Carcinus maenas, the Latin name for the European green crab, translates to “raving mad crab.” This may as well be their nickname as these crabs are ecologically destructive and are considered one of the world’s 100 worst invaders by the International Union for the Conservation of Nature. European green crabs are native to Europe and North Africa, but they made their way to the United States in the mid-1800s, likely through ballast water. In 1989, they were first discovered on the West Coast in San Francisco Bay, likely hitching a ride in a live bait shipment. Ten years later they were discovered on Washington’s outer coast and more recently have moved into Puget Sound. These crabs can quickly reproduce beyond what can be controlled by state-wide trappers, especially as they reach sexual maturity within one year. In other locations, European green crab invaders claim habitat that disrupts other native species that are culturally, ecologically, and economically vital. Examples of destruction on the East Coast include the depletion of a soft shell clam fishery in New England, behavioral changes among mussels in Massachusetts, and loss of eelgrass beds in Maine. For these reasons, this issue has caught the attention of many citizens, shellfish growers, tribal communities, researchers, state agencies, and the Washington state Governor’s office. Unfortunately, European green crabs in Puget Sound are continuing to invade habitats. Current efforts to keep the population under control in Washington state include monitoring, early detection, removal trapping, and research focusing on dispersal. 

Washington Sea Grant’s Crab Team is an early detection and monitoring program that brings citizen scientists together to collect data to better understand salt marsh and pocket estuary organisms, including European green crab. I joined their team this past year and have been learning a lot about crabs found in Washington. Through this article, I hope to share how you can identify European green crabs while conveying the importance of this issue. With more eyes on shorelines and in the water, we can help those working on the issue by expanding the available data on habitats that European green crabs occupy. Read on to learn about European green crab characteristics, so you can accurately spot and report these feisty invaders.

European Green Crab Identification

European green crabs have a few identifying features, so take note of the following. These crabs have a pentagon-shaped shell that can be up to four inches wide, making it wider than it is long. Another key characteristic is their five marginal teeth which look like spines on the outer edge of each eye. They also have three rostral bumps between their eyes. If you have a good grasp, you can also look at their claws and walking legs. European green crab claws are not rounded and their fifth leg is slightly flattened. While this flattened leg is not the best for swimming, it aids in quick movement. While European green crabs are typically green in color, they cannot be confidently identified by their color as it can vary from mottled green, to brown, to reddish orange. 

Native Crab Identification

European green crabs are commonly misidentified and can be mistaken for the following native crabs, likely due their similar coloration, so here are some distinctions to consider. The hairy shore crab, Hemigrapsus oregonensis, only has three marginal teeth which is less than the five marginal teeth European green crabs have. Additionally, the hairy shore crab has two rostral bumps between their eyes compared to the three rostral bumps European green crabs have. Hairy helmet crabs, Telmessus cheiragonus, are covered with bristles and bumps throughout their diamond-shaped carapace. While European green crabs have hairs on their walking legs, their pentagon-shaped carapace is rather smooth. Northern kelp crabs, Pugettia producta, have a pronounced pointy rostrum that extends past its eyes with a shell that is longer than it is wide. This differs from the small rostral bumps and wider shell that European green crabs have. Look for these distinctions in the photos below.

Impacts of Misidentification

Accurate identification and reporting are essential to effective monitoring and estimation of population size. Additionally, misidentification has the potential to lead to mortality among native crab species if observers kill suspected invaders. This is why the Washington Department of Fish and Wildlife is currently not asking the public to kill any European green crabs. Instead, they only ask for photos. This way if a crab was misidentified, the reporter gets information about the look-a-like native species and tips to differentiate the two species without any harm to native crabs.

Join the Effort and Get Involved 

To report European green crabs, take photos and if possible, include objects of a standardized size next to the European green crab for comparison. Take multiple pictures of the suspected European green crab(s) from various angles. Submit sightings directly to Washington Department of Fish and Wildlife through their website or by calling 1-888-WDFW-AIS (1-888-933-9247). You can also submit sightings through the Washington Invasive Species Council app. This council is made up of biologists, entomologists, epidemiologists, and invasive species managers who review the report and connect the reporter to the appropriate Washington Department of Fish and Wildlife staff. You can also submit sightings to Washington Sea Grant Crab Team staff by emailing crabteam@uw.edu.

When fishing, remember to keep an eye out on boats and in bait buckets, fishing gear, and other equipment — adult European green crabs can survive up to five days out of water. Properly drain water from the boat itself and do not forget to also empty anything that can hold water. Lastly, clean your gear before heading to a new area to avoid inadvertently transporting European green crab larvae and juveniles. European green crab planktonic larvae can survive up to 80 days and juveniles can change their color when molting to match their environment. Do not let these sneaky invaders come with you on your next adventure. 

When walking on the beach, keep an eye out for any molts in beach wrack or vegetation. As the tide drops, algae, marine debris, and molts will wash up, so be on the lookout. Often, the colors of molts are much paler than living crabs, so be sure to use shape characteristics to identify European green crab molts.

Although smaller in size than other crabs, European green crabs are voracious predators that are able to crack open clams and mussels which are culturally, ecologically, and economically vital shellfish in Washington. These bivalves are already facing physical stressors due to climate-induced conditions such as ocean acidification. Thus, the added biological implications from predation by European green crabs warrant concern. Research indicates that European green crab feed on 22 clams per day which may destroy the $270 million shellfish industry. Furthermore, European green crabs burrow into muddy habitats in salt marshes and pocket estuaries, leaving holes which degrade these ecosystems even faster. They have also been seen disrupting ecosystems such as eelgrass beds as they pull the roots out to find their prey. Eelgrass beds in Washington are a home for many other culturally, ecologically, and economically vital species such as juvenile salmon and halibut that the state cannot afford to lose. For these reasons, we must keep our eyes open for these destructive invaders and properly report sightings through photos to help others monitor, track their spread, and remove as many individuals as possible. This will allow us to stay on top of the European green crab issue in Puget Sound, Washington, and the greater West Coast region.

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