Aquatic animals are useful as indicators of many kinds of pollutants in the aquatic environment. The presence of pollutants in the general environments of air, land and water is reflected in their accumulations in and effects on aquatic organisms because the aquatic portion of the biosphere is often the 'sink' for human-generated pollutants. Aquatic animals lend themselves particularly well to the study of special phenomena, such as carcinogenesis and teratogenesis. The use of aquatic animals and systems adds an important dimension to research on those pollutants that may affect wildlife and human health.