Chinese mitten crabs-Long distance crawling

by Matthew Norton

Most living creatures have to be able to move at some point during their lives, to get food, to escape predators or to find new places to live. Some species take this one step further and live their lives while constantly on the move, such as the Chinese mitten crab. These little critters will travel over 1,000km upriver from their birthplace in the sea and then come all the way back down again to make their own baby crabs who will repeat this long voyage.

Chinese mitten crab image 1 — Chinese mitten crab is a fitting name for these little critters given that they are native to China and have very hairy claws.

As tiny floating larvae, these little crabs cannot swim very far, but can still make their way from the coastal seas where they hatched from and into the estuary (the space between the sea and the rivers) by using the tides. The movement of the tides is strongest at the water’s surface and then gets weaker as you go down and the crab larvae take advantage of this by swimming up when the tide is coming in and then swimming down when the tide goes out. The result is that they get themselves pushed more by the incoming tide, which takes them further inland.

Manipulating the tides to propel yourself up rivers only works when the incoming tides can push against the river flow. After this point, crawling on the bottom of the river becomes a much easier way for Chinese mitten crabs to travel. Even so, they may still have hundreds of kilometres to go, but it can take them up to five years to be ready to travel back down and breed. At least they have time on their side.

On their travels, these little crabs will also have to cope with the challenges of moving between different environments. The change in salinity (saltiness of the water) they experience as they move from the sea to the river is particularly tricky to get around. All animals need a certain amount of salt in their bodies dissolved in a certain amount of water, too much salt or too little water and their cells will shrivel up, too much water or too little salt and their cells will get bloated and burst, neither are good outcomes. Aquatic animals use various methods to control the levels of salt and water in their bodies, such as moving salts over their gills, drinking, or not drinking water and using dissolved, non-salt chemicals to make up for a low salt content.

Chinese mitten crab image 2 — These diagrams show how fish from freshwater (left) and seawater (right) regulate the levels of water and salt ions in their body. Both use very similar mechanisms, but in very different ways because for freshwater fish there is the risk of getting too much water and not enough salts whereas in seawater fish it is the other way round.

Regulating salt and water levels in your body is easy enough when the salinity of your environment doesn’t change much, but for Chinese mitten crabs a change in tactics is necessary when they move from sea to river and vice versa. As larvae, they are very skillful at cope with a wide range of salinities, but as they grow and move towards the river, where salinity is low, but stable, this becomes an unnecessary expense. This is probably why adult Chinese mitten crabs are not so flexible with salinity, at least until they return to the sea.

It does seem strange that these little make such long and difficult journeys, why not just stay in the sea close to where they were born. They may find more food, fewer predators or fewer competitors in the rivers, but their eggs and larvae may have a better chance in the sea, but this is just one of many possible reasons. Chinese mitten crabs are not the only species who seem to live a difficult life, but this can be necessary for them to live in a natural world where the ‘easy’ roles are already taken.

From a human perspective

Humanity has caused some significant damage to the natural world over the past few decades through global warming, pollution, invasive species and so on. That last one has certainly come back to haunt us in some parts of the world. In theory any species can become invasive if it makes it way to a new area by clinging to our boats, our floating rubbish, or if it is driven there by climate change. Once the species arrives and gets a foothold, it can play havoc with the local ecosystem, driving out species that actually belong there by hunting them, depriving their food or modifying the environment.

Chinese mitten crabs are one such invasive species who have spread through Europe and America since the start of the 20th century and they have made their presence known. There is evidence to suggest that they will forcibly evict shore crabs (Carcinus maenas) from their shelters on the shore, which is ironic given that they are an invasive species themselves in other parts of the world. They can also carry pathogens (disease causing microorganisms) which are deadly to native species, such as the fatal crayfish plague caused by the bacteria Aphanomyces astaci. Unfortunately, by the time we found out that Chinese mitten crabs were a carrier they had already occupied much of Europe.

Chinese mitten crab image 3 — For European crayfish, Chinese mitten crabs are a significant threat, not only through competition for food and other resources, but also because they carry deadly diseases which crayfish have no resistance to.

There are problems for us as well, Chinese mitten crabs have a tendency to burrow into river banks, which can damage crops and destabilise anything we build close to the river, and interfere with fishing activities. Their larvae can also clog up our water pipes. However, some entrepreneurs have suggested turning the presence of these destructive crabs to our advantage, by harvesting and eating them. This has already been done in their native China, where they are farmed and ultimately served in seafood restaurants, and has been proposed in America and around the river Thames.

Chinese mitten crab image 4 — Chinese mitten crabs are already farmed, harvested and cooked in China so it is possible to eat them in regions where they are an invasive species and get their populations under control at the same time.

Properly managed, this could be an ideal solution for keeping numbers of Chinese mitten crabs under control. However, there is a risk that profit may take priority over protecting native species and habitats and this could mean crabs being deliberately imported and farmed in areas where they don’t belong. Clearly, I am not the only one who is concerned because in most of the United States it is illegal to import and farm Chinese mitten crabs with the only exception being California, where some fishing is allowed.

There has been a lot of doom and gloom in this article, but at least we are coming up with solutions to the problems we are causing to the natural world. Some of these solutions, such as harvesting and eating Chinese mitten crabs, are particularly attractive because we can also make money from them. This can be a cause for concern, but I am still optimistic that we can still protect the underwater habitats we share with the natural world, so long as we don’t get carried away.