1 Introduction
The green mussel Perna viridis had been known as a good biological indicator for heavy metals concentration by many researchers (Khan et al., 2015; Ali et al., 2014; Putri et al., 2012; Widmeyer and Bendell-Young, 2007; Yap et al., 2002; Sivalingham, 1977). From the literature, the heavy metal concentrations measured in the soft tissues of mussels could be used as biomonitors of heavy metal bioavailabilities and contamination in the coastal environment, in which the mussels live (Yap et al., 2006). The mussel species have been widely used in pollution monitoring programs such as ‘mussels watch’ program. It is an ecologically and economically important bivalve with outstanding potential as sentinel organism.

The green mussel fulfills the important criteria to be a good biomonitoring agent for heavy metal pollution studies in the coastal waters because of its wide geographical distribution, sedentary lifestyle, easy sampling and identification, availability throughout the year, capacity to accumulate pollutants in their tissues, tolerance but relative sensitivity to chemical pollutants and commercially important protein source (Yap et al., 2004).Due to its stress tolerant capacity and ability to accumulate pollutants in their tissues, present research work focused on determination of essential and non-essential heavy metals such as Hg, Pb, Cu, Ni, Zn, Co, Fe, Cr, Cd and Mn concentrations in the soft tissue of common bivalve (P. viridis) which were collected from the Manora channel of Karachi, Arabian Sea, Pakistan. However, the concentration of heavy metals accumulated by marine organisms are not only depending on the water quality but also seasonal factor, temperature, salinity, diet or food intake, spawning and individual variation (Hamed and Emara, 2006). On the other hand, periodical evaluation of heavy metals level in the tissues of organisms and water from expected polluted area are of major importance (Ali et al., 2014; Yap et al., 2009; Aktan and Tekin-Özan, 2012; Kock and Hofer, 1998)

The metal pollution in the ocean is a major problem that is directly affecting the marine life and indirectly affects human health and resources (Qari and Siddiqui, 2008). Karachi, being the natural seaport and the largest industrial city of Pakistan, located in the north eastern border of the ArabiaSea, lying between latitudes 24o48´ N and 66o59´ E longitude (Ashraf et al., 1992). The port and several industries are located in the Karachi city. Untreated or partially treated industrial and domestic waste waters are discharged directly or c