Primed to Detect the Green Crab Invasion

On the British Columbia coast, an insidious invasion is happening below the waves, one poised to substantially alter nearshore ecosystems if left unchecked. 

The invader? A shore-dwelling crustacean called the European green crab.

At first glance, these interlopers look innocuous: with shells measuring 10 centimeters across, they’re not much bigger than BC’s native shore crabs. But they have a well-earned reputation as one of the most destructive marine invasive species in the world, with a plethora of direct and cascading impacts, both ecological and economic. 

Hardy, adaptable, and prolific, European green crabs are thought to have first arrived in North America in the 1800s in the ballast water of ships, and they have since been found at specific locations from Mexico to Alaska. These efficient predators feed on a wide range of native species, from juvenile fish to other crustaceans, and they out-compete native crabs for food. They also can destroy eelgrass meadows and shellfish beds while feeding or burrowing for shelter.

For most people, this invasion is out of sight and out of mind, but for those whose lives intertwine with coastal ecosystems it’s a growing concern, especially as the crabs continue to colonize new areas. To give coastal communities and ecosystems a better chance of responding to and managing the threat of invasive green crabs, a new tool is being deployed by scientists at the Hakai Institute—one that will make it easier for a network of community partners along the coast to detect and get ahead of these invaders, even before laying eyes on them.

This early warning tool hinges on environmental DNA: the bits of genetic material that green crabs, like every organism, leave behind in their environment. 

“We work a lot with citizen science groups using eDNA to measure biodiversity,” says Hakai Institute scientist Matt Lemay, the science lead for the Integrated Coastal Observatory (ICO). ICO is a Hakai-led network of partner organizations that includes Parks Canada, First Nations and citizen science groups.

ICO partners collect monthly seawater samples along the BC coast and send them to the Hakai Institute’s Quadra Island Ecological Observatory for sequencing and analysis.

“The focus of ICO’s work has been on marine fish diversity since the initiative began in 2019,” says Lemay, “but from the outset partners were looking for ways to monitor this invasion.”

Now Lemay, ICO network coordinator Sue Velazquez, and the rest of Hakai’s ICO project team are nearly ready to deliver on that request.

Coastal waters teem with eDNA. Even the smallest organisms shed genetic traces of their presence through things like skin cells, scales, or mucus. The trick, however, is being able to accurately detect DNA of a single species—in this case, green crab—amongst the ocean’s swirling soup of genetic material. 

That’s where part one of this important initiative comes into play. Hakai’s ICO team has been working to optimize a targeted genetic tool to isolate and detect any traces of eDNA from green crabs—and only green crabs—in a sample. 

Known as qPCR (quantitative polymerase chain reaction), what distinguishes it from traditional approaches is the “quantitative” aspect:  these tests can go beyond revealing the simple presence/absence of a species to shed light on its relative abundance in an area. 

A key part of the tool, called the primer, binds to and amplifies the target DNA, and a new piece of tech called a digital droplet PCR instrument helps the ICO team detect it amidst the melange of background DNA. 

The team has been busy putting the new green crab tool through its paces, using the archive of seawater samples collected by ICO partners over the past few years—specifically those samples from areas where the species is known to exist, notably on western and southern Vancouver Island. Parks Canada and the T’sou-ke Nation, respectively, are actively monitoring European green crab in each of those locations. 

“It’s exceeding my expectations,” says Lemay. “If there’s any shred of green crab DNA in the water this tool should detect it. At this point I suspect that we can get to really low, hopefully first wave, detections.”

“Many ICO members have heard of a green crab being spotted or captured in areas where they weren’t before, and they’re quite concerned about it,” says Velazquez. “Early detection is the most valuable thing for a lot of groups because most aren’t out there observing it in the field.”

Green crab DNA shows up in ICO seawater samples year-round, says Lemay. This indicates that qPCR testing picks up on green crab DNA from all life stages—meaning the Hakai ICO team won’t need to time their sample analysis to the crab’s life cycle. 

“You don’t know what that genetic material is–if it’s coming from adults, if it’s coming from larvae, if it’s coming from spawning,” says Lemay. “I was surprised to see how abundant the DNA was year-round. Clearly the adults