Methylmercury: An organic form of mercury that is highly toxic and is the main culprit in mercury poisoning. Methylmercury is easily absorbed into the living tissue of aquatic organisms, is not easily eliminated, and therefore accumulates in fish that are predators. The degree to which mercury is transformed into methylmercury and transferred up the food chain through bioaccumulation depends on factors such as water chemistry and the complexity of the food web.
Methylmercury is highly toxic to mammals, including people, and causes a number of adverse effects. The brain is the most sensitive organ. The population at highest risk is the children of women who consume large amounts of fish and seafood during pregnancy. The risk to that population is likely to be sufficient to result in an increase in the number of children who have to struggle to keep up in school and who might require remedial classes or special education because of brain damage.