Volunteers just found an invasive crab in Puget Sound. Now, scientists want to know how many there might be.

The European green crab has a distinctive feature.

"From the edge of the shell, 1-2-3-4-5," counted UW Research Scientist Sean McDonald.

The bumps make it easy to spot, an unwelcome find on San Juan Island this week.

"If they became established down there it would be fairly detrimental," McDonald said.

A group of volunteer scientists made the discovery. The volunteers are part of Washington Sea Grant’s (WSG) Crab Team, a monitoring program set up to look for European green crabs and collect information on local marine life.

The crab is invasive, likely a hitch-hiker on ships and El Nino currents. They're skilled at killing shellfish and have voracious appetites. They populate rapidly and tear up fragile eel grass beds.

"It's not just shellfish. The impacts won't be limited to shellfish people like to eat. Birds rely on shellfish. These small estuaries and marshes are important nursery habitat for a lot of animals," McDonald said.

The crabs were discovered on the coast in the late 1990s. This is the first time anyone's documented one in Puget Sound. Scientists are increasing monitoring patrols to see just how many there are. After that, they'll decide on a plan to get rid of them.

"We're most concerned about the inland waters of Puget Sound. We have a lot of suitable habitat for them," McDonald said. "There are a lot of places where they can find a niche."

Scientists are asking for help from the public to track the crab's progress. Here are a few ways to get involved:

1. Learn how to how to identify green crab. Check out the Crab Team website or Facebook and Twitter @WAGreenCrab.

2. Take a photo and report sightings to the WSG Crab team at crabteam@uw.edu.

3. Attend a public presentation by the Crab Team at UW Friday Harbor Labs on San Juan Island Tuesday, Sept. 13, 7 p.m. See the Crab Team webpage for additional details.