south- 

 east trades from the Southern Hemisphere cross 

 the Equator and gradually turn into southerly and 

 southwesterly directions in the Northern Hemi- 

 sphere. Winds are especially intense during this 

 period near the African, Somali, and Arabian 

 coasts. The SW monsoon is well established in 

 May and persists through September. It is most 

 intense in July. 



The onset of monsoon periods in the Indian 

 Ocean is found to be symmetrical in time (Wooster 

 et al., 1967; Taft, 1971). Each period lasts for 5 mo; 

 April and October are transition periods. Because 

 of these short periods of transition in the monsoon 

 system, it was decided to divide each April and 

 October period into two halves and add the first 

 half of April and the latter half of October to NE 

 monsoon. The latter half of April and the first half 

 of October were similarly added to the SW mon- 

 soon period. The overall winds and surface cur- 

 rents are far stronger in the SW monsoon than in 

 the NE monsoon. 



The surface circulation plays an important role 

 in the distribution of planktonic animals in the 

 ocean. In the Indian Ocean the surface circulation 

 is subjected to a longer seasonal change than in 

 other oceans. The outstanding features of the sur- 

 face circulation are: 1) currents that follow in a 

 more or less zonal direction far from the conti- 

 nents, and 2) currents near continental coasts. 

 During the NE monsoon the North Equatorial and 

 the South Equatorial currents flow westward and 

 the Equatorial Countercurrent flows eastward be- 

 tween these two currents. Unlike those in the 

 Pacific and Atlantic, the countercurrent in the 

 Indian Ocean is situated in the Southern Hemi- 

 sphere. This is related to the position of the dol- 

 drums south of the Equator. The South Equatorial 

 Current, after reaching the African coast, con- 

 tributes water to the Agulhas Current flowing 

 southwest and also to the eastward flowing Coun- 

 tercurrent. During the NE monsoon the Somali 

 Current, flowing southwest with a speed of about 

 100 cm/s, merges into the Mozambique Current 

 (Neumann and Pierson, 1966), thereby bringing 

 many tropical species as far south as lat. 35°-40°S 

 along the coast of Africa. At this time the surface 

 current off the southwest coast of India is also 

 weak and flows in a southeast direction. In the Bay 



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