Invasive green crab. (Photo by Ryan Munes, USFWS)

Invasive green crab. (Photo by Ryan Munes, USFWS)

Crabby about invasive green crabs

They’re described as one of the biggest threats to Alaska’s marine environment, and you’ve probably never heard of them. This year, they were found in the state for the first time. Say hello to the invasive green crab.

Invasive green crabs (also known as European green crabs, Carcinus maenas) are shore crabs native to Europe and North Africa. Though they’re new to Alaska, they’re not new to the United States. They have been wreaking havoc on the Atlantic Coast since the 1800s, likely transported across the ocean by ships in their ballast water.

Over a century later, they made their way to the Pacific Coast, likely stowed in live seafood or bait shipments. Since then, these invasive green crabs have slowly expanded north and south, either hitchhiking or floating in larval stages on ocean currents. Until, voila, they arrived in Southeast Alaska!

To see a detailed map of their spread, check out the work of the Washington Sea Grant Crab Team.

Despite their small size, ranging from approximately 2.5 to 4 inches in shell width, invasive green crabs have outsized impacts on ecosystems. They’re generalist predators with a fierce appetite.

Researchers Floyd and Williams found a single crab can consume upwards of 20 clams per day! They feed on mussels, clams, juvenile oysters, native crabs and juvenile salmon.

Recently, other scientists, Howard and others, documented them eating eelgrass in British Columbia. They compete with native crab species, such as Dungeness crabs (Metacarcinus magister) and red rock crabs (Cancer productus) for forage and habitat.

As if their appetite wasn’t enough, invasive green crabs destroy eelgrass beds. Eelgrass beds are important for:

• Juvenile salmon and other fish as nursery habitats.

• Herring and other fish as spawning habitat.

• Bivalves, such as clams and mussels, and crustaceans as sediment habitat.

• Migratory and resident shorebirds and sea ducks as foraging habitat.

• Marine mammals, such as sea otters, as foraging habitat for prey.

The consequences of the degradation and destruction of eelgrass beds have the potential to cascade throughout the entire ecosystem.

Invasive green crabs damage eelgrass meadows by tunneling, burrowing into sediment, and clipping blades of seagrass, searching for food and shelter.

In Newfoundland, researchers Metheson and others found between a 50% and 100% decline in eelgrass cover after green crabs invaded these habitats. Furthermore, the more crabs and the longer they had been there, the greater the negative effects on eelgrass.

The negative impacts can happen fast. For example, within just four weeks of infestation, researchers Howard and others in British Columbia found eelgrass density declined up to 81% in areas of high invasive green crab density.

Ultimately, the arrival of invasive green crabs in Alaska has potentially huge implications for subsistence, recreational and commercial fisheries. Even before they arrived in Alaska, the state had warned of the damage they could cause.

It’s no wonder they are listed on the International Union for Conservation of Nature’s 100 of the World’s Worst Invasive Alien Species.

It’s not all doom and gloom, though!

Fortunately, agencies and partners across Alaska have been surveying for invasive green crabs since the 2000s. This preparation allowed the Metlakatla Indian Community Department of Fish and Wildlife to first detect the crabs during a survey on Annette Islands Reserve in July 2022.

Since detection, activities on Annette Islands Reserve have shifted from early detection surveying for invasive green crabs to intensive trapping to minimize the population before it can further expand its range.

Despite the common name, invasive green crabs aren’t always green. They can range from dark brown to green and even be yellow or orange. The telltale sign for identifying an invasive green crab is the five spines behind each eye.

Thankfully, no native Alaska crabs have five spines. Juvenile Dungeness crabs are the species most likely to be mistaken for the invaders, but they have 10 spines behind each eye.

Check out Kachemak Bay Research Reserves’ crab identification guide for more detailed information. You can test your identification skills with Washington Sea Grant Crab Team “Pick the European green crab guide.”

We need your help!

• Report any green crab sightings. If you’re on Annette Islands Reserve, call 907-886-FISH. If you’re anywhere else in Alaska, report it to the Alaska Department of Fish and Game’s online Alaska Invasive Species Reporter or the Invasive Species Hotline at 877-INVASIV.

Don’t keep or kill the crab; rather, take plenty of photos with a standard-sized reference item, such as a coin or key. Then, send your photos to the Alaska Department of Fish and Game (tammy.davis@alaska.gov).

• If you’re interested in getting your hands wet and dirty, you can volunteer to be a citizen scientist by joining the marine invasive species citizen monitoring network. Contact ADF&G Invasive Species Program coordinator at 1–877-INVASIV to get involved!

• If you’re beachcombing, keep an eye out for carapaces while strolling the coast. The carapace, the hard upper shell of a crab, is shed during molting.

Looking for carapaces is a great way to detect the presence of the invasive green crab, as well as other native species. Not only is it a fun treasure hunt, but you are helping to track and detect invasive species!

Just make sure to count the spines — remember, five behind the eyes = invasive!

Ashley Lutto is Administrative Support at Kenai National Wildlife Refuge, spending every chance she gets outside recreating and exploring on the Kenai Peninsula. She wrote this with partners at Metlakatla Indian Community, Alaska Department of Fish and Game, and the National Oceanic and Atmospheric Administration, and editing by Sabrina Farmer. Find refuge events, recreation, and more at kenai.fws.gov or Facebook: https://www.facebook.com/kenainationalwildliferefuge and more Refuge Notebook articles (1999–present) at https://www.fws.gov/kenai-refuge-notebook.

Eelgrass beds of Colby Creek Estuary. (Photo by Linda Shaw, NOAA)

Eelgrass beds of Colby Creek Estuary. (Photo by Linda Shaw, NOAA)

Invasive green crab carapace and a whole live crab. (Photo by Linda Shaw, NOAA)

Invasive green crab carapace and a whole live crab. (Photo by Linda Shaw, NOAA)

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