Rob Jordan / 2020VISION / View of the River Till, part of area where The Tweed Foundation are monitoring the population and spread of Signal Crayfish (Pacifastacus leniusculus), Northumberland, England, UK, October

Rob Jordan / 2020VISION / Signal crayfish (Pacifastacus leniusculus) in a defensive posture after being caught by The Tweed Foundation monitoring the species population and spread on the River Till, Northumberland, England, UK...

Rob Jordan / 2020VISION / Signal crayfish (Pacifastacus leniusculus) in a defensive posture after being caught by The Tweed Foundation monitoring the species population and spread on the River Till, Northumberland, England, UK...

Paul Hobson / Signal Crayfish {Pacifastacus leniusculus} Leicestershire, UK

Michel Roggo / Signal crayfish (Pacifastacus leniusculus) Europe, introduced from North America

Andy Sands / Signal Crayfish (Pacifastacus leniusculus) defence posture in chalk stream, Hertfordshire, UK

Michel Roggo / Signal crayfish (Pacifastacus leniusculus), fighting for burrow built in the soft limestone lake bed, Lake Neuchatel, Switzerland, October. Photographed for The Freshwater Project.

Michel Roggo / Signal crayfish (Pacifastacus leniusculus), Lake of Neuchatel, Switzerland, October. Photographed for The Freshwater Project.

Michel Roggo / Signal crayfish (Pacifastacus leniusculus), fighting over burrow dug in soft limestone ground. Lake Neuchatel, close to Boudry, Canton of Neuchatel, Switzerland. October. Photographed for The Freshwat...

Michel Roggo / Signal crayfish (Pacifastacus leniusculus), a North American species of crayfish, introduced to Europe in the 1960s. Lake Neuchatel, close to Boudry, Canton of Neuchatel, Switzerland. October. Photo...

Cyril Ruoso / Signal crayfish (Pacifastacus leniusculus), non-native invasive species. On riverbed in woodland. La Cure river, Morvan, Bourgogne-Franche-Comte, France. May.

Cyril Ruoso / Signal crayfish (Pacifastacus leniusculus), non-native invasive species, on riverbed. La Cure river, Morvan, Bourgogne-Franche-Comte, France. May.

Cyril Ruoso / Signal crayfish (Pacifastacus leniusculus), non-native invasive species, on riverbed in woodland. La Cure river, Morvan, Bourgogne-Franche-Comte, France. May.

Paul Hobson / Signal crayfish (Pacifastacus leniusculus) on river bed near bridge, split level image. River Rivelin, Sheffield, England, UK. September 2018.

Paul Hobson / Signal crayfish (Pacifastacus leniusculus) displaying aggression with claws up, split level. River Don, Sheffield, England, UK. September 2018.

Paul Hobson / Signal crayfish (Pacifastacus leniusculus) displaying aggression with claws up. River Don, Sheffield, England, UK. September 2018.

Nick Upton / White-clawed crayfish (Austropotamobius pallipes) collected from a well-stocked stream being released at an ARK site stream, safe from Signal crayfish (Pacifastacus leniusculus) and Crayfish plague, E...

Nick Upton / Alan Sumnall of GWT releasing White-clawed crayfish (Austropotamobius pallipes) collected from a well stocked location at at an ARK site stream, safe from Signal crayfish (Pacifastacus leniusculus) an...

Nick Upton / John Field placing White-clawed crayfish (Austropotamobius pallipes) in an inspection tray for sexing and health checks ahead of translocation to an ARK site, safe from Signal crayfish (Pacifastacus l...

Nick Upton / GWT team placing White-clawed crayfish (Austropotamobius pallipes) collected from a well-stocked stream in buckets for release at an ARK site, safe from Signal crayfish (Pacifastacus leniusculus) and...

Nick Upton / John Field and a GWT volunteer inspecting White-clawed crayfish (Austropotamobius pallipes) caught in a well-stocked stream to check their size, sex and health ahead of translocation to an ARK site, s...

Nick Upton / White-clawed crayfish (Austropotamobius pallipes) collected from a well-stocked stream in a bucket ready for release at an ARK site, safe from Signal crayfish (Pacifastacus leniusculus) and Crayfish p...

Nick Upton / Emma Settle of GWT disinfecting her boots before helping to release White-clawed crayfish (Austropotamobius pallipes) at an ARK site, safe from Signal crayfish (Pacifastacus leniusculus) and Crayfish...

