Cut and pasted from https://en.wikipedia.org/wiki/Methylmercury
Because methylmercury is formed in aquatic systems and because it is not readily eliminated from organisms it is biomagnified in aquatic food chains from bacteria, to plankton, through macroinvertebrates, to herbivorous fish and to piscivorous (fish-eating) fish. At each step in the food chain, the concentration of methylmercury in the organism increases. The concentration of methylmercury in the top level aquatic predators can reach a level a million times higher than the level in the water. This is because methylmercury has a half-life of about 72 days in aquatic organisms resulting in its bioaccumulation within these food chains. Organisms, including humans, fish-eating birds, and fish-eating mammals such as otters and whales that consume fish from the top of the aquatic food chain receive the methylmercury that has accumulated through this process. Fish and other aquatic species are the only significant source of human methylmercury exposure.
This last statement is false.
The problem with Mercury is that it binds with just about anything and everything and can easily change forms. This is why it's so dangerous. When a school has a broken thermometer they have to close it and scrub everything down. Otherwise it will come right off the walls where it's landed looking for a better binder and that tends to be sulfur based organsims. It enters through the skin and when it finds materials with stronger bonds it changes forms. If it comes in contact with the right kinds of bacteria inside the body it switches to the most deadly kind of mercury, methyl mercury.