The aim of the present study is to investigate LC50 values in three seasons of TBTO on L. marginalis. The active substance TBT is highly toxic damaging to non target species. The bivalves have been bioindicators to determine aquatic pollution status. The adverse effect of Tributyltin oxide has been studied on freshwater bivalve mollusk Lamellidens marginalis in summer, monsoon and winter season during June 2014–may 2015. To evaluate the median lethal concentration of TBTO, bioassay was performed in all seasons on fresh water bivalve for 24hr, 48hr, 72hr and 96 hrs. The LC50 values in ppm were 1.61, 1.11, 0.85, 0.39in summer, 2.26, 1.67, 1.27, 0.88 in monsoon and 2.63, 1.92, 1.45, 1.09 in winter season. All values are after 24, 48, 72and 96 hours respectively. Results showed decrease in lethal concentration as exposure period increases and this was observed for all three seasons.

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