Benthic (bottom-dwelling) invertebrates and certain fish and amphibian species may face exposure to mercury by absorbtion through direct contact with sediments via skin and gills. They may also release mercury bound in the sediment by direct ingestion. Having thus entered the food chain, mercury bioaccumulates as bottom dwellers are consumed by others.

Toxic methylmercury can inflict increasing levels of harm upon species near the top of the aquatic food chain. These likely victims include: predatory fish such as freshwater trout and salmon, ocean swordfish and tuna; turtles; fish-eating birds such as loons, cormorants, pelicans, ospreys, and eagles; and humans.

There is a great deal of uncertainty as to the sensitivity of various organisms to mercury. The exposure levels believed to be safe for humans may protect certain species but not others. The modes of exposure experienced by many species, particularly in the aquatic realm, differ drastically from those of humans. This makes it diifficult to ascertain the methylmercury threshold dose for adverse effects to the many species of wildlife.

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