Invasive Species Invertebrates

Signal crayfish (and other non-native invasive crayfish species) - Pasifasticus leniusculus

Description and Identification
Freshwater crayfish look like miniature lobsters and live in streams, rivers and still waters. There is one native species (White-clawed crayfish Austropotamobius pallipes) and several non-native species which have become established in the wild, of which Signal crayfish is the most widespread and troublesome. Signal crayfish may reach a length of 30cm from the tip of the rostrum (projection between the eyes) to the end of the tail, when outstretched. More typically they reach about 15 cm. (White-clawed crayfish reach
10-12 cm.) The body is bluish-brown to reddish-brown and smooth. In adult males the undersides of the claws are red and there is a white-tuquoise patch on top at the base of the claw hinge (hence the common name – signal).  (Photo; Signal crayfish. Image Environment Agency)

Key features:
Rostrum has smooth, more-or-less parallel sides
Rostrum apex is very pointed with prominent shoulders
Two pairs of post-orbital ridges
Adult males have claws red underneath and with white-turquoise  patch above
Body smooth and blue-brown to red-brown   (Photo; Signal crayfish. Image Environment Agency)

Other invasive species established in the wild are:
Narrow-clawed or Turkish – rough thorax, rostrum has toothed margin, narrow claws
Noble crayfish – claws rough above and red below, rostrum has row of spines on top of apex
Red swamp – body red to reddish-brown, claws covered in spines, rostrum has small triangular apex
Spiny-cheek or Striped crayfish – body brown with stripes across abdomen, spines on sides of rostrum

There is also a species - Marbled crayfish - currently being kept in captivity which could cause significant problems if it did escape because it has a voracious appetite and can reproduce asexually. This means one female can start a whole population.

Problems caused
Alien crayfish species are mainly a problem because of the threat they pose to our native species. This occurs through predation, competition and because of a fungal disease called crayfish plague carried by signals and other american species (Red swamp and Spiny-cheeked). The Amercian species carry the disease but are not affected by it whereas in many areas of the country populations of White-clawed crayfish have been wiped out by it. It can be spread either from infected individuals or by spores carried on equipment such as fishing equipment.

Signal crayfish are also known for their voracious appetite. They predate on White-clawed crayfish directly, especially the younger lifestages, and out-compete them for food and shelter. It is suspected that dense populations of signals can deplete plant and animal life in rivers, competing with fish in the process.

Signal crayfish originate from North America and were introduced to Britain and continental Europe for farming, from where they have escaped and established populations in the wild. They are established widely across southern England and in scattered localities elsewhere in Britain. In Essex, there are records from scattered localities throughout the county plus a known well-established population in the upper River Stour. Turkish crayfish, originally from western Asia and eastern Europe, are established in the lower River Stour with single records from the River Colne in Colchester and the lower River Blackwater. There is a single record of a Red swamp crayfish (species originates from southern USA) from Tilbury but no records of Noble or Spiny-cheeked crayfish.

There may be more widespread populations, but special surveys need to be carried out to confirm this. The Environment Agency licences such surveys and carried out some surveys itself, along with other organisations.

Control measures
Signal crayfish and other species are impossible to eradicate – they burrow deep into banks making massive excavation necessary, so destroying the habitat. Control measures concentrate on trying to stop further spread. Under the Wildlife and Countryside Act 1981, it is illegal to release or allow to escape into the wild any non-native crayfish species. Licences for crayfish farming in Essex can be granted (by Natural England), but keeping alien crayfish is not encouraged as escapes to the wild are hard to prevent. They are also not very profitable. It is illegal to keep any species of crayfish other than the tropicNative white-clawed crayfish. Image Environment Agencyal red-clawed crayfish for ornamental purposes.

The Wildlife and Countryside Act also makes it illegal to take native crayfish from the wild or offer them for sale without a licence from Natural England. Under bylaws, a licence must also be obtained from the Environment Agency for the specific means of capture. In Essex licences for trapping are only issued for surveys for conservation purposes.

The Environment Agency operates a water transfer scheme from the upper River Stour to the River Blackwater. In order to limit the number of signal crayfish dispersed by this route, trapping is carried out at the pumping station on the River Stour.

