June 9, 2025

The molt at False Bay was textbook, opening at the “molt line” between the top and bottom of the shell, and showing a clean interior. When crabs molt, they even leave the covering to their gill tissue behind! Photo courtesy of Megan Cosand. Click to enlarge.

Students at the University of Washington Friday Harbor Labs (FHL) found a green crab molt in False Bay on San Juan Island last weekend. This molt is the first evidence of green crabs found in False Bay and the first evidence since 2019 of green crabs in the San Juan Islands. In 2016, San Juan Island was the site of the first green crab confirmed along the Salish Sea shorelines of Washington, and since then, ongoing monitoring and additional high-intensity trapping efforts have only caught a total of five additional live crabs and a single molted exoskeleton. Despite broad searching, all previous detections were made at a single site – the marsh at the head of Westcott Bay, also on San Juan Island. False Bay does not have any current monitoring efforts, but has been trapped periodically going back two decades to work done by Oregon State University researcher Sylvia Yamada. The crab that shed this shell was a 49mm male – likely arriving at the site in 2024, which was a big recruitment year at many sites throughout the Salish Sea. 

The students found the molt opportunistically, when they were visiting the site for an independent research project – but they knew how to identify European green crabs in part because of their coursework at FHL this spring. The class, “Novel Ecosystems”, taught by Dr. Marjorie Wonham, focused on how ecosystems are changing due to human-induced pressures on physical and ecological characteristics and included a heavy emphasis on green crab. The class also partnered with Crab Team to assess monitoring protocols in use across the network, including minnow trap opening size and molt hunt efficacy. They even conducted outreach at the FHL Open House, creating innovative demonstration materials like a 3D-printed green crab carapace (shown in the post header) and laser-cut models of native crabs to get word out to the public about green crab.

Can you spot the green crab? Because color can be confusing, these laser-cut wood carapaces could be an excellent teaching tool for learning to distinguish green crabs from the shape of their shell alone. Photo courtesy of M. Wonham. (Answer below).

Dr. Wonham shared, “We were surprised and impressed to learn that the students had gone out on a field survey, spotted the molt, identified it correctly, and reported it to WSG – all of their own initiative. We are proud to have a group of students finishing up this program who can help keep an eye out on Washington beaches for green crabs and other introduced species.” Megan Cosand, who was among the students out in the pouring rain that day reflected on her concern about the health of False Bay if green crabs establish a population there. “Eelgrass on San Juan Island is doing poorly enough without green crabs digging it up. There are also a lot of juvenile Dungeness crabs at False Bay, which would be impacted by green crabs.”

FHL Students Luke Peyerwold, Noah Breiter, and Megan Cosand with the green crab molt they found in False Bay. Photo courtesy of M. Cosand.

The False Bay detection highlights how molts can be the first way evidence of green crab is found for a site, particularly locations that can’t be trapped frequently. Washington has more than 3,000 miles of shoreline, more than can be covered by trapping alone. Anyone with safe and legal access to marine shorelines can help find beaches where green crabs might be present by participating in the Molt Search program. Partners at WSG Crab Team and WSU Extension are hosting the second annual Salish Sea-wide Molt Blitz this month and encouraging as many people as possible to get to their favorite beach and do a survey on June 20th. It’s the longest day of the year, so there’s no shortage of opportunities. Training events are happening around the region and online in advance, and staff and partners will be hosting beach events day-of. Learn more about how to plug in on the Molt Blitz event page.

– Emily Grason

*In the photo of the demonstration carapaces above, if you thought the carapace in the bottom right looked suspicious, you were right! The five teeth or spines to the outside of each eye is all you need to ID a green crab. The purple and hairy shore crabs in the top row (left and right, respectively) have only three teeth in their back shell, while the hairy helmet crab (bottom left) actually has 6, and a different shape of carapace.