Nick Upton / White-clawed crayfish (Austropotamobius pallipes) found in shallow stream by GWT team for translocation to an ARK site, safe from Signal crayfish (Pacifastacus leniusculus) and Crayfish plague, Englan...

Nick Upton / Large White-clawed crayfish (Austropotamobius pallipes) caught in a well stocked stream ahead of translocation to an ARK site, safe from Signal crayfish (Pacifastacus leniusculus) and Crayfish plague,...

Nick Upton / Very small White-clawed crayfish (Austropotamobius pallipes) beside a much larger one caught in a well stocked stream, with the smaller one not large enough for translocation to an ARK site, safe from...

Nick Upton / Large male White-clawed crayfish (Austropotamobius pallipes) with claspers visible under the tail, caught in a well stocked stream ahead of translocation to an ARK site, safe from Signal crayfish (Pac...

Nick Upton / GWT team processing White-clawed crayfish (Austropotamobius pallipes) in a well-stocked stream to check their size, sex and health ahead of translocation to an ARK site, safe from Signal crayfish (Pac...

Nick Upton / Richard Spyvee lifting a baited trap set for White-clawed crayfish (Austropotamobius pallipes) under license in a well stocked stream for translocation of healthy Crayfish to an ARK site, safe from Si...

Nick Upton / Richard Spyvee inspects a baited trap that has caught many White-clawed crayfish (Austropotamobius pallipes) under license in a well stocked stream for translocation of healthy Crayfish to an ARK site...

Will Watson / Scientist measuring a crayfish head and thorax of a non-native Signal Crayfish (Pacifastacus leniusculus), River Lugg SSSI, north Herefordshire, England, UK, August.

Will Watson / Scientist castrating Signal Crayfish (Pacifastacus leniusculus) during a control programme, River Lugg, SSSI, Herefordshire, England, UK, August 2017.

Remi Masson / Signal crayfish (Pacifastacus leniusculus) in a river. Haute-Savoie, Alps, France, October.

Remi Masson / Signal crayfish (Pacifastacus leniusculus) in a river. Alps, Haute-Savoie, France. October. Invasive species.

Stephen Dalton / Signal crayfish (Pacifastacus leniusculus) seeking new habitat walking across land. Claws raised in defense, Rookery Wood, Sussex, England, UK. August.

MYN / Dirk Funhoff / Signal crayfish (Pacifastacus leniusculus) waving claw, Idar-Oberstein, Hunsruck, Germany, January. Meetyourneighbours.net project

MYN / Dirk Funhoff / Signal crayfish (Pacifastacus leniusculus) Idar-Oberstein, Hunsruck, Germany, January. Meetyourneighbours.net project

MYN / Dirk Funhoff / Signal crayfish (Pacifastacus leniusculus) claw, Idar-Oberstein, Hunsruck, Germany, January. Meetyourneighbours.net project

MYN / Dirk Funhoff / Signal crayfish (Pacifastacus leniusculus) Idar-Oberstein, Hunsruck, Germany, January. Meetyourneighbours.net project

MYN / Dirk Funhoff / Signal crayfish (Pacifastacus leniusculus) Idar-Oberstein, Hunsruck, Germany, January. Meetyourneighbours.net project

Visuals Unlimited / Signal Crayfish (Pacifastacus leniusculus) a species that has been introduced from the USA into Europe and Japan, seen here feeding on a fish.

Visuals Unlimited / Signal Crayfish (Pacifastacus leniusculus) a species that has been introduced from the USA into Europe and Japan

Visuals Unlimited / Signal Crayfish (Pacifastacus leniusculus), a species that has been introduced from the USA into Europe and Japan, captive

Visuals Unlimited / Signal Crayfish (Pacifastacus leniusculus), a species that has been introduced from the USA into Europe and Japan

Alex Hyde / Signal Crayfish (Pacifastacus leniusculus), a species introduced to the UK from North America. They are far larger and more aggressive than the native species, and can easily out-compete them. Photog...

Linda Pitkin / 2020VISION / Signal Crayfish (Pacifastacus leniusculus) on lake bottom, Wraysbury Lake, Middlesex, England, May

Linda Pitkin / 2020VISION / Signal Crayfish (Pacifastacus leniusculus) on lake bottom, Wraysbury Lake, Middlesex, England, May

Rob Jordan / 2020VISION / Signal Crayfish (Pacifastacus leniusculus) placed on a population spread monitoring map, River Till, Northumberland, England, UK, October