Environmental DNA tracks invasive Green Crabs

Researchers believe a new method that combines water testing with genetic analysis could be used to help prevent future European green crab invasions.

European green crabs are a invasive species that eat shellfish, devastate marsh habitats through their burrowing in the soil, and decimate seagrass beds. The invasive species also reproduces quickly, making it a nightmare for wildlife managers seeking to control its spread in Washington state’s marine waters.

Last month, Governor Jay Inslee issued an emergency order in response to more than 70,000 crabs caught on Lummi Nation land as well as dramatic increases in crab populations on Washington’s outer coast and other locations in Puget Sound in recent years.

The journal Ecological ApplicationsResearchers have shown that the new DNA-based method works just as well in detecting green crabs as setting traps for live animals. This is a more laborious process. Results suggest these two methods could complement each other as approaches to learn where the species’ range is expanding.

The new method uses genetic material found in the environment, called eDNA. This is after organisms have passed through it. Scientists can take a bottle of water and extract DNA from it to determine which species are present in the area.

“We have limited resources to be able to combat this problem, and it’s important to think about how to allocate those resources efficiently and effectively,” says lead author Abigail Keller, who completed the work as a master’s student in the University of Washington School of Marine and Environmental Affairs. “Knowing the best situations for using eDNA to detect invasive green crabs is important, and that’s what our study tried to tackle.”

The data was gathered from 20 green crab traps located throughout Puget Sound and the coast over three months in 2020. Trapping at these locations was done by a large number of partners participating in statewide efforts to monitor and control European green crab, including multiple tribes, Washington Department of Fish and Wildlifethe state lead for green crab managementWashington Sea Grant’s Crab Team, and other state and federal agencies.

Environmental DNA

Researchers visited each location to collect water samples. Then, they ran genetic analyses to determine whether or not there was a European green crab present at each location. They were able to validate the eDNA data against the actual presence of the crabs. They discovered that eDNA was as sensitive and accurate as counting live crabs and trapping them to determine their abundance and presence.

This is significant, the researchers say, because eDNA as a detection method is new, and it hasn’t always been clear how to interpret eDNA detections in past scenarios. This study shows how eDNA techniques can be combined with traditional monitoring methods, such as counting and trapping crabs, to more effectively detect and control invasive species outbreaks.

“Here’s a really well-validated example of how to use eDNA in the real world. To me that’s really exciting,” says coauthor Ryan Kelly, associate professor in the University of Washington’s School of Marine and Environmental Affairs. “There are lots of invasive species, and many imperiled and endangered species that are hard to monitor, so this is one significant way forward on all of those fronts.”

The study also examines when eDNA is useful in monitoring for invasive crayfish, and when traditional trapping or counting still make the most sense. For example, taking water samples and testing for green crab DNA in remote locationsor in areas where outbreaks haven’t yet been identifiedcould save time and resources instead of deploying traps. Alternatively, eDNA probably wouldn’t be helpful in locations where large numbers of green crabs are already living and where community scientists and managers are already trapping and controlling those populations, the researchers explain.

“From a management perspective, the value of this tool just really comes to life in places that are more remote or have a lot of shoreline to cover, like Alaska, where green crabs haven’t yet been detected,” says coauthor Emily Grason, a marine ecologist who leads the Washington Sea Grant Crab Team. “I see eDNA as another tool in the toolkit, and we can imagine scenarios where it can be used alongside trapping, especially as an early detection method.”

Stop the green crabs

It is crucial to find these crabs quickly after they move to a new area. Managers can prevent new invasions by testing water from multiple locations and then following up with water testing, monitoring on the ground, and trapping if green-claw DNA is detected.

The paper identified green crab DNA in one location where the species hasn’t yet been captured, near Vashon Island. The researchers followed up one year later with intensive trapping and retested water. No green crabs were found or additional green crab DNA was discovered. The researchers think the earlier positive sample likely was picking up green crab larvae, which weren’t present in that location a year later. This effort was a significant test case for how eDNA combined with traditional trapping can be used for green crab management.

“The reason we pursued this project in the beginning is that early detection of green crabs is difficultit’s like finding a needle in a haystack,” says coauthor P. Sean McDonald, associate teaching professor in environmental studies and aquatic and fishery sciences and the university’s principal investigator for Crab Team research. “So if adding eDNA to our toolkit helps us detect those needles, then that’s great to have at our disposal.”

Ana Ramn Laca, from the Cooperative Institute for Climate, Ocean and Ecosystem Studies, serves as an additional coauthor. Washington Sea Grant provided funding.

Source: University of Washington