A marine shrimp caught near the mouth of a river smacks less of iodine than one caught in unadulterated seawater because the seawater has a higher iodine content. Even marine shrimp that live far away from rivers can differ in flavor because iodine content in seawater and the food shrimp eat vary geographically. Shrimp acquire an iodine flavor when they eat algae. (These plants are iodine-rich because they concentrate within their cells the iodine in seawater). In addition, shrimp get iodine by eating sea creatures, such as sand-dwelling worms, that also eat the algae. Once digested, a fair portion of the iodine remains in the shrimp's bodies.
Sometimes the iodine flavor is intensified when processors use the additive sodium bisulfite. This chemical can amplify the iodine's effect on our taste buds. Although its use to prolong the storage life of shrimp is prohibited in the United States, it is no secret that some foreign processors surreptitiously taint their shrimp with it before exporting them to America.