For the first time this year I’ve been to the seaside – not a long sandy beach, but the boulder-strewn former harbour entrance at the bottom of the crumbling cliffs of Penarth Head where many rocks on the lower half of the shore have bladder wrack growing on them. Fucus vesiculosus is probably the most common seaweed to be found on British shores, and grows on the coasts of most of the oceans and seas in the northern hemisphere.

As its common name implies, this member of the kelp family uses air bladders for buoyancy, to help float its fronds upwards towards the light. When growing in an area with more violent wave action, it grows less bladders as the wave movement helps elevate its fronds instead. It is intolerant of drying out, so its fronds flop together in a moist heap which helps keep water loss to a minimum between tides.

Though herbalists have used bladder wrack for centuries to stimulate thyroid function, and for the treatment of rheumatism and some skin diseases, it was only in 1819 that J. F. Coindet validated its efficacy scientifically, when he discovered that bladder wrack contained iodine. Extracts were subsequently used in medicines to treat goitre and other thyroid diseases.