The European Green Crab: An Invasive Species on our Shores

Last week, you may have seen an article in The Daily World newspaper about European Green Crabs. They are an invasive species that should not be in our region. Unfortunately, they've made their way to our shores, and their numbers are increasing. They are on the state Department of Fish and Wildlife’s (WDFW) list of “aquatic invasive species of greatest concern", and for good reason. Emily Grason, the crab team lead at Washington Sea Grant, told The Daily World that…

The [European] green crab is often called one of the most damaging invasive species worldwide. As we’re seeing numbers on the rise in Washington there is concern because of the damage that’s been caused where it’s been established outside its range.

In this post, we will introduce you to the species, look at the damage that they are doing and why it is a problem, and share how you can help remove them from our environment.

Meet the European Green Crab

The European Green Crab (also known as the European Shore Crab) is a small, aggressive marine shore crab. They tend to make their homes in shallow waters along the shores of diverse coastal areas, including bays, rocky intertidal areas, and estuaries. On the West Coast, they are most commonly found on estuarine mudflats. They are tolerant of diverse climatic conditions, which makes them able to survive at a range of different temperatures and salinity (the concentration of dissolved salts in the ocean).

The European Green Crab is a Crustacean. This means that it sheds its exoskeleton in its growth process. This also means that it has a segmented body with what the State of Alaska describes as "jointed appendages." Other features that European Green Crabs, and all other Crustaceans for that matter, have include a pair of jaws (also known as mandibles) and two pairs of mouthparts (also known as maxillae) that are used for sensory reception, feeding, and ventilation from a gill chamber. European Green Crabs have one pair of claws and four pairs of walking legs. Males typically have larger claws and longer legs than females.

Female European Green Crabs mate when their shells are soft. This occurs after they molt (shed their hard shell). They typically mate with males that are larger than they are. Once they mate, male crabs will guard female crabs until her shell has hardened. The female carries her brood of fertilized eggs on the underside of her body for several months. A female crab can produce more than one brood. Young European Green Crabs go through six larval stages: 1 Protozoea stage, 4 Zoea stages, and 1 Megalopa stage. Each stage is separated by its own molting event. These stages can last anywhere from four to eleven days depending on the temperature of the water (warmer temperatures mean shorter stage durations). During these early life stages, European Green Crabs travel on currents, as part of coastal plankton. This is potentially what allows them to expand their range, and may be how they've ended up on our shores.

Identification

While the name "European Green Crab" may lead you to believe that the crabs are always green, their coloration actually varies. The dorsal (top) shell, also known as a carapace, is generally mottled and multicolored. The color of the dorsal ranges from grey or brown with yellow patches to olive or dark green. The color of the ventral surface (underside) is generally green outside or orange. However, during molting season, it can change to red. Additionally, several native crab species, including the Dungeness Crab, also have green shells. Therefore, it is not advisable to identify a European Green Crab by color alone. Color can certainly help you identify them, but you need something more than that to make a positive identification.

The characteristic of European Green Crabs that is the most readily identifiable is a set of five triangular teeth (also known as spines) that are spaced out evenly on each side of the carapace margin, which can be found between the widest part of the shell and the eyes. Additionally, European Green Crabs have a trio of rounded lobes (also known as bumps) that are between the eyes. The carapace width of an adult European Green Crab is normally about 2.5 inches (6.35 centimeters) long. However, outside of their native range, they can grow up to 4 inches (10.16 centimeters) long. Since Ocean Shores is not part of their native range, you can expect to see the longer length crabs here. Also, while European Green Crabs may look a lot like juvenile Dungeness crabs (scientific name: Cancer magister) in shape, you can tell them apart by counting the number of spines behind their eyes. Dungeness Crabs have ten small spines. European Green Crabs have five larger spines. Furthermore, you can tell European Green Crabs apart from Helmet crabs (scientific name: Telmessus cheiragonous) by looking for stiff hairs on their bodies. European Green Crabs don't have any, but Helmet Crabs do.

The Damage they are Doing and Why it is a Problem

To understand the damage that European Green Crabs are doing, we must first look at what they eat. Nothing is really off the table for them. Basically, if they can break it apart, they will eat it. They feed on "small fish, including salmon smolt, juvenile razor clams, juvenile oysters, [and] native crab species." They will also eat what Grason described to The Daily World as a "vegetarian diet [consisting] of seaweed or eel grass." The fact that they eat Eel Grass is what is causing much of the concern.

Ecologically, Eel Grass is a foundational species. Salmon, juvenile Dungeness Crab, and a wide range of other organisms rely on Eel Grass as food source. But what's even more worrisome is that Eel Grass is also important for the survival of humans because it helps store carbon. Carbon is essential for all life to be sustained on Earth. Therefore, we have to treat European Green Crabs not just as a threat to our environment, but also to our very survival.

The good news is that efforts are being made to control the crabs in an effort to reduce the amount of damage that they are able to cause. Volunteers, tribes, shellfish growers, and state agencies have joined forces in an all-out battle against what The Daily World correctly dubbed "the destructive invaders." A trap was recently set up across the harbor from Ocean Shores, near the old Saginaw Mill site, to see if any European Green Crabs could be found there. We don’t know if any have been found there yet. However, we do know that, in the last year, a few other local sites have been monitored, including several here in Ocean Shores. It appears those efforts are working. 915 traps were set in 2020 at 21 different locations in Grays Harbor, Willapa Bay, and Makah Bay. 1,762 European Green Crabs were caught and removed. The crabs seem to like Ocean Shores more than other locations. In our fine city alone, 562 crabs were removed for every 100 traps that were set. Additional trapping that was conducted by the Makah Tribe and Pacific County's Weed Management department led to the removal of a further 1,500 crabs. That is excellent news as it means a combined total of nearly 3,300 fewer crabs are around to cause damage. However, more will be coming to take their place. According to Grason, "…they are more abundant in Grays Harbor than they have been since they were first detected in the late 1990s, but that said they are not as abundant as we think they can get.” Additionally, Grason believes that “we haven’t seen the worst of it yet." Therefore, now may be the best time we have to intervene and prevent the damage from getting worse than it already is.

How You Can Help

If you see a live European Green Crab or a European Green Crab molt, visit wsg.washington.edu/crabteam to report it. Additionally, since the number of these crabs is higher here in Ocean Shores, you might also want to consider reporting sightings on the Coastal Interpretive Center Facebook page to help increase awareness of the problems among residents and visitors alike. Working together, we have the best shot at removing European Green Crabs from our environment and stopping their destruction before it's too late.

References

1. https://www.thedailyworld.com/news/state-volunteers-tribes-shellfish-growers-work-together-to-combat-invasive-european-green-crab/
2. https://www.futurelearn.com/info/courses/the-biology-of-bugs-brains-and-beasts/0/steps/68848#:~:text=Life%20on%20earth%20would%20not,of%20life%3A%20growth%20and%20replication.
3. http://www.adfg.alaska.gov/index.cfm%3Fadfg%3Dinvasiveprofiles.europeangreencrab_characteristics
4. http://www.adfg.alaska.gov/index.cfm?adfg=invasiveprofiles.europeangreencrab_distribution
5. https://science.nasa.gov/earth-science/oceanography/physical-ocean/salinity

© Ian D. Caldwell, May 2021