Methylmercury is the organic mercury compound that may accumulate in seafood.
Methlymercury exposure typically has less serious effects on exposed adults, though these include development of cardiovascular and autoimmune disease. By contrast children born to mothers exposed to methyl mercury, which is able to cross the placenta, show severe neurological issues and other developmental problems; prenatal exposure has also been associated with ADHD symptoms. Methylmercury exposure has been associated with a reduction in circulating thyroid hormones. There is some research that the mercury content of some fish essentially counteracts the benefits of Omega-3 fatty acids also found within.
Methylmercury accumulates up the food chain, i.e. is typically found in lower levels among algae and small fish, and in higher levels among larger, fish-eating fish. Older and larger fish will likely have higher methylmercury content.
Shark, swordfish, marlin, largemouth bass, northern pike, large species of tuna
Monomethylmercury, monomethylmercury(II) cation