A new threat to the regional ecology is starting to show up in Puget Sound, and scientists are calling it one of the worst invasive species known.

Washington Sea Grant and Washington Department of Fish & Wildlife recently announced that a single European green crab (Carcinus maenas) was found in Padilla Bay by staff of the Padilla Bay National Estuarine Research Reserve (Padilla Bay NERR). This is the second sighting of the global invader. The first sighing of the crab in Washington waters was in late August at Westcott Bay, San Juan Island, where volunteers caught one.

“We were relieved to find very little evidence of a larger population of invasive European green crab in Westcott Bay,” said Padilla Bay Reserve staff member, Emily Grason. “But finding an additional crab at a site more than 30 miles away suggests that ongoing vigilance is critical across all Puget Sound shorelines.”

Grason works with P. Sean McDonald, both crab experts with Washington Sea Grant.

After the first sighting, the Washington Department of Fish & Wildlife’s (WDFW) Aquatic Invasive Species Unit and Washington Sea Grant formed a monitoring and detection program to track the green crab population and the extent of the spread. They also aim to better understand the impact the crab has in Puget Sound.

To look for more green crabs after the first sighting, responders scoured a two-mile radius near Westcott Bay. It took them three days. When they finished they reported finding only one single green crab molt (cast-off shell).

“The findings indicate that there was likely more than one crab in the Westcott Bay Marsh, but that the invasive crabs are not yet firmly established,” Washington Sea Grant reported in a statement.

Since 1997 green crabs started showing up in Oregon and made its way up to Washington and British Columbia estuaries with the aid of El Nino currents.

As to how the crab has turned up in the Salish Sea experts can only speculate. One idea is that green crab larvae were brought to Puget Sound by higher than average surface currents from outside waters. Another likely reason is unintentional transportation of the crab from infested waters by humans.  

So what could an invasion mean for Salish Sea ecosystems? According to the WDFW, the crab is a threat to native salt marsh and pocket estuary organisms such as clams and oysters. The green crab preys on clams, oysters, mussels and native crabs. The crabs cannot break through the shells of mature oysters but can feed on the younger, more vulnerable ones. Furthermore, adult green crabs can borrow down 6 inches to get to clams and will consume 40 half-inch clams a day, as well as other crabs its own size. The Dungeness crab could be threatened by the invasive species because the green not only occupies their habitat but also feeds on the same foods. Seabirds would also compete for the same food. Looking longterm, with no major predator, the green crabs could devastate marsh and estuary ecosystems in Puget Sound and disrupt the greater ecosphere.

To prevent an outbreak here, the WDFW and Washington Sea Grant are asking volunteers and beach goers to be one the lookout for green crabs.

“Once volunteers or the public raise the alarm, a trapping effort can be initiated. The situations in Westcott Bay and Padilla Bay are excellent examples of how an early detection and rapid response program should operate,” said McDonald.

The green crab color can vary from a dark mottled green to reddish green. Adult green crabs can be about the size of a fist. On each side of the shell are five spines and three rounded lobes between the eyes.

“WSG’s Crab Team is committed to continuing the efforts of volunteer monitoring as resources allow, but we also rely on beachgoers to keep a watchful eye out for this invasive species,” said Grason

For more information visit wsg.washington.edu/crabteam