Desmarestia, also known as acid weed, acid kelp, acidweed, sea sorrel, mermaid's hair, and landlady's wig is a genus of brown algae found worldwide. However, 'sea sorrel' can also specifically refer to Desmarestia viridis. Members of this genus can be either annual or perennial. Annual members of this genus store sulfuric acid in intracellular vacuoles. When exposed to air they release the acid, thereby destroying themselves and nearby seaweeds in the process. They are found in shallow intertidal zones.

Ingesting sulfuric acid can cause severe digestive disturbances, but since sulfuric acid tastes extremely sour, members of the genus are unlikely to be eaten in harmful quantities.

The genus was named in honor of Anselme Gaëtan Desmarest.

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