Green Crab: The Tiny Invader with a Big Appetite
WFRC Researchers are developing innovative tools to detect and prevent the continued spread of this invasive species in the Pacific Northwest.
Many people are surprised to learn that an invasive crab may be eating the very seafood they enjoy. The European green crab—a small but aggressive crustacean—is one of the most widespread marine invasive species in the world. A single crab can consume up to forty half‑inch clams per day, and it also preys on other crabs that are similar in size. These feeding habits allow European green crab populations to rapidly grow in abundance and quickly disrupt ecosystems.
Here in the Pacific Northwest, European green crabs can consume farmed oysters and clams, compete with native crabs and other crustaceans for food, and disturb coastal sediments in ways that damage eelgrass beds. They may also traffic diseases among crustacean species along the coast. These characteristics put culturally, economically, and ecologically valuable species such as shellfish, Dungeness crabs and Pacific salmon in harms way. Because controlling an established green crab population is incredibly difficult, preventing their spread remains the most effective long‑term management strategy.
Historically, European green crabs have been detected by physically trapping them or by finding their shells along the shoreline. These approaches continue to be critical for control and monitoring; however, they have their limitations. Green crabs often burrow into sediments or occupy habitats where traps are less effective. Trapping is also costly and labor intensive for the broad type of monitoring needed to detect these crabs before they grow into large, damaging populations.
Scientists at the U.S. Geological Survey’s Western Fisheries Research Center (WFRC) are contributing to efforts to control the spread of European green crabs by developing and optimizing new, cost-effective methods for broader surveillance. Environmental DNA sampling offers an important complementary tool to physical trapping that gives managers and scientists the ability to monitor green crabs across large landscapes and even when the adult invaders are difficult to find.
WFRC scientists are studying how factors such as sampling method, season, tide phase, and habitat type influence detection probability. Their data show that eDNA concentrations can vary depending on whether water samples are filtered actively or passively, whether sampling occurs during high, low, flood, or ebb tide, and whether the nearby habitat includes streams, tide channels, shorelines, or marinas. For example, eDNA can become diluted during flood tides in low‑salinity streams, while low tide in marinas may concentrate eDNA and improve detection success. These findings help determine when and where sampling is most effective.
WFRC is also studying the use of light traps—used to count Dungeness crab larvae and forecast their future adult abundance for harvest management—to also help detect the presence and dispersal of European green crab larvae.
To learn more about USGS research on European green crab, explore the publication “Development and Refinement of Methods for Early Detection of European Green Crab,” which includes detailed findings from eDNA and larval detection studies.