<P> Fish and shellfish concentrate mercury in their bodies, often in the form of methylmercury, a highly toxic organic compound of mercury . Fish products have been shown to contain varying amounts of heavy metals, particularly mercury and fat - soluble pollutants from water pollution . Species of fish that are long - lived and high on the food chain, such as marlin, tuna, shark, swordfish, king mackerel and tilefish (Gulf of Mexico) contain higher concentrations of mercury than others . </P> <P> Mercury is known to bioaccumulate in humans, so bioaccumulation in seafood carries over into human populations, where it can result in mercury poisoning . Mercury is dangerous to both natural ecosystems and humans because it is a metal known to be highly toxic, especially due to its ability to damage the central nervous system . In human - controlled ecosystems of fish, usually done for market production of wanted seafood species, mercury clearly rises through the food chain via fish consuming small plankton, as well as through non-food sources such as underwater sediment . This mercury grows in concentration within the bodies of fish and can be measured in the tissues of selected species . </P>

What kind of seafood would you expect to have the highest levels of mercury and why
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