Fish and shellfish have a natural tendency to concentrate mercury in their bodies

... often in the form of methylmercury, a highly toxic organic compound of mercury.

Species of fish that are high on the food chain, such as shark, swordfish, king mackerel, albacore tuna, and tilefish contain higher concentrations of mercury than others. This is because mercury is stored in the muscle tissues of fish, and when a predatory fish eats another fish, it assumes the entire body burden of mercury in the consumed fish. Since fish are less efficient at depurating than accumulating methylmercury, fish-tissue concentrations increase over time.

Thus species that are high on the food chain amass body burdens of mercury that can be ten times higher than the species they consume...

Source: Fish and shellfish have a natural tendency to concentrate mercury in their bodies
Author: Wikipedia

Link: Asian Seafood Raised on Pig Feces Approved for U.S. Consumers
Link: Fish and Fishery Products Hazards and Controls Guidance Fourth Edition
Link: Shellfish Poisoning
Link: Shrimp Food Poisoning Signs
Link: The FDA Seafood List - Updates for 2012