1. INTRODUCTION Utilization of marine resources for human consumption has increased rapidly in worldwide. Seafood products are currently in high demand as they are considered healthy, nutritional and possess medicinal values. The oceans offers a large biodiversity of fauna and flora which is estimated to be over 5,00,000 species, which are more than double of the land species (Anand et al., 1997; Kamboj, 1999). There are approximately 5,000 species of Sponges, 11,000 species of Cnidarians, 9,000 species of Annelids, 66,535 species of Molluscs and 6,000 species of Echinoderms (Laxmilatha et al., 2006). Molluscs are widely distributed throughout the world and have many representatives such as slugs, whelks, clams, mussels, oysters, scallops, squids, and octopods in the marine and estuarine ecosystems. Among the Molluscs, 50,000 species of Gastropods, 15,000 species of bivalves and 600 species of Cephalopods have been reported (Alfred et al., 1998). Molluscs occupy a variety of habitats ranging from mountains fresh waters to sea. These are more abundant in the littoral zones of tropical seas. Gastropods and bivalves constitutes 98 per cent of the total populations of molluscs. Molluscs are delicious and protein rich food among the sea foods (Jagadis, 2005). Bivalves belong to Molluscs, which is the second largest phylum among the invertebrates. They have been exploited worldwide for 2 food, ornamentation, pearls etc. for the the human welfare. Bivalves of orders, Orterids, Mytulids and Pectinids are harvested for food globally. The bivalves in the coastline could form an important source of food, raw material for village industries, indigenous medicine etc. and it is widely used as a cheaper food source for coastal area people. Shell fish such as mussel and bivalves contain approximately 20 to 28 per cent calories of fat. Bivalves also provide high quality protein with all the dietary essential aminoacids for maintenance and growth of the human body. For this reason, bivalves could be considered a low fat, high protein food that can be included in a low-fat diet (King et al., 1990). In India, 5,070 species of Molluscs have been recorded, of which 3,370 species are from marine environment (Venkataraman and Wafar, 2005; Ramachandran et al., 2011). These are included among the economically important edible species of bivalves such as oysters, Crassostrea madrasensis and C. gryphoides, bivalvas, Meretrix casta, M. meretrix, Paphia malabarica and Villortia cyprinoides and green mussel, Perna viridis (Chatterji et al., 2002). In Indian coastal marine ecosystems, 17 species of bivalves have been exploited (Ramachandran et al., 2011). Molluscs with rich diversity of marine organisms assume a great opportunity for the discovery of new bioactive compounds. Thus, the marine environment is an exceptional reservoir for bioactive natural products, many of which exhibit structural features that are not found in terrestrial natural products (Joshua, 1999). A wide variety of bioactive 3 substances are being isolated and characterized from the food that is derived from the marine environment, several with great promise for the treatment of human and fish diseases. There is a vital interest in discovering new antimicrobial compounds with fever environmental and toxicological risks and the resistance developed by the pathogens (Challeram et al., 2004; Periyasamy et al., 2012). The marine environment comprises complex ecosystem with a plethora of organisms and many of these organisms are known to possess bioactive compounds as a common means of defense (Indap and Pathare, 1998). The marine natural products have been investigated predominantly for their antimicrobial, cytotoxic, anti-leukamic, anti-tumour, anti-viral and anti-inflammatory properties (Kamiya et al., 1984; Anand et al., 1997; Anand and Edward, 2001; Naganuma et al., 2006; Liyan Song et al., 2008; Chandran et al., 2009; Shanthi et al., 2011; Ramasamy and Balasubramanian, 2012; Periyasamy et al., 2012). The marine ecosystem remains as untapped resources for discovery of many drugs and contemporary experimental studies which indicates that pharmacologically active substances could be isolated from marine organisms (Naganuma et al., 2006; Liyan Song et al., 2008). In the last two decades alone, structures of over 6,500 marine natural products have been elucidated (Kamboj, 1990; Wright, 1998; Naganuma et al., 2006). More than 100 pure compounds of known and new structural types have been isolated and characterized (Kamboj, 1999; Ramasamy and Balasubramanian, 2012). The screening of marine 4 organisms, especially bivalves for therapeutic drugs are of greater interest now-a-days. They have been recognized as a potential source of antibacterial and antifungal substances. The