The Gulf of Mannar (GoM), located between India and Sri Lanka, has astonishing faunal richness and diversity1. Two oceanographic data sets are discussed here to show the unique ecological setting in the GoM sustaining a rich and diverse fauna. We tested the hypothesis that a specific stretch of a large marine environment behaves differently from the rest of the region due to its peculiar geographical positioning. Primarily, unlike the adjacent Indian southwestern shelf, coastal upwelling and the associated seasonal oxygen deficiency, potentially imparting physiological stress to marine fauna2-7, does not occur in the GoM. Secondly, the GoM along the Indian coastline receives adequate amount of primary (plankton) food from the Arabian Sea and the Bay of Bengal through the seasonally reversing coastal currents8-12. Thirdly, the GoM water has high transparency, aerated and sandy seafloor conducive for the growth of diverse corals and many sensitive fauna1,4. Our studies show that oxygen deficiency sheltered geography has a significant role in facilitating significantly high faunal diversity and richness in the GoM. We suggest that other similar coastal environments worldwide, protected from the extended oxygen-deficient region, might be functioning as a refuge for marine life and thereby increased faunal diversity6,7.