We all worry about mercury in fish these days. Elemental or ionic mercury is released into the environment from a variety of natural processes and as a result of human activity.
Bacteria in the oceans change this form of mercury into the organic monomethylmercury (CH3Hg) form via methylation. Following a bio-magnification process it accumulates in the muscle tissues of large, long-lived predatory species that feed on other fish, including sharks.
The mercury in fish is almost entirely monomethylmercury which is present in muscle tissue, not in fat. Monomethylmercury binds to cysteine in body fluids, and while not very lipophilic (ability to dissolve in fats and oils) for passive membrane transport, it is actively transported across the cell membrane by amino acid transporters (by mimicking methionine). Transport is efficient and that is why the mercury in fish is almost entirely in the monomethylmercury form. This also explains why it is efficiently absorbed and can move around the body of animals and humans. It accumulates in muscle tissue rather than fat because it has a very high affinity for (and therefore binds strongly to) the thiol (-SH) groups on the amino acid cysteine and hence to proteins containing cysteine that are located there.