Currently there are no known poisonous seaweeds but there are some that produce acid and cause digestive discomfort. 

Here are a few examples:

  1. Wireweed (Sargassum muticum): Wireweed, also known as Japanese wireweed or sargassum, is an invasive seaweed species that can be found along the UK coastline. While it is not typically lethal, it can cause digestive discomfort and is generally considered unpalatable. It's important to avoid consuming wireweed.

  2. Dead Man's Fingers (Codium spp.): Dead Man's Fingers is a dark green seaweed that resembles a hand emerging from the sea floor. While not generally considered toxic, it is not commonly consumed in the UK and is best left alone.

  3. Desmarestia spp.: Some species of Desmarestia, commonly known as acid kelp, can produce sulfuric acid when crushed. This can lead to skin irritation and should be handled with care. These seaweeds are typically not consumed.

  4. Fucus vesiculosus (Bladderwrack): While bladderwrack is not necessarily poisonous, it can sometimes accumulate heavy metals and other contaminants from the water. It's essential to source bladderwrack from reputable sources to ensure it is safe for consumption.

Foragers and individuals interested in harvesting seaweed for consumption should be well-informed about the seaweed species they are collecting and should follow safety guidelines to ensure that they are gathering edible and safe varieties. It's also a good practice to consult local authorities or experts in marine biology to learn more about the seaweeds in your specific area.