NON-NATIVE INVASIVE SPECIES – THE CHINESE MITTEN CRAB

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The United Kingdom - as well as almost every other country on the planet - has often suffered from the effects of environmental damage through the proliferation of non-native species. For millennia, mankind has travelled the world, followed quickly by the establishment of trade routes and the movement of valuable animal and plant commodities. Unfortunately these routes have also brought their fair share of problems such as the globalisation of small pox, influenza and the infamous ‘black death’.

Today similar problems exist and while modern medicine had made great strides in the prevention of such epidemics there is still an on-going problem with the deliberate and accidental introduction of non-native plant and animal species into sensitive environments. Recent history has already shown us the terrible destruction that can be reaped through the experiences of Hawaii, Australia and New Zealand.

Invasive non-native plant and animal species are now the second greatest threat to biodiversity worldwide after habitat destruction. This is because they can have a negative impact on native species, as well as for the damage caused to the environment, and as a secondary issue - local economies.

The Chinese mitten crab is just the latest in a long line of alien introductions that have caused havoc in our country’s waterways. First recorded in the River Thames in 1935, populations of the Chinese mitten crab remained localised for decades, but since the 1990’s their populations have seen a dramatic increase in both size and range of habitat. It is now well-established in the Rivers Thames, Humber, Medway, Tyne, Wharfe and Ouse.

They are aggressive, have few predators and will eat almost anything that they can get their claws into – aquatic plants, native fish eggs, and molluscs. Their opportunistic lifestyle has made them a hugely successful introduction, but this success has been at a high cost to the local environment. To make things worse, they can burrow a metre or more into river banks, affecting their integrity. At a time of increased flooding risk, areas where there are significant numbers of Chinese Mitten crabs have seen their once stable river banks collapse, causing considerable damage.

Chinese mitten crabs will happily survive in both fresh and saltwater. In fact, their young are born in coastal regions or estuaries, and then they migrate up the river to spend their adult lives in freshwater. Years later - when they reach sexual maturity - they will return back to the saltwater to breed. However, once they are in freshwater, they can move huge distances.

Dr. Paul Clark from the Natural History Museum had this to say on the matter:

‘...studies have shown they can migrate up to 1,500km. The crab can even leave the water, cross dry land and enter a new river system. Its phenomenal ability to disperse is of concern to scientists in the UK because the crab could infiltrate many of the country's rivers...’

For these reasons it has now been placed on the IUCN (the International Union for Conservation of Nature) 100 of the world’s worst alien species list.

The Chinese mitten crab was given its intriguing name because its large claws - which are covered by soft bristles - resemble mittens. Its scientific name ‘Eriocheir sinensis’ derived from the Greek, means wool hand.

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