Actions that the public can take
Prevent spread of crayfish plague by treating all fishing nets, waders etc. used in waters where crayfish are known to be present before using in other waters. This involves thorough drying followed by treating with a proprietary disinfectant.
Inform Essex Biodiversity Project if any crayfish species is found.
If Marbled crayfish are known to be offered for sale, or kept, please contact the Fish Health Inspectorate at Cefas on 01305206673 or email

Sources of information and References
Crayfish Strategy Document 2015 - Download (PDF 220KB)
BUGLIFE Species Management Sheet; White Clawed Crayfish
National Biodiversity Network – records of locations of native and non-native species
Global Invasive Species Database – general information
E-fish business – guide to licensing of fish (including crayfish) movements and capture
CEFAS – Centre for Environment, Fisheries and Aquaculture Science – information on marbled crayfish
Defra – information pages
Freshwater Crayfish in Britain and Ireland. D Holdich and D Rogers. Environment Agency, 1999.
Impact of Signal Crayfish on the Aquatic Macroinvertebrate Fauna, Macrophytes and Fish of the UK and the Impact of Trapping on Crayfish Population Dynamics. David Rogers Associates, Environment Agency, 2005

Chinese Mitten Crab  Eriocheir sinensis

Description and Identification
The Chinese Mitten Crab is a large crab with a shell width of approximately 80 mm and legs that are one and a half times its width. It has "mittens" on its claws which are actually lots of soft bristles or setae.
The male and female of the species are easily distinguished, the female having a wide abdominal plate on its underside, that of the male being much narrower. (Photo: Environment Agency)
Problems caused
The mitten crab is a very successful "invader" and is known to burrow into river banks and represents a hazard to river and other freshwater engineering projects. It generally out-competes many native species.
Mitten crabs burrow into intertidal mud banks, which could pose a potential problem in the Thames tideway if crab numbers continue to increase. Areas of natural bank are generally of muddy clay, and these are rare because much of the Thames is lined with concrete flood defence walls. Such mud banks are being actively conserved by the Environment Agency, and much time and effort has been taken to protect and enhance them.
The crab is capable of emerging from water and crossing dry land to enter new river systems. This invasion could eventually threaten freshwater habitats currently occupied by populations of our native crayfish (Austropotamobius pallipes).
The mitten crab is the secondary host of a parasitic lung fluke that can infect mammals including humans, causing symptoms similar to those of tuberculosis, if they eat a raw or poorly cooked infected crab. The crab is a delicacy in Asia and is eaten raw. The Port Health Services are currently investigating the Thames population for parasites.
The Chinese mitten crab (Eriocheir sinensis) originates in south-east Asia and is thought to have been introduced to Britain in 1935 through ballast water on ships. The Chinese mitten crab is found in estuaries, lakes, riparian zones, water courses and wetlands. Its distribution is increasing.
Mitten crabs spend the first five years of their lives upstream in fresh water and then migrate seawards and gather in large numbers to breed in estuaries.

The largest populations in Britain are found in the Humber, Thames and Medway estuaries. The population in the Thames probably runs into millions and they occur throughout the estuary and lower freshwater reaches of the river.
Control measures
The spread of Chinese mitten crabs is difficult to control because they colonise new river systems via the sea, either naturally or through ship ballast or most likely both.
A Natural History Museum led project is looking at the possibilities of commercially exploiting the Chinese mitten crab as a way of providing an effective method of controlling population size, while providing financial benefits to local Thames fishermen. The project proposes to investigate the population structure and density of the species around the year and at various places along the River Thames, using commercial fishery techniques, to determine whether commercial exploitation of the mitten crab population is feasible. The project also proposes to determine whether the crabs from the Thames are fit for human consumption.
Mitten crabs are not fish and do not fall within the Environment Agency’s fisheries powers/duties (unlike crayfish, which do). Therefore they can not directly regulate fishing for Mitten crabs. Essex Biodiversity Partnership and the Environment Agency are promoting awareness of the problem and reducing the chances of humans transferring the crabs to other catchments.
Actions that the public can take
Report their findings to the Environment Agency, tel: 08708 506 506, email
Sources of information and References
Natural History Museum

