Biogeographical Change in the Latitudinal Boundary of a Bisubtropical Pteropod Styliola subula (Quoy et Gaimard) in the Indian Ocean

Abstract

The geographical distribution and relative abundance of the bisubtropical pteropod Styliola subula is given in Fig. 1. This species has a wide range of distribution from 10° N to 40° S and is very rare or absent in the Indian Ocean north of 10° N and south of 40° S. The sparse occurence of this species between 5°N and 10° N, and 30° S and 40° S is noticed in the present study. There were 579 collections from north of 10° N: except for a single record of a single specimen from AB 175 at 17° 26′N and 56° 57′E, none of them contained this species. Therefore, the extreme scarcity of S. subula in the northern part of the Arabian Sea is borne out by previous and present findings. There were also 268 Zooplankton collections from north of 10° N in the Bay of Bengal, none of which contained this species. The southern boundary of S. subula in the South Indian Ocean further supports previous findings in all 3 oceans. Clearly, the boundary of subtropical convergence which runs for the most part between 38° S and 40° S acts as a thermal barrier to the distribution of S. subula further south than 40° S.