Most Unwanted: Chinese mitten crab
Eriocheir sinensis
Habitat: Freshwater
Threat: Threatens ecosystem functioning and protected species
Status: Established
What is it
The Chinese Mitten Crab is very distinctive crab, light brown in colour, with hair growing on equal sized claws that are often referred to as ‘mittens’. It is currently the only crab to live in freshwater in Ireland and Britain and therefore may have significant impacts on the functioning of this environment. This species is also able to migrate across land unlike all other crab species in Ireland. Mitten Crab are an intermediate host for the mammalian lung fluke Paragonimus ringer, known to infect humans
Habitat
It spends most of its life in rivers, but must migrate tothe sea to breed. Once the crabs have mated the males are thought to die, leaving the females to brood the eggs. In the spring the eggs hatch into larvae and after about six to seven weeks these metamorphose into juvenile crabs, which then migrate back up the river into freshwater to complete the life cycle (NHM).
Impact
When population densities are high, E. sinensis causes considerable damage to soft sediment banks through burrowing which increases erosion and might affect flood defences. This has been documented in Europe and North America.
In Ireland there is concern that the species may impact on the native and endangered White Clawed Crayfish as well as fish species such as the protected Twaite Shad fish. Additional negative impacts, such as loss of biodiversity and recruitment of commercial species are expected.
There are economic impacts associated with introduction of this species. To date, in Germany, the species is known to have cost at least 80 million Euro in monetary value.
The Mitten Crab is also an intermediate host for the mammalian lung fluke Paragonimus ringer, known to infect humans.
Where is it from
The species is originally from Asia where it is considered a delicacy in local cuisine. Now found in Europe, where it invaded nearly a centaury ago, Mitten crabs have now spread to North America. For reasons that are not understood, the populations of mitten crabs in England and North America did not at first expand like those in mainland Europe. But numbers are currently rising, prompting concern.
Where is it in Ireland
Chinese Mitten Crab was first found in Waterford Estuary by fishermen catching them in their net.
How did it get here
It is unclear at present how exactly the Chinese Mitten Crab arrived in Ireland. Likely vectors for introduction include ship ballast water and hull fouling. Other human mediated vectors such as the live food trade and smuggling may be possible as the mitten crab is a delicacy. Historic evidence elsewhere has shown that crabs are imported live illegally to markets
How does it spread
Once the species is present it can easily colonise new river systems by walking over land. The species also has a planktonic stage in its life cycle which can facilitate further distribution.
What you can do
Report all sightings.
Additional reading
Habitas Invasive Alien Species in Northern Ireland
