Wireweed is an olive-brown seaweed. It may sound unappetizing, but this is my favourite species of seaweed to forage! Far from wiry, it has a fresh, salty taste that is great in stir fries or salads. An added bonus is that it turns green when you cook it, especially if you use a little lemon. If you would like to try eating some wireweed, you can help to minimize its spread by taking care not to drop any when harvesting it. You should also practice responsible foraging - check for any creatures and wash your seaweed thoroughly before eating!

Where is the species found in the UK? How did it get to the UK?

After making its way across the channel from France in the1970’s, wireweed was first recorded on the Isle of Wight in 1973. If you snorkel or rockpool along the south coast of the UK today, you’re bound to see lots of this seaweed in the water. It is thought to be native to the North-west Pacific Ocean, so it is considered a non-native species here. This seaweed anchors itself to rocks in shallow waters and estuaries, growing over two meters long!

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Once a frond of wireweed has been established, it spreads in two main ways. For short distances they release tiny seeds/spores into the water, but to travel further whole fronds of the seaweed must break off to be carried by the currents. This seaweed does not need other individual wireweed growths nearby to reproduce – one piece of wireweed is enough to start an army!

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A third, very slow method of transport has been reported more recently: carrier limpets! Scientists discovered limpets can help the seaweed invade vulnerable seagrass beds in the UK, by carrying it in on their shells. This is not limited to limpets, as some people think wireweed could have been introduced on the shells of another invasive species, the Pacific oyster.

Why should we track the species? What is the impact on native species?

Now that we know how wireweed reproduces and travels, let’stalk about the concerns that scientists have with this species. Wireweed growsand spreads much more rapidly than a lot of native UK seaweeds and seagrasses.It could beat native species in the competition for light and space.

The branches of wireweed are covered in small “gas bladders”– berry like spheres filled with gas that make it float up towards the surface of the water. In a shallow area – like a rockpool – wireweed can cover the surface. Native seaweed species in this rockpool might not be able to photosynthesise as much when the tide is out, as the wireweed will block out the light. However, this cover could be very helpful for keeping the rockpool animals cool during our increasingly hot summers. If wireweed did take over and cause harm to the balance of our shallow water ecosystems, it would be considered a true invasive species in UK waters - rather than a non-native.

Whilst there is no conclusive evidence that wireweed is bad for our shallow water habitats, it may affect which animals and seaweeds are present in some areas. We need to keep an eye on how widespread wireweed is, and how it effects the ecosystem. This way we can manage populations if they start to negatively impact local species.

How to spot it? Any key features?

You can help to monitor wireweed by recording it on iNaturalist during one our bioblitzes, or whenever you see it! It is quite distinctive for a seaweed, growing in thin, tall fronds with branches covered in oval shaped leaves and air-filled spheres.

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Sources

1.     Bunker. F., Brodie. J., Maggs. C. & Bunker,A. (2010). Seasearch Guide to Seaweeds of Britain & Ireland.

2.    Hardy,F.G. & Guiry, M.D., (2003). A check-list and atlas of the seaweeds of Britain and Ireland. London: British Phycological Society

3.    Hayward,P., Nelson-Smith, T. & Shields, C. (1996). Collins pocket guide. Sea shore of Britain and northern Europe. London: HarperCollins.

4.    Howson,C.M. & Picton, B.E. (1997). The species directory of the marine fauna and flora of the British Isles and surrounding seas. Belfast: Ulster Museum. [Ulster Museum publication, no. 276.]

5.    JNCC(Joint Nature Conservation Committee). (1999). Marine Environment Resource Mapping And Information Database (MERMAID): Marine Nature Conservation Review Survey Database. [on-line] http://www.jncc.gov.uk/mermaid

6.    NBNAtlas (2025) Sargassum muticum. Available at: https://species.nbnatlas.org/species/NHMSYS0021058667(Accessed: 8 May 2025).

7.    NNSS(n.d.) Watersipora subatra. Non-native Species Secretariat. Available at: https://www.nonnativespecies.org/non-native-species/information-portal/view/3141 (Accessed: 14 May 2025)

8.    Norton, T.A. (ed.), (1985). Provisional Atlas of the Marine Algae of Britain and Ireland. Huntingdon: Biological Records Centre, Institute of Terrestrial Ecology., Pages 41-50, https://doi.org/10.1093/aob/mcad143

9.    Firth,L.B., Foggo, A., Watts, T., Knights, A.M., & DeAmicis, S. (2024). Invasive macroalgae in native seagrass beds: vectors of spread and impacts. Annals of Botany, 133(1), 41-50,  https://doi.org/10.1093/aob/mcad143.