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    These water-dwelling invasive species are experts at hitching a ride from lands a far and pose a serious threat to the health of our rivers, canals and streams, and all that live within them. Help us stop their spread – Check, Clean and Dry!

    1. Zebra Mussel

    GB Non-native Species Secretariat / Via

    Zebra mussels are found in rivers, lakes and canals and can breed very quickly forming large colonies which block pipe-work and affect lock gates. They can smother native species and quickly take nutrients from the water changing what can there.

    Zebra mussels are very small - usually about 30mm in length but can grow up to 50mm. They have zebra-like light and dark bands of colour, usually blue or brown and yellow-white, and are a distinctive 'D' shape. They attach, usually in groups, to anything solid underwater, like masonry, stones or tree roots.

    2. Quagga Mussel

    David Aldridge, Cambridge University

    These watery hitch-hikers have recently found their way to UK waters and have the potential to cause problems wherever they go. They are cousins of the zebra mussel and look very similar, but are possibly even more invasive! They can survive in some places that the zebra mussel can’t and can even beat them at their own game.

    They breed exceptionally fast and block pipes causing a headache for water companies as well as disrupting the life in our waterways due to their incredible filtering capacity. They are a similar size to the zebra mussel (about the size of a thumbnail), but with duller stripes and a more rounded shape.

    3. Killer Shrimp

    GB Non-native Species Secretariat / Via

    This little shrimp which grows to just 30mm long is one of the most invasive species in Europe. It will kill a range of British species, including young fish and our native shrimp, and can spread very quickly. It lives in freshwater habitats, such as rivers and streams, but can survive in some estuaries. You can tell a killer shrimp because it has distinctive cone shaped bumps on its tail, usually has a striped back, and is often larger than British shrimps. Watch out for these on your water sport clothing or equipment and make sure you don’t take them with you!

    4. Signal Crayfish

    GB Non-native Species Secretariat / Via

    The signal crayfish comes from the US and is having a drastic impact on our native white-clawed crayfish, driving it towards extinction. They can strip rivers and streams of all other life until they have to resort to cannibalism to survive! Burrowing by crayfish can cause riverbanks to wear away and structures built at the edges of rivers to become unstable. Fishing companies have also been affected because they eat fish eggs and compete with native fish for places offering protection. This crayfish looks similar to a mini-lobster and is about 6-9cm long.

    5. Slipper Limpet

    GB Non-native Species Secretariat / Via

    This peculiar looking limpet, which looks a bit like a gnarly old toenail, has an opening in its shell which has a shelf extending half its length. It can smother creatures that live on the seabed and dramatically change the environment down there. It competes for food and space with other species that feed by filtering water, including native mussels and oysters, and preys on shellfish. It often attaches to the shells of dead and living hard-shelled creatures, such as scallops, crabs, whelks and mussels, and has a habit of clinging on to important species that are farmed and man-made structures and equipment.

    6. Carpet Sea Squirt

    GB Non-native Species Secretariat / Via

    With a name like sea squirt, it’s hard to see why these seemingly inoffensive little water dwellers from the Pacific Ocean are on the list. But these little 1mm long creatures have a habit of clubbing together in huge groups up to several square kilometres big which overgrow native species and don’t leave much real estate left for anything else! They can also clog up fishing equipment, boats and smother reefs.

    7. Australian Swamp Stonecrop

    GB Non-native Species Secretariat / Via

    Australian swamp stonecrop was originally introduced to Britain as a pond plant because of its ability to oxygenate water, but has since spread rapidly in the wild. It grows in dense mats that out-compete most native freshwater plants and blocks light which affects fish and other creatures. A tiny fragment of this or the following plants accidentally carried on damp clothing or equipment can be all it takes to start a new outbreak!

    8. Floating Pennywort

    GB Non-native Species Secretariat / Via

    Originally from North America, Floating Pennywort is a freshwater plant with distinctive kidney shaped leaves that form huge floating mats across the water surface, suffocating the whole area and increasing the risk of flooding. At the peak of its growth Floating Pennywort can grow at up to 20cm a day! It was introduced to Britain in the 1980s and within ten years had spread into the wild. It is now known to be present in at least 150 different places in England and Wales.

    9. Water Primrose

    Trevor Renals

    A highly invasive freshwater weed from South America, introduced to Europe for its attractive yellow flowers. While it may look pretty, in France Water Primrose blocks waterways and overgrows ponds and lakes forcing out native wildlife and causing flooding. If it were to establish widely in Britain, Water Primrose could cost as much as £240 million to manage!

    10. Water Fern

    GB Non-native Species Secretariat

    This tiny floating fern from North America may look harmless but can form a thick layer up to 30cm deep on standing or slow-flowing water! This blocks out light, affecting the quality of the water below. The dense carpets can sometimes look like solid ground, making them potentially dangerous. Water Fern is usually green, but look out for reddish mats in the autumn.

    11. Parrot Feather

    GB Non-native Species Secretariat

    Parrot’s feather is native to Central and South America and has been grown in water gardens in the UK since 1878. In 1960 it was found growing in the wild where it can rapidly take over whole areas of water and cause flooding. Parrot’s feather can live through the winter and when ponds dry out it can even be found growing on land!

    How Can You Help?

    GB Non-native Species Secretariat

    Check, Clean, Dry! ( If you’re a water user, you may unknowingly be helping to spread invasive species from one water body to another in equipment, shoes and clothing. These three really simple steps will help prevent you spreading these invasive species and help protect our wildlife and natural environment – Check, Clean, Dry.

    Be Plant Wise! ( If you have any of these plants in your garden pond make sure not to let them escape. Compost all unwanted plant material carefully in your garden; never exchange invasive species with friends and never dump them in the wild!

    If you spot any of these species, please report it:


    •or go to: Non-native species alert

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