INTRODUCTION

The Molluscs are soft bodied invertebrates with or without an external protective shell. They inhabit usually water bodies, marine, estuarine, as well as fresh water; many are also terrestrial, often associated with moist shaded lands. If the body of the Molluscan taxa is enclosed by a pair of shells hinged in the middle it can be classified under the Class Bivalvia. Bivalves, which include clams and oysters contribute to the livelihoods of many people in India. The first Mollusc appeared at the end of the Pre-Cambrian period, approximately 550 million years ago (Sturm et al., 2006). It is the second largest phylum in the invertebrates comprising more than 100,000 species worldwide of which, 5070 species are present in India (Venkataraman and Wafar, 2005). Molluscs have been exploited worldwide for food, ornamentation, pearls, lime, and medicine (Nayar and Rao, 1985). Geologic evidence from South Africa indicates that systematic human exploitation of marine resources had started about 70,000 to 60,000 years ago (Volman, 1978).

Of the 5070 species of molluscs recorded from India, very few of them, especially of the bivalves, are exploited for food and other economic purposes. In the estuarine villages of the Karnataka three clam genera, Meretrix, Paphia and Villorita, and some oysters are used as food and sustain the livelihoods scores of people (Rao and Rao, 1985; Rao et al., 1989; Boominathan et al., 2008). Even these few edible bivalves are threatened of late due to shell and sand mining, over-exploitation, and salinity changes brought about in the estuaries due to constant releases of fresh water from hydel projects upstream in the rivers. A study conducted in Kali estuary by Boominathan et al. (2012) revealed that the edible estuarine bivalves lost about 15 kms of their occupational territory, pushed more westwards towards the Arabian Sea, due to water releases from upstream dams. Such a study was necessitated, to begin with, in all the estuaries of Uttara Kannada, which are getting subjected to ever increasing human pressures. The present study covered the situation of the edible bivalves, their diversity and its distribution, in Kali, Gangavali, Aghanashini, and Sharavathi estuarine areas.