The Solent Forum

Working in partnership for the future

Solent Marine Invasive Species

Marine Invasives ID Guide
Marine Invasives ID Guide
Marine Invasives ID Guide

The Solent, much of which is designated as a Special Area of Conservation (SAC), Special Protection Area (SPA), and Marine Conservation Zone (MCZ), is a key entry point for marine invasive species into the UK due to its high volumes of international shipping and recreational boating, both major pathways of spread. These species pose a threat to the Solent’s native biodiversity and fishery industries, especially shellfisheries, due to their potential to threaten native species, habitats or whole ecosystems. The biofouling they can cause affects the performance of vessels and infrastructure creating additional cleaning and maintenance costs for marine businesses and their customers.

What is a Native vs Non Native vs Invasive Species?

A native species originated and developed in its current surrounding habitat, it “belongs” in the location and usually causes no harm but can be a nuisance under certain conditions. The oyster, Ostrea Edulis, is a native species to the Solent. Non-native species originated somewhere other than their current location and have been introduced to an area usually by human activity, however they have no noticeable negative ecological and/or socio economic effects. They do not “belong” here but do not cause a problem. Approximately ten to fifteen percent of non-natives are harmful and cause ecological and/or socio-economic harm. They do not “belong” and cause a problem, for example, outcompeting or predating native species, reducing biodiversity, fouling waterside equipment, infrastructure, shellfishery gear, and vessel hulls. These species are termed 'invasive'.

What Research has been done in the Solent?

Although Natural England has undertaken monitoring of marine invasives within harbours and marinas, the adjoining areas of intertidal habitat, have been largely ignored. However, there is a strong suspicion that such species are much more prevalent on natural shores than has been documented. An EMFF funded project aimed to respond to this identified data and information gap by developing a replicable survey methodology to map marine invasives within Marine Protected Areas in the Solent.

Rapid assessment surveys of 14 ‘Clusters’ of three sites, one marina/harbour site, one nearby shore and one more distant shore, were completed in the Solent, recording non-native species and native species from target lists. This new data has also been compared with previous surveys of marine invasives in Solent marinas to provide a timeframe for assessment of the risk of colonisation of natural shores by invasives from nearby marinas. See: Mapping Invasive Alien Species in intertidal habitats within Natura 2000 sites in the Solent (JP042).

View the Solent maps of the findings (zip file).

Are Marine Invasives Impacting on the Solent's Designated Sites?

Monitoring has shown that high numbers of marine invasive species have been recorded across all designated features of the Solent Maritime Special Area of Conservation (SAC) and they have the potential to outcompete native species for space and food resource. The 2016 SAC condition assessment failed the attribute non native species and pathogens because of the high number of non native species recorded, however, it indicated that further monitoring was required to determine impact. Surveys in both 2018/19 and 2022 demonstrated presence and onward spread of non native species across the Solent and found that in some areas non native species could now be shown to be colonising intertidal habitats and so contributing to the unfavourable status of the site.

Marine Invasives Identification Guide

The Marine Biological Association produced an Identification Guide for Selected Marine Non-Native Species. This shows 38 species of non-native seaweeds and marine animals that may be found in ports and marinas, on boat hulls, on fishing gear or on natural shores. If you would like a free hard copy of this booklet please contact the Solent Forum Office

Further down this page we give more information on the key species of concern in the Solent.

Social Media

If you would like to keep up to date with the latest development on this topic then you can follow The Marine Biological Association on Twitter or follow their Facebook page. During May each year the GB Non Native Secretariat organise an Invasive Species week.

An Introduction to Solent Marine Invasive Species

Recorded after the marine invasive species workshops in March 2023 by APEM Ltd.

Key Solent Invasive Species

There are a number of marine invasive species currently in the Solent that we need to watch out for and help prevent their spread, we have set out in more detail below some of the key ones, you can find others using the Identification Guide for Selected Marine Non-Native Species. If you do see an invasive species please report it through iRecord. There is lots of information on these species on the Non Native Species Secretariat website's information portal, we have provided a link to the relevant portal page for each of the species below, just click on the species name.

