Bladder wrack (Fucus vesiculosus)
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Fucus vesiculosus, known by the common name Bladder wrack, is a seaweed found on the coasts of the North Sea, the western Baltic Sea, and the Atlantic and Pacific Oceans, also known by the common names black tang, rockweed, bladder Fucus, sea oak, black tany, cut weed and rock wrack. It was the original source of iodine, discovered in 1811, and was used extensively to treat goitre, a swelling of the thyroid gland related to iodine deficiency. In the 1860s, it was claimed that bladder wrack, as a thyroid stimulant, could counter obesity by increasing the metabolic rate and, since then, it has been featured in numerous weight-loss remedies.
The species is common especially on sheltered shores from the middle littoral to lower intertidal levels.  It is rare on exposed shores where any specimens may be short, stunted and without the air vesicles.
Fucus vesiculosus is one of the most common algae on the shores of the British Isles. It is recorded from the Atlantic shores of Europe, the Baltic Sea, Greenland, Azores, Canary Islands and Madeira. On the Atlantic coast of North America from Ellesmere Island, Hudson Bay to North Carolina.
Fucus vesiculosus is a very variable alga. It can grow to 100 cm or more and is easily recognised by the small gas–filled vesicles which occur in pairs on either side of a central midrib running along the centre of the strap-like frond. Ascophyllum nodosum also has air vesicles, but rather than being paired, they are arranged in series along a frond which is not flattened and without a midrib. Both are common species on most shores of the British Isles.
The large brown algae have similar life cycles. At maturity the reproductive bodies form in conceptacles sunken in receptacles produced towards the tips on the branches. In these conceptacles oogonia and antheridia are produced and after meiosis they are released. After fertilisation, the zygote develops, settles and grows to form the diploid sporophyte plant.
The large vegetative phase is diploid and gametophytic. Meiosis occurs during the formation of the gametes. The egg cells are formed on the female plants in conceptacles embedded in receptacles. The spermatozoids and egg cells are extruded from the conceptacles. Once fertilised, the zygote settles and grows to form the new diploid gametophyte.