Bladder wrack is a seaweed found on the coasts of the North Sea, the western Baltic Sea, and the Atlantic and Pacific Oceans. It was the original source of iodine, discovered in 1811. Bladder wrack (sometimes spelled bladderwrack) is also known by the names black tang, rockweed, bladder Fucus, seawrack, sea oak, black tany, cut weed, and rock wrack. Directions: As a dietary supplement, take one capsule two or three times a day with meals or a glass of water.