(Main image photo credit: Kathryn Birch)

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Asparagopsis armata, more commonly known as Harpoon Weed, is an invasive non-native species of red algae, or Rhodophyta. It gets its common name due to the harpoon-like barbs on the thallus, or ‘branch’ of the algae, that it uses to attach itself to other seaweed or the substrate like rocks and sand. It has two distinct life stages: the sexual form (gametophyte) and the asexual form (tetrasporophyte), known as Falkenbergia, which were once thought to be two different species.

It is found growing in the intertidal zones and subtidal areas, in deep rock pools and sandy pools. It can also be epiphytic (growing on other seaweeds), or free floating, unattached.

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

It is native to Australia and New Zealand but has since spread far beyond its original habitat, along the Atlantic coasts of France, Spain, and Portugal. First recorded in Europe in 1925, it made its way to the UK being recorded at Lundy in the Bristol Channel in 1949. Today, the asexual form is commonly found along the western coast of the UK and Ireland as far upas Shetland but isn't reported as much on the east coast. Currently, the sexual form is, slightly more restricted to southwest England and western Ireland.

It is thought that it originally arrived from oyster imports or by attaching to the hulls of ships; however, harpoon weed can also spread by drifting or rafting up. The sexual stage uses its hooks to attach to other materials, including seaweed fragments, and is carried by surface currents. The asexual phase float as balls of seaweed. Both stages are thought to be able to reproduce vegetatively (cloning itself), meaning it can spread quickly and far.

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

This seaweed is regarded as an invasive species as it negatively impacts native marine flora by outcompeting them and forming dense growth belts. It has also been shown to reduce the biodiversity of seaweeds in infralittoral rock pools (deep rock pools which stay submerged even at low tide).

Harpoon Weed has chemical defences that deter most predators with the exception of the Sea Hare (Aplysia parvula), but these toxins can alter the water's chemical composition, affecting other marine organisms' growth and survival.

Seagrass beds are also threated by increased growth of Harpoon Weed because it can form dense mats or grow directly on the plant, smothering the seagrass and limiting access to light and nutrients. Although it doesn't have an immediate impact on humans, it can clog fishing nets and attach to swimwear and clothing.

Harpoon Weed is used as a component in skincare products and cosmetics, and recent evidence suggests that adding it to cattle feed could help reduce methane production by cows by up to 67% - opening the potential to reduce emissions for this potent greenhouse gas.

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How to spot it? Any key features?

Harpoon Weed’s two life cycle phases have different but distinct appearances. The asexual, phase is rose-pink to brownish-red in colour, highly branched creating dense cottonwool-like balls 1-3cm in size which, although present all year round, are most noticeable from October to March. These are often found with Ulva species. and Coral Weed (Corallina officinalis).

The sexual, gametophyte stage is a delicate rosy-pink to yellowish-pink, appearing bushy or tufted towards the top with the lower parts appearing barer. It is found between June and September reaching up to 30cm with the ‘harpoon’ barbs on the lower branches about 10mm long.

It can be confused with Hook Weed (Bonnemaisona hamifera) which has recurved barbs rather than straight ones.

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Sources

1.    M.D. Guiry in Guiry, M.D. & Guiry, G.M. (2025). Algae Base. World-wide electronic publication, National University of Ireland, Galway. https://www.algaebase.org/search/species/detail/?species_id=6

2.   Non-native species secretariat. (2011). Available at: https://www.nonnativespecies.org/non-native-species/information-portal/view/373 (Accessed: 13 May 2025).

3.   Roque, B. M., Salwen, J. K., Kinley, R. & Kebreab, E. (2019). Inclusion of Asparagopsis armata in lactating dairy cows’ diet reduces enteric methane emission by over 50 percent. Journal of Cleaner Production, Vol. 234, P132-138, https://doi.org/10.1016/j.jclepro.2019.06.193

4.   Silva, Carla O., Lemos, Marco F.L., Gaspar., Rui., Gonçalves., Carlos. & Neto, J.M. (2021). The effects of the invasive seaweed Asparagopsis armata on native rock pool communities: Evidences from experimental exclusion., Ecological Indicators Vol 125 https://doi.org/10.1016/j.ecolind.2021.107463

5.   Skewes, M. 2003. Asparagopsis armata Harpoon weed. In Tyler-Walters H. and Hiscock K. Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [cited 13-05-2025]. Available from: https://www.marlin.ac.uk/species/detail/1619

6.   Soler-Vila, A., Edwards, M., Whelan, S., Hanniffy, D., Heesch, S., Hernández-Kantún, J., Moniz,M., Quéguineur, B., Ratcliff, J., Wan, A.H.L., and M.D. Guiry. (2022). Macroalgae Fact sheets (2nd Edition), Galway, Ireland. 66 pp.