Little crabs with furry pincers that look like mittens might make the Chinese Mitten Crab sound cute and friendly but it is a real threat to our river systems and the organisms that live within them.
Chinese Mitten Crabs originated from Asia and came to Europe about a century ago in the ballast waters of ships. Despite entering Europe, they didn’t reach Britain for seventy years and only recently have they been officially confirmed in Southampton Waters. Their dark brown bodies are about 10cm with four pairs of legs also up to 10cm in length and a pair of fur-covered pincers at the front.
These creatures start life in river systems and migrate seawards, reaching sexual maturity in estuaries. The males enter the estuary first and mating occurs as soon as the females arrive. The adult crabs then migrate to the sea over the winter, where the females spawn because it is warmer and milder. By the summer the eggs have been transported back into the estuary by the tides and the adult crabs have followed to watch the eggs hatch. By the end of the egg hatching the adults have died out and the juveniles travel upstream to the rivers where they bury into the river cliffs – which is where the problems begin again.
Juveniles can also travel across land between rivers. The crabs remain classified as juveniles for two to three years but in some places they can remain juveniles until they are five, depending on the temperature and salinity of the water. Whilst they are juveniles, they bury into river cliffs and cause land to erode and collapse into the rivers. They also eat vegetation and small fish but have no natural predators in Southampton Waters. This is a major problem, because it means the crabs can reproduce to form large populations.
The Environment Agency are currently looking at ways to reduce numbers but initial methods have come under scrutiny, since the nets used are banded in the UK due to their threat to eels, which are protected. Although eels are not present in all the same locations, the Environment Agency are looking for a generic scheme that can be implemented nationally. Once caught, they are likely to be sold in Asia where their reproductive organs are a delicacy, whilst some may be kept in captivity or used for research.
So it looks like it’s not just humans whose juveniles cause problems! So do the Chinese Mitten Crabs – the only difference is that the Chinese Mitten Crabs aren’t doing anything about repairing their damage. Until we find a solution these furry invaders are at risk of taking over not just Southampton’s waterways, but soon enough all of Europe’s