This book is concerned with the heavy metal toxicity of estuaries, estuaries are important resources for coastal aquaculture, whereby they act as natural filters for suspended materials and pollutants. Human activities have drastically altered natural concentrations of many substances in estuaries. Prolonged bio-monitoring of pollutants has been given increasing attention due to ability of various aquatic organisms to accumulate these heavy metals. Among all types of pollution, aquatic pollution is of greater concern as each and every kind of the life depends on water. Millions tons of pollutants are introduced into environment by various industries by way of gaseous, aqueous and solid waste. These waste changes the natural conditions of aquatic medium, in turn causing behavioral response in crustaceans, mollusks, fishes and other aquatic organisms, ultimately leading to physiological adjustment., toxicity testing is an essential component of evaluating water pollution.This study change in ecological degradation, reflecting environmental pollution. Toxicant like heavy metal ions are non degradable, resulting into bioaccumulations with ecosystem and its biotic components