1. Introduction
The northern sector of the GoM is the first Indian Marine Biosphere Reserve1. The GoM, along its Indian coastline, has a spread of ~1500 km2 that extends typically from Tuticorin to Mandapam8 over a distance of ~ 190 km (Figure 1). The GoM has astonishingly rich and diverse fauna compared to the adjacent regions, and the ’Kurusadai’ Island, one of the 21 small islands situated close to the Indian coastline in the GoM, is traditionally known as ’Marine Biologist’s Paradise’. During 2010 - 2011, researchers from the National Institute of Oceanography, India including us have carried out oceanographic observations in the GoM and the adjacent Palk Bay (PB) as a feasibility study to link these water bodies through the Sethusamudram Ship Channel proposed by the Govt. of India. As an outcome of this, a series of peer-reviewed publications have come out dealing with their general environmental setting 8-12. Even though a large number of studies are available on the faunal diversity in the GoM, convincing explanation on its oceanographic causes is completely absent1. Since the GoM is an extension of the Southeastern Arabian Sea (SEAS), differentiating the features of the GoM from the SEAS has great importance. The main objective of this paper is to establish the oceanographic and ecological reasons that facilitate an unusually rich and diverse fauna in the GoM. For which we examined here, the general characteristics of the GoM8-12 besides elaborating upon a crucial environmental signal found in our 2010-11 in-situ studies in the GoM. Also verified these results using a recent oceanographic survey in 2018 covering the southwest coast of India up to the western boundary of the GoM (Tuticorin) augmented with satellite remote sensing data sets and Regional Ocean Model System (ROMS) projections.