Bladder wrack (Fucus vesiculosus) seaweed on the shore. The characteristic air bladders (round) act as floats when the plant is underwater. This seaweed is a good source of iodine and was used in the nineteenth century to treat goitre, a swelling of the thyroid gland caused by iodine deficiency. Bladder wrack is a common food in Japan and is used as a flavouring throughout Europe. It grows on rocky shores around the middle of the littoral zone.