Global invasive species database

Zebra mussel  Driessena polymorpha

Description and identification
Zebra mussels (Driessena polymorpha) are small black and white striped shellfish, which live in freshwater and feed by filtering the surrounding water (up to 260L/Hr/kg) and removing the plankton from it. They can grow up to 5cm long. (Photo: Environment Agency)
Problems caused
Zebra mussels congregate in very large numbers up to as many as 100,000 per square metre. They can clog intake pipes, drains and screens causing reduced water delivery to power plants, industries, fish hatcheries and waterworks. To combat this, expensive filtration or sterilisation systems may be installed and some companies have resorted to chemical control using chlorine. However zebra mussels can detect chlorine and close their shells as a response. This requires prolonged exposure before they are killed which has environmental drawbacks. The impact of zebra mussels on the ecosystem is not always predictable, and depends on the nutrient status of the invaded system and the community of organisms present.
Zebra mussels are filter feeders and in high densities could cause a reduction of phytoplankton denying food for other filter feeders e.g. native bivalves, zooplankton.
Zebra mussels can smother native freshwater bivalves leading to decreases in and possibly, removal of native bivalve populations.
They could have serious impacts on fisheries, fish spawning grounds and aquatic plant life. They may directly effect fish populations by decreasing the amount of food available for juvenile fish and cause increased predation due to greater water clarity.
At low densities, natural predators (e.g. fish, crayfish, coot and tufted ducks) can control zebra mussels. However, at high densities their dense byssus (bundle of fibrous, tough strands secreted by the mussel) makes them much harder to detach.
Although zebra mussels have been present in the UK for over 150 years a secondary spread has been noted since 2000, with an increase in both abundance and distribution (Aldridge et al., 2004).
Adult zebra mussels live in clusters on hard surfaces in rivers, canals and lakes, It persists in waters that are high in calcium and where the pH is greater than 7.5. It can tolerate low salinity and reproduces best in water temperatures between 17 – 25oC.
They were formerly found only in the freshwater areas of the Black and Caspian Seas but expanded their range to include much of Europe following the building of canals. The original introductions to Britain were through importation of Baltic timber. They can also be relocated by ballast water between freshwater ports (they cannot survive full seawater conditions). The zebra mussel was first reported in the British Isles in London’s Surrey docks in 1824. By 1850 they had spread by canals and rivers, over most of central England. It has caused serious problems to industry and the natural environment in the USA.
Control measures
Cambridge University have developed a ‘biobullet’ that kills the mussel with low doses of salt, without stimulating the mussel to close up. This is deadly to zebra mussels but doesn't affect most other organisms.
The Environment Agency is taking an active role in assessing the risk of zebra mussels to UK aquatic biodiversity. For example, protection of the depressed river mussel (UK BAP priority), which is at risk from zebra mussel smothering.
It is developing policies to prevent the spread of zebra mussels to both new and existing waterbodies and gives advice to anglers, boat users, and sampling staff. This is especially relevant where zebra mussels have been recorded at new sites that could result in a big spread in the next few years.

Action that the public can take
Report new findings to the Environment Agency, tel: 08708 506 506, email

Sources of information and references
The recent and rapid spread of the zebra mussel (Dreissena polymorpha) in Great Britain David C. Aldridge, Paul Elliott and Geoff D. Moggridge (2004)

Quagga Mussel, Dreissena bugensis rostriformis,

What is it?
A highly invasive non-native species, similar to Zebra Mussel. Significantly alters whole ecosystems by filtering out vast quantities of nutrients. Also a serious biofouling risk, blocking pipes, smothering boat hulls and other structures.
What is the urgency?
Quagga Mussels, like Zebra Mussels, are very small. Unlike Zebra Mussels this species has not yet been found in Great Britain, but based on its spread through Europe is expected to arrive soon. It is essential to identify any new invasion as early as possible.

How do you identify it?
➤ Similar to Zebra Mussel (small with a stripy shell)
➤ More rounded in cross section, unlike Zebra Mussel which is triangular
➤ Has an undulating seam between the valves (shells)
➤ Rolls to the side when placed on its front, Zebra Mussel tends to lie flat
➤ Has a small byssal groove, unlike Zebra Mussel which has a large
groove towards its middle (this may not always be obvious)

The Non-Native Species Secretariat has a page on the Quagga Mussel and we have two factsheets to download.

An identification guide (PDF 760KB)

A Biosecurity Guide (PDF 78KB)

If you suspect you have found this species it is essential to report it. Please send details, along with a photograph, to: