Fucaceae
Fucus vesiculosus
Bladderwrack · Black tang · Rockweed
- Kingdom
- Chromista — not a plant
- Part used
- whole
- Native range
- Rocky intertidal shores of the north Atlantic and north Pacific
- Morphology
- A brown seaweed to a metre, with flat forked fronds, a distinct midrib and paired gas-filled bladders that hold it upright in water. Anchored by a holdfast, not a root.
What is in it
Evidence
No supported evidence for the weight-loss and thyroid claims it is marketed for. Its iodine content is real and is the reason for both its historical use and its modern hazard.
Safety
Iodine content is high and enormously variable between samples — enough to cause both hyperthyroidism and, paradoxically, hypothyroidism, and cases of thyroid dysfunction from bladderwrack supplements are documented. Brown seaweeds concentrate arsenic and other heavy metals from seawater, and elevated arsenic has been found in retail products. Contraindicated in thyroid disease and in pregnancy.
Interactions: Interferes with thyroid medication; Additive with anticoagulants through fucoidan; Iodine interacts with lithium and amiodarone
Regulatory: Permitted as a listed ingredient with iodine limits. Several regulators have warned about arsenic in seaweed products.
Historical use
Used around north Atlantic coasts as fertiliser, fodder and medicine. Burned seaweed was the source from which iodine was first isolated in 1811, and bladderwrack was used for goitre before anyone knew why it worked.
Recorded as history and attributed where possible. Long use establishes that a plant was widely used, not that it worked, and not that it was safe.
Worth knowing
Bladderwrack is not a plant. Brown algae are chromists, whose ancestors acquired chloroplasts by swallowing a red alga that had itself already swallowed a cyanobacterium — so they are separated from land plants by an evolutionary distance vastly greater than that between a mushroom and a human.
It matters here because the collection kept meeting the same question with Ganoderma, and the answer is the same: a materia medica organised by what people used cannot be organised by what a strict botanist would accept.
Its medical history is genuinely important. Burnt seaweed ash was where iodine was first isolated, by Bernard Courtois in 1811, and the connection between seaweed and goitre had been noticed long before anyone could explain it. Bladderwrack is one of the very few entries here where a folk use led directly to the discovery of an essential nutrient.
The same iodine is why it is filed as a serious hazard. Content varies wildly between samples, and too much iodine disrupts the thyroid as reliably as too little.
Nomenclature verified against GBIF backbone taxonomy.