What Is a Mitten Crab?
Mitten crabs (Eriocheir sinensis), so-called because their furry claws look like ‘mittens’, are native to Korea and parts of Southern China. Their natural habitat is in rivers, and they have an omnivorous diet which usually consists of worms, snails, muscles, small fish and some aquatic plant material.
Mitten crabs have spread to Europe and parts of North America, and are known to be a significant pest which can clog up sewage systems. Some were even allegedly sighted in the River Thames in London a few years ago!
The Mitten Crab Life cycle
Mitten crabs will instinctively migrate downstream towards the sea before reproduction. They will reach puberty and then begin to breed. Mitten crabs die shortly after reproduction, and consequently will often produce numbers of eggs in excess of one million.
The life stages of the mitten crab are as follows:
- Once the eggs are exposed to salt water, they mature
- Larvae will hatch from the eggs
- The larvae move into fresh water
- The larvae gradually develop into fully-fledged mitten crabs in fresh water.
How to Identify a Mitten Crab
Mitten crabs have a distinctive appearance from other types of crab. They have the ‘mitten’ appearance of their claws, which are usually furry and equal in size. They have a slight gap in between their eyes. Their body is roughly circular and smooth and have a brown coloured exterior.
The Impact of Mitten Crabs
The ecological impact: mitten crabs have an exceptional ability to burrow, and can accelerate coastal erosion. This could also break the food chain in local aquatic systems, as they will eat the food supply of multiple organisms through their omnivorous diet. They could also potentially out compete many local species for their food, potentially killing off local populations of entire species.
The economic impact: the key economic impact of mitten crabs is on local fishing communities. They can damage nets and equipment, which can be extremely costly for fisherman, especially if the fishing gear is more expensive. In parts of Asia, mitten crabs have been known to consume rice crops before they are allowed to grow, exhausting food supply and having a negative impact on the well being of smaller rice farmers. It remains to be seen whether this will have similar effects elsewhere.
The threat to the public: mitten crabs can host a parasite, which, if consumed, can be dangerous to humans or other mammalian animals. If humans eat poorly cooked mitten crabs then this can be of particular concern.
The impact on endangered fish: in the late 1990s, mitten crabs were a nuisance to endangered fish in the United States. They ended up suffocating fish because they blocked water tanks that were intended to house the endangered species of fish. This could be of concern to people in the United States and in parts of Europe if they are infested by mitten crabs.
Origin of Mitten Crabs
Mitten crabs are native to South Eastern Asia. They were introduced to Europe in the early 1900s and spread rapidly, proving to be very successful and well-suited to European conditions. Since the 1990s, they have been found in the West of the US, and have since spread further inland, as they did in Europe.