Marine Ecotoxicology is the study of contaminants in the marine environment and their toxicity to the marine organisms. This study also covers the mechanisms of contaminant accumulation and its effects on the living organisms at all biological levels (from molecules to ecosystem). Complete characterization of the contamination of a marine ecosystem would require analysis of contaminants in seawater, sediments, and organisms. All these components present the positive and largely negative effects of contamination and the need for monitoring. Analysis of seawater and sediments would provide the actual concentrations in the environment and information on sources and fluxes of contaminants. However, analysis of seawater and sediments may not show the true effects of the contaminants on organisms in view of their low concentrations. Their bioavailability also cannot be measured correctly. Seawater concentrations are also subjected to fast temporal fluctuations raising questions on the true representative value of these analyses. In this background, living organisms are of significance as they present the advantages of bioconcentrating the contaminants that can be easily analyzed. The concentration levels serve as direct measures of the bioavailability as the biological organisms integrate the environmental concentrations over a longer period than seawater. At the same time, it is impossible to measure contaminants in all the organisms of an ecosystem although the levels may present wide variations in their response to the same contaminant. In view of this reason, monitoring programs make use of bioindicator organisms that are of help in quantifying the contaminants and their bioavailability. The main objectives of such programs are:

• to identify the nature of contamination, concentration of contaminants, their distribution and sources;
• to determine the dynamics of the contaminants: fate of contaminants in abiotic and biotic compartments of the ecosystem;
• to assess the effects of contaminants on individuals, populations, and communities and
• to evaluate the impact on human health and activities (food resources, commercial interests, nature preservation)