Crabs may be a charming sight for beach-goers, but for local wildlife, the green variety could pose a serious threat to their ecosystems.
Last month, a team comprised of European green crab experts from Washington Sea Grant’s (WSG) Crab Team and UW scientists at Friday Harbor Laboratories discovered evidence of a green crab in Westcott Bay, San Juan Island.
After surveying a two-mile radius over three days, the team only found a single molt, or cast-off shell. Their findings suggest that there is likely more than one crab in the Westcott Bay Marsh, but that the invasive species is not yet firmly established in the Pacific Northwest’s pocket estuaries, or smaller isolated habitats.
“The Pacific Northwest has a pretty complex ecosystem and I wouldn’t expect green crabs to occupy all of the Pacific Northwest,” said P. Sean McDonald, lecturer and Capstone instructor in the UW Program on the Environment. “They could get a foothold in pocket estuaries and that could cause some serious problems.”
A native of the northeast Atlantic Ocean and Baltic Sea, the European green crab has established populations in Australia, South Africa, and North America. Immigrating to the United States during the 19th century aboard European trading vessels, the species has caused a serious impact on the environment by consuming shellfish, clams, and smaller shore crabs. The green crab can dig as far as six inches into marshland habitats and can eat up to 40 half-inch clams a day.
Often confused with native helmet crabs or the hairy shore crab, green crabs are more easily identified by the shape of its shell than its color, which can range from a mottled green to a crimson red. The invasive species is typically identified by the five short teeth along each side of its shell and the three rounded lobes around the eyes. The carapace, the bony shield on the crab’s back, is three to four inches wide.
Green crab populations in Washington state were limited to Pacific coastal estuaries until they were discovered in Canadian waters across the Strait of Juan de Fuca in 2012, which encouraged the state’s wildlife department to invest in Salish Sea monitoring and early detection.
WSG currently monitors just 26 out of the 400 sites potentially threatened by green crab invasion. The organization’s monitoring efforts are performed in part by a group of 100 volunteers, from academics to wildlife enthusiasts, who are trained to independently identify and document green crabs.
Emily Grason, a UW marine ecologist who has been involved with the study since fall 2014, advised that locals should remain aware of the green crab’s unpredictability.
“Invasive species have a tendency to surprise us and do things that we aren’t predicting,” Grason said. “It’s entirely possible that green crabs could find a beach that we aren’t carefully watching because there are 2,500 miles of shoreline.”
The extermination of green crabs can be achieved through a variety of methods, including the use of pesticides and baited traps.
While green crabs may not pose a significant threat to the American west coast any time soon, McDonald cautioned that they could still impact local wetlands.
“The important thing to remember, from my perspective, is that green crabs are likely to find their way into Puget Sound,” McDonald said. “Collecting data is extremely helpful in taking early action before there’s a serious problem.”
McDonald further advised Washington state residents to stay vigilant and to alert scientists upon spotting green crabs.
“If you see a crab, don’t kill it,” McDonald said. “Take a picture, document it, and alert us to the situation.”
Reach reporter Tim Gruver at email@example.com. Twitter: @T_TimeForce