As the weather continues to warm, hopefully you will find yourself on our local beaches more and more often. This year, while you are beach combing, take some time to observe the crabs around you. The European Green Crab (Carcinus maenas) is one of the most invasive species in the world and it has, unfortunately, been detected in many parts of British Columbia. Successful populations have now been reported in the Gulf Islands near Salt Spring Island, and as concern continues to grow, learning how to recognize and report this invader could help protect the surrounding marine habitat.

How did European Green Crabs get here?

As is the case for many other aquatic invaders, this resilient species was spread primarily through human transport. It is very likely that European Green Crab larvae were accidentally transported in the ballast water of ships. This water was then discharged in foreign oceans bringing with it a very successful invasive species.

tanker ballast water
Photo taken by Darren Hillman

Although it is believed that the European Green Crab have been in Eastern Canada since the 1800s, they did not become a problem in BC until the end of the 1990s. Since then, there have been considerable populations located in inlets of western Vancouver Island and the central coast of BC. Luckily, cold water temperatures and the outward direction of flow from the Juan de Fuca Strait have created an unsuitable environment for larval growth. This has protected the inner Salish Sea (including the Gulf Islands) for the past few decades… until now.

Why are we worried?

European Green Crab have been found in 2020 near White Rock in Boundary Bay and near Salt Spring Island in the Southern Gulf Islands. In 2016, a European Green Crab was caught in Wescott Bay on San Juan Island. These discoveries have led to increased monitoring throughout American and Canadian waters, including efforts led by the greater public.

What are the ecological impacts of European Green Crabs?

As is the case with many resilient invasive species, European Green Crab cause havoc in intertidal ecosystems. They disrupt the balance of ecosystems by eating and outcompeting various intertidal species such as molluscs and native crabs. This leads to the decline of these species that have ecological roles to fulfil. They are also known to destroy incredibly valuable eelgrass beds, leaving mudflats in their place.

eelgrass beds
Photo taken by Rob Underhill

This transformation is detrimental as many invertebrate and juvenile fish species live in eelgrass meadows. The loss of these species will have impacts all the way up the food web. Our oceans are already dealing with many challenges and many populations are suffering. The invasion of the European Green Crab would create more damage in an already fragile ecosystem.

How can we identify the European Green Crab?

The best way to identify this species is by counting its marginal teeth (the spines on either side of the eyes). The European Green Crab is unique with the five marginal teeth present on either side of each eye. Check out the photo below! Can you locate the ten marginal teeth?

anatomy European green crab

The size and shape of the shell can also help with the identification process. The shell of this species is no larger than 10 centimetres and the front of the shell is wider than the back of the shell. You may be wondering if colour can also help. Unfortunately, it cannot. The name is quite misleading and, although this species can be green it can also be other colors such as red or yellow.

Keep your eyes open this summer and, while you’re at it, challenge yourself to learn about other crab species that you come across. If ever you do spot a European Green Crab, here is what you can do: take a photo, take note of the date, specific features, and location (as exact as possible) and report it to the Department of Fisheries and Oceans.

References

1.Yamada, S., Thomson, R., Gillespie, G., & Norgard, T. (2017). Lifting barriers to range expansion: The European Green Crab Carcinus Maenas (Linnaeus, 1758) enters the Salish Sea. Journal of Shellfish Research, 36(1), 201-208.

2. Department of Fisheries and Oceans. (2021). European Green Crab. Retrieved from: https://www.dfo-mpo.gc.ca/species-especes/profiles-profils/europeangreencrab-crabevert-eng.html

3. Grasen, E & Martin, K. (2018). Building Capacity to Protect the Salish Sea FromEuropean Green Crab. Retrieved from: https://wsg.washington.edu/egc-building-capacity/

4. Duncombe, L & Therriault, T. (2017). Evaluating trapping as a method to control the European green crab, Carcinus maenas, population at Pipestem Inlet, British Columbia. Management of Biological Invasions, 8 (2), 235-246.

5. University of Washington: College of the Environment. (2020). Identifying European Green Crab. Retrieved from: https://wsg.washington.edu/crabteam/greencrab/id/


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