Invasive species alert! Wire Weed and Watersipora

25 May 2023  /  By Sarah Pearson

Whilst we work to conserve and restore our seagrass beds, we need to watch out for invasive species that threaten the seagrass ecosystem.
From May-July you might spot the up to two metres-long, olive brown fronds of wire weed (Sargassum muticum). Wire weed interferes with seagrass bed regeneration. It spreads through the dispersal of spores, or through “stone-walking”. This is when the buoyancy of the plant exceeds the weight of the stone it is anchored to, allowing it to disperse across the seabed.

Credit: Keith Hiscock

Wire weed is super-fast-growing, self-fertilizing and has a long-life span. The invasion began when plants hitchhiked on oysters being imported to Britain from the Pacific. Wire weed was initially found in Bembridge lagoons, Hampshire, with just thirty plants spotted in 1973. Over the past fifty years, wire weed has successfully invaded our coastal waters and estuaries.
Wire weed expels spores in synchronised pulses around spring tides, coinciding with new and full moons. It thrives in shallow waters. 

Credit: Keith Hiscock

Herbivores such as sea hares and gastropods don’t appear to control adult wire weed plants. However, these natural grazers may thin down populations by consuming newly settled wireweed sporelings. The grazers can also help later on in the summer, when the plant is in a period of slow growth.

Other invasive species

There are other non-native species that occur on seagrass and may be a threat, including Antithamnionella ternifolia and the very aggressive Watersipora subatra, which are spread attached to the hulls of boats and other surfaces such as intertidal bedrock and marina pontoons. They also attach themselves to seagrass leaves. The image below was taken last summer (August) in Falmouth Harbour, where a significant number of seagrass fronds had Watersipora growth like this. Additionally, ephemeral filamentous algae blooms that respond to sunshine and warming waters in the spring are often linked to eutrophication and that may threaten seagrass.

Watersipora on seagrass, credit Matt Slater, Cornwall Wildlife Trust


How can you help?

Boat users can check their boats, ropes, bilges, and trailers. Drain the water, as some species can survive in damp conditions. Dispose of material in landfill – do not return to the sea. Hose and brush or water-blast the remaining matter off the hull. Allow to dry and repeat annually.

Credit: Keith Hiscock