Solent Marine Invasive Species

Slipper limpet

The slipper limpet (Crepidula fornicata) has a domed shell, oval or kidney-shaped, up to 5 cm long, with an internal flat shelf. The outer surface is pale, with growth lines and brown patches. Sedentary as adult, it often aggregates into chains or leaning stacks of individuals, larger towards base. It attaches to solid surfaces, or small objects such as stones or shells on sediment, in shallow coastal waters or low intertidal. It is present year-round, and broods during a long breeding season. The GB INNS have produced a good management guide for this species.

Carpet sea squirt

The Carpet sea squirt (Didemnum vexillum) is pale orange, cream or off-white colonies, lacking brown or black pigment, and forms extensive, thin (2-5 mm) sheets; it can form long, pendulous outgrowths. It is firm, has a leathery texture and a veined or marbled appearance. Numerous small pores close when disturbed to produce tiny whitish spots; larger water exits occur at intervals. It lives in shallow water in harbours and marinas but also in natural habitats, and potentially in deeper water. It often overgrows other attached organisms. It dies back in harsh winters, and produces larvae in summer and autumn. Watch a video about this species. The GB INNS have produced a good management guide for this species.

Pacific oyster

The Pacific oyster (Magallana gigas) has thick, rough, hinged shells up to 18 cm long with lower half often cemented to a solid surface with strong raised ribs leading to a markedly wavy or saw-toothed shell margin, often with dark-purple brown markings. It may grow upright when crowded, presenting sharp edges of shells. It can be found on lower shores and coastal waters; on fixed artificial structures in harbours and marinas, and on natural shores. It is present year-round, spawning in the warmer months. The GB INNS have produced a good management guide for this species.


Wireweed (Sargassum muticum) is a large olive brown seaweed with fronds often over 1m long. A main axis or stipe bears alternating secondary branches, giving it a characteristic 'washing line' appearance out of the water. It has branches with small flattened leaflets and spherical gas bladders. It grows on hard surfaces in rock pools and in shallow water rarely deeper than 5m. It is perennial but the braches die in autumn and only the small basal holdfast remains over winter.

Trumpet tube worm

The Trumpet Tube-worm (Ficopomatus enigmaticus) are clumps or reefs of upright, white, intertwined chalky tubes, one to 3mm diameter with flared collars at intervals, attached at base to a solid surface. The collars are largely absent if growth is recumbent over substrate. Recently formed tubes are pale horn-brown. Each tube houses worm with crown of banded, feathery, feeding tentacles; the spiny plug closes tube when animal is withdrawn. Found in sheltered, shallow coastal sites with reduced or fluctuating salinity such as ports, harbours, marinas, channels, and lagoons. Tubes, at least, present year round reproducing in the warmest months.

Asian Date Mussel

The Asian Date Mussel (Arcuatula senhousia) is a small (10-30mm) plump, greenish mussel. It has radiating reddish lines and thicker purple-brown wavy lines. It prefers enclosed intertidal and shallow subtidal flats (to 20m) on soft or hard substrates and can attach to seaweeds or artifical structures.  In shallow coastal waters it can form dense, extensive mats. In GB this species is currently restricted to the central part of the south coast, having only been reported from Southampton Water, Chichester Harbour and the Isle of Wight.

Devil's Tongue Weed

Devil's Tongue Weed (Grateloupia turuturu) ia a red alga that has red blades up to 1m long, it has a short stem before the blade widens and is slippery to the touch. It is most frequently found on marina pontoons and navigation buoys and on pebbles in shallow subtial and lower intertidal pools at sheltered sites. It can be found throughout the year. It was first recorded in the UK at Southsea beach in the Solent, in 1969.

Information source: Identification Guide for Selected Marine Non-Native Species

Identifying Invasive Seaweeds

The MCS Big Seaweed Search first launched in 2009 and records data around the UK coasts. It comprises 14 seaweed species including seaweeds affected by sea temperature rise, non-native seaweeds and seaweeds affected by ocean acidification.

Back to home