Invasive Species: Chinese Mitten Crab

A Chinese mitten crab (Eriocheir sinensis), which goes by numerous other common names.

As the name implies, this species is a native of Asia: mainly China, extending into Taiwan and parts of Russia. Since it was first sighted in Germany in 1912, the Chinese mitten crab has become invasive in most of Europe. Since the species prefers a warmer climate, they are rarely seen, but still occasionally pop up in the colder waters off the coats of Norway. They are also found in the United States on the West coast, in the Great Lakes, and the Mississippi delta. Sightings have also occurred in parts of Canada and Japan.

The pale claw tips are their "mittens"

Methods of introduction have included trade for both food and aquarium purposes. The species is considered a delicacy in some areas, so they have even been spread through smuggling. These crabs have also been spread through ballast water and in the communities of fouling organisms that will coat the bottom of boats. Larvae will also disperse naturally to nearby environments.

The damaging affects that have lead to the species' designation as an invasive vary widely. Their migration to and from freshwater systems strips biomass from freshwater communities, which will result in serious changes to the food web. Mitten crabs also burrow into sediment, which can damage levees, dykes, and other man-made structures. They also eat just about everything, which results in a drop in the native biodiversity of a habitat. Direct competition with species who have similar diets to the crab is also of concern. They are known to impact native species of salmon-type fish in California. These crabs are also known to damage fishing gear, steal bait, and block pipes due to their sheer numbers. They also take over tanks meant to protect fish from turbines that move water, causing the fish the tanks are meant for to die. As for humans coming into direct contact with these crabs, they are a known intermediate host for a species of lung fluke. They are also known for bioaccumulation, a process by which toxic chemicals collect in food collects in the animal's tissue. If the crab is then eaten, all of those toxins are then passed on to whatever ate it. Considered the species' classification as a delicacy, this could be of direct concern to humans.

This species is currently on the list of 100 World's Worst Invasives at #34. 

Images are from Wikimedia Commons under Creative Commons licenses: one, two.