potential of marine bivalves as sources of biologically active products remain largely unexplored. Therefore, a broad screening of bivalves for bioactive compounds is necessary. The Indian bivalves have protein content ranging from 5-80 per cent, carbohydrate 3-24 per cent, fats 0.5 – 3 per cent, calcium 0.04 – 1.84 per cent, phosphorus 0.1 – 0.2 per cent and iron 1-29 mg/100 g of the fresh weight (CSIR, 1992). Fatty acids in marine invertebrates have been studied in many habitats because of their significance in human life (Ackman, 2000). Bivalves have a great importance because of their fatty acid components and their variability in different areas. The polyunsaturated fatty acids (PUFA) have been recognized as effective factors in human health and nutrition (Bruckner, 1992). The quality of protein is usually assessed by its amino acid composition. The amino acid composition in turn is helpful in assessing the nutritive value of an organism. Bivalves, Anadara granosa and Meretrix casta commonly occur in intertidal areas of Indian coasts, particularly in South East and West coast of India (Salaskar and Nayak, 2011; Ramasamy and Balasubramanian, 2011). As bivalves form good protein food, a comprehensive knowledge of their biochemical constituents during different seasons of the year would be valuable for large-scale exploitation from natural resources and to promote culture. Cyclical changes in 5 biochemical composition of bivalves tissue and mainly studied to assess the nutritive status of an organism. Several species of bivalves are found in the estuaries, coastal and backwaters of India coastal ecosystems (Laxmilatha et al., 2006) which are exploited for their meat and shells. Information on the distribution and exploitation of bivalves from India was reported very earlier also (Rao, 1963; Jones, 1968; Alagarswami and Narasimham, 1973). Studies on the resource characteristics, exploitation and biology of bivalves from Karnataka, Kerala and Andra Pradesh were made by Harkantara (1975), Rao (1984), Nayar et al. (1984), Rao and Rao (1985), Achery (1988), Laxmilatha and Narasimham (2002), Laxmilatha et al.(2006) and Ramasamy and Balasubramanian (2011). Laxmilatha et al. (2006) felt that the existing potential for bivalve culture and distribution was immense and stressed the need for organizing culture programs and exploitation of distribution to augument production. At present there is a great demand for bivalves meat as a delicacy in developed countries (Appukuttan, 1996). Considering the demand for meat and shells by the various industries in Tamil Nadu State, India, from the coastal ecosystems of Muthupet estuary and Adirampattinam coastal waters of South East of India, a broad screening of bivalves for the distribution, and seasonal variation in physico-chemical characteristics, status and elucidation of bioactive compounds, nutritional quality, biochemical compositions in whole body of bivalves and their antibacterial activity against bacterial pathogens is necessary. 6 Estuaries are highly productive, dynamic and unique ecosystems providing food, transport, recreation etc. and estuaries play a predominant role in the socio-economics of the coastal regions by providing valuable resources like fishes, bivalves, molluscs, crabs, shrimps etc. Water quality studies are important and have been taken up because they play a key role in aquaculture. The maximum production of bivalves is obtained, when physico-chemical factors are at optimum level. Therefore, water quality is a permanent factor in an ecosystem productivity. The reproductive cycle of bivalves influenced by exogenous or endogenous or both factors. Among exogenous factors, the habitat is one of the factor for breeding behaviour of bivalve organisms. The rainfall, temperature, photoperiod, humidity, salinity, pH, dissolved oxygen, free CO2, solids, hardness and nutrients of water also influence the breeding, distribution and biology of the bivalves (Ramasamy and Balasubramanian, 2011). Though works are available on biodiversity of bivalves, seasonal variations happen in physico-chemical characteristics of coastal waters of South East coast of India and their elucidation of bioactive compounds, and nutritional quality. Most of the studies carried out so far are related to anti-bacterial activity against bacteria and fungal pathogens. However, there is no work on economically important bivalves like A. granosa and M. casta from Muthupet estuary and Adirampattinam coastal waters of South East coast of India, and their elucidation of structural bioactive compounds and nutritional quality and their antibacterial activity against pathogenic bacteria are important. 7 Therefore, the present study was carried out on bivalves, Anadara granosa Linn. and Meretrix casta