The Green Crab and its Invasive Effects on the Ecosystem

My Invertebrate Zoology class was taking people down for trips to the Aqualab here at the University of Guelph.  I browsed some of the marine and freshwater animals, and held some others like the sea urchin and the starfish. Eventually, I got to hold the green crab, which struck my interest since I am particularly fond of crustaceans. I noticed that it had a very hard shell, or carapace, which was splotched with patterns of black and lime green colouring. I also had to hold it by the sides of the shell to prevent it from pinching me with its claws, as it most likely thought I was a predator. It used all of its legs to try to gain the advantage and wriggle out of my grasp; however I held on tight and avoided its two main claws. It wasn’t until after I had put the crab back into the tank that I was informed it was actually an invasive species in North America, coming from its native Europe and North African countries by attaching themselves to the bottom of boats. I decided to investigate and wonder what made this impressive specimen such a dangerous invader.

I found out that the green crab is considered one of the top 100 worst invasive species in the world, according to Fisheries and Oceans Canada. This is due to their hard carapace, which makes them very durable and almost indestructible. They can also rotate their claws over their back to protect from predators (something I found out when I held one), and can live out of water for over a week, with both of these increasing their survivability and endurance dramatically. This video explains more about the history of the green crab, and their morphology as well:

Their diet consists of clams, oysters, and other smaller crabs, and because they are so aggressive they have driven out almost all the other native crab species. Monaca Noble documents this in her blog, “Small Lagoon Fights Off Occupation”, in which a team of volunteers attempt to remove these crabs by setting up traps, and are then shipped to a farm for use as fertilizer. The link is right here:

It was also shown in an experiment by Haarr and Rochette that adult green crabs will out compete and prey upon juvenile American lobsters. Crabs from different regions of Canada were tested (the Northumberland Strait, the Bay of Fundy and the Scotian Shelf), with all of them having negative effects on the crabs. However, some crabs from certain regions had an even greater impact on the mortality of the lobsters than others. The link to that article is here:

After reading that article, I wondered what the correlation between the specific geographic origins and the aggressiveness of the crabs was. More broadly, I wonder what causes the aggressiveness of the crab in the first place. Is it due to the unlimited potential of free reign in their North American marine ecosystem, and they are simply capitalizing on it? Or are these crustaceans hard wired to be aggressive from the start? These questions still elude me, but it is these types of questions that make the topic of Zoology so appealing.

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