ZOOGEOGRAPHY OF THE GENUS NEMATOSCELIS 

 (CRUSTACEA, EUPHAUSIACEA) 



K. GiOPALAKRISHNAnI 



ABSTRACT 



The International Indian Ocean Expedition provided zooplankton samples to study the distribution 

 and seasonal changes in numerical abundance of Nematoscelis in the Indian Ocean. Distributional 

 boundaries of species of this genus in the Atlantic and Pacific oceans were determined mainly on the 

 basis of mid- water trawls. All seven species of this genus occur in the Pacific, whereas only five species 

 are present in the Indian and four in the Atlantic. Two forms ("old" and "new") q{N. gracilis , considered 

 to be ecophenotyp)es, but distinguished on the basis of morphological differences observed in the 

 petasma, occupy the tropical Indo-Pacific subregion. The "old form" is most abundant in the oxygen- 

 poor waters of the Arabian Sea, Bay of Bengal, and also in the eastern tropical Pacific. The "new form" 

 is mostly confined to areas of the South Equatorial Current. The present study indicates that N. 

 gracilis does not occupy the equatorial zone of the Atlantic Ocean, but "new forms" are transported 

 from the Indian Ocean up to the southern tip of South Africa. Nematoscelis atlantica is distributed in 

 the central water masses of the Pacific and Indian oceans, whereas in the Atlantic its distribution 

 extends also to the equatorial zone. Nematoscelis microps and N. tenalla are warmwater species and 

 are distributed between lat. 40°N and 40°S, although lacking in most areas of low oxygen water. 

 Nematoscelis lobata has restricted distribution in the region of the Philippines. Nematoscelis megalops 

 occupies the circumpolar transitional regions of the Southern Hemisphere and also the subarctic and 

 transition subregions of the North Atlantic. Nematoscelis difficilis is endemic to the North Pacific 

 transition zone. The basins of Timor and Banda Seas and their associated straits in the Indo- Australian 

 Archipelago appear to allow inter-ocean gene flow among populations of N . gracilis ,N . Microps, andN. 

 tenella. A similar communication exists between Atlantic and Indian Ocean populations of these 

 species through the oceanic waters around the tip of South Africa. 



Zoogeography of the order Euphausiacea is 

 reasonably well known for the Pacific Ocean 

 (Brinton, 1962). The study on the distribution of 

 euphausiids in the Indian Ocean is still in its pre- 

 liminary phase. Available data from this ocean 

 are adequate to make possible estimates of species 

 ranges only. The role of the biological program 

 during the International Indian Ocean Expedition 

 (IIOE) (1960-65) was to provide materials from 

 wide areas of the ocean to facilitate a better under- 

 standing of distributions of many zooplankton 

 taxa in the Indian Ocean. Therefore, in the pres- 

 ent study it was decided to put more effort into 

 understanding the geographical distribution and 

 seasonal abundance of Indian Ocean species of 

 Nematoscelis than those from the Pacific and At- 

 lantic oceans. Preliminary observations on the 

 distribution of Euphausiacea as a whole were 

 made by Gopalakrishnan and Brinton (1969). 

 Subsequently, Brinton and Gopalakrishnan 

 (1973) brought the distributional information up 



'Scripps Institution of Oceanography, La Jolla, CA 92037; 

 present address: The Hawaii Institute of Marine Biology, Uni- 

 versity of Hawaii, P.O. Box 1346, Kaneohe, HI 96744. 



Manuscript accepted February 1974. 

 FISHERY BULLETIN; VOL. 72, NO. 4, 1974. 



to date on the basis of IIOE samples studied up to 

 that time. 



Ocean-wide records of species of euphausiids in 

 the Indian Ocean come from three major expedi- 

 tions: the Percy Sladen Trust Expedition (Tatter- 

 sall, 1912), the Deutsche Tiefsee Expedition (Illig, 

 1930), and the John Murray Expedition (Tatter- 

 sall, 1939). Studies on the regional fauna are 

 available for the eastern waters of South Africa 

 (Boden, 1951), Red Sea, Arabian Sea, and Bay of 

 Bengal (Ponomareva, 1964, 1968). Species of 

 Nematoscelis of the southwest coast of India were 

 provided by Sebastian (1966). Only four of the 

 seven species of this genus have been reported 

 from the Indian Ocean: A^. gracilis, N. megalops, 

 N. microps, and A^. tenella. The present study 

 confirms the presence of a fifth species, A^. atlan- 

 tica, in the southern Indian Ocean. Nematoscelis 

 megalops was described by Boden (1951) from the 

 southern African waters. Three females of this 

 species were reported by Illig (1930) from south- 

 west of Ceylon in the northern Indian Ocean. The 

 present investigation does not confirm its dis- 

 tributional range in that region. 



Nematoscelis difficilis is an endemic species of 



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