The European shore crab, scientific name Carcinus maenus, is in the process of taking over the world. This little creature, also frequently referred to as the green crab, features all the characteristics of your average crab, but now also has a feature most animals do not: it occupies five of the world’s seven continents. Native to the Atlantic coasts of Europe and Northern Africa, individuals can now be found on both coasts of North America, the Atlantic coast of South America, Eastern Australia, both the East and West coasts of Africa, the Mediterranean, the Indian Ocean, and parts of the Pacific around Burma and Japan, according to the International Maritime Organization.
Looking at the green crab, nothing unusual is immediately apparent. Its mottled green and brown shell is wider than it is long, with ribbed edges and little stalked eyes at the front edge. As one would expect, its front two legs have pincers. The underbelly of the crab looks freakishly like a human six-pack, but still nothing gives off the impression of an evil mastermind making its way toward world domination.
The term for organisms that have such widespread impacts is “invasive species.” According to the National Invasive Species Information Center, part of the United States Department of Agriculture, an “invasive species” is non-native to the ecosystem it inhabits and causes economic, environmental or health-related harm. But how do invasive species get to so many places? The most frequent answer in the marine environment is shipping. Organisms can be transported in ballast water (water in ships used for stability), by attaching to the ship in what is referred to as hull fouling, or in contaminated packing material (like seaweed).
After disembarking, organisms that go on to become dominant in a wide variety of places are able to do so because their behaviors are flexible and they can adapt well to different conditions. For example, the green crab is euryhaline and eurythermal, meaning it can live in large ranges of salinity and temperature. Furthermore, these creatures excel at finding prey and avoiding predators in their new homes. Often, invasive species will have no natural predators when brought to a new ecosystem and will prey on smaller animals that have no defense against them since the two species didn’t evolve together. Thus the invasive species is able to pursue unlimited growth.
The pursuit of unlimited growth is not an immediately evident characteristic of the green crab. But the species is silently infiltrating seashores around the world and causing harm to native species. According to the Global Invasive Species Database, these crabs are competing with native organisms for food and resources, and “voraciously devouring” mussels and other crustaceans. This disrupts ecosystems as well as human economic systems because many of these organisms are commercially important. Essentially, the crabs are either eating or crowding out all of the organisms that already made their homes in these places.
Native species are totally unprepared for this invasion, but calling the invaders “evil masterminds,” as I did earlier, gives the wrong impression. The same goes for calling them “invaders.” Even the term “invasive species” is a little misleading. These phrases imply a sort of malintent, and, in doing so, transfer responsibility and blame away from humans and onto the organism. In reality, invasive species are just individuals who happen to have been transported to a new ecosystem and now are doing their best to survive, as all living things spend their lives trying to do.
In that case, where do we draw the line between invasive species and others like, for example, us? Homo sapiens evolved in Africa, making it non-native to all six other continents, yet here we are. Are we causing harm to our ecosystems? The answer is clearly yes if we look to pollution, climate change, resource use, and the many other anthropogenic sources of harm. In fact, “invasive species” exist largely because of human uses of the earth.
Moreover, there is an element of xenophobia tied into the idea of invasive species. Instead of taking this phenomenon as a side effect of global connectivity, it can turn into a fear and hatred of outsiders and the place from which the organism came. Shipping is a vital part of the global economy. Focus needs to shift away from finding a culprit to blame, whether it be the shipping industry or the organisms themselves.
The green crab is just one of many marine hitchhikers, but despite their and others’ quest for world domination, there is still hope for ecosystem survival. New technologies, such as stronger paint for ship hulls that prevents organisms from adhering to it, are being developed to prevent the transfer of non-native species.
Just as important as regulations and new technology is education about this phenomenon. Invasive species are not just a weed, or a pest, or an organism that we find aesthetically displeasing or difficult. The definition is more complex, but it is clear that the ecological impacts of its arrival defines an invasive species as such. Homo sapiens may seem to have already achieved world domination, but we will not last without understanding the interactions of all of our co-inhabitants.