DESMARESTIA

Desmarestia is a genus of brown seaweed found in cold waters worldwide, including around the British coast. It's best known by its common names — acid weed, acid kelp, sea sorrel — and for a genuinely unusual trait: it stores sulphuric acid in its tissues.

That single fact has given it a reputation it half deserves. It's the seaweed foragers are told to avoid, and the one most often described as poisonous. It isn't.

What It Looks Like

Two species turn up regularly in British waters. Desmarestia aculeata, sometimes called witch's hair, is stiff and wiry with fine branching fronds, olive-brown, and often found in tide pools and shallow water. Desmarestia ligulata is broader and strap-like, flatter and softer, growing to a metre or more.

Both feel different from the kelps growing near them — less leathery, more brittle — and both discolour and go slack quickly once out of the water, which is a useful field clue.

The Acid

Desmarestia stores sulphuric acid in vacuoles inside its cells. While the seaweed is alive and intact in the water, this causes no problem. Once the tissue is damaged or the plant starts to break down out of the water, the acid is released — and it will digest the seaweed itself, turning it sour, slack and eventually to mush within hours.

The purpose appears to be defensive: grazing animals that take a bite get an unpleasant surprise, which is why Desmarestia often stands untouched in areas where sea urchins have cleared everything around it.

Is It Dangerous?

No. Desmarestia isn't poisonous and won't harm you if you eat it — it simply tastes acidic and unpleasant, which is deterrent enough. Handling it is fine, though it's worth washing your hands afterwards and keeping it away from your eyes.

The practical problem is what it does to a harvest. Put Desmarestia in a bag with good seaweed and the acid it releases will spoil the lot. That, rather than any toxicity, is why foragers learn to recognise it and leave it where it is. More on this in our guide to