FISHERY BULLETIN: VOL. 72, NO. 4 



Figure 17. — Locality records and nighttime abundance of Nematoscelis tenella adults in the Indian Ocean: a - NE Monsoon period. 



However, the absence of A^. atlantica from the 

 region of the Indo-AustraHan Archipelago sug- 

 gests the lack of inter-ocean communication of 

 this species. Moreover, N. lobata, w^hich is mor- 

 phologically most similar to A^. atlantica, is en- 

 demic to a part of this region, mainly the Sulu and 

 Celebes seas. These basins are connected with the 

 open ocean by relatively shallow channels. This 

 region is well known for its endemic species. Gil- 

 bert and Hubbs (1920) pointed out that the pecu- 

 liar characteristics of the isolated basin of the 

 Sulu Sea might have caused the evolution of many 

 fish species. Because of morphological similarities 

 and non-overlapping distribution (geographical 

 isolation), N. lobata and A^. atlantica may have de- 

 rived from the same stock. 



The North Pacific central population of A'^. at- 

 lantica is also isolated from its southern counter- 

 part. However, it is not known whether the dis- 



juncts are genetically different. Analyses per- 

 formed by both numerical and conventional ways 

 indicate no morphological differences. 



Another region of importance in considering 

 gene flow between Indian and Pacific populations 

 is the South Australian Basin and the Tasman 

 Sea. The Bass Strait between Australia and Tas- 

 mania is probably too shallow to allow passage of 

 adult oceanic euphausiids. However, larvae and 

 juveniles may be transported across this passage. 

 The only route for adults is south of Tasmania, but 

 this is almost certainly too far south for the 

 tropical-subtropical species including N. tenella, 

 N. microps, and N. atlantica. Evidence from south 

 of Australia (Monsoon Expedition, long. 

 120°E-175°W) indicates that populations of A^. 

 megalops are in communication between the 

 South Pacific and Indian oceans. The Pacific popu- 

 lation evidently does not mix with the South At- 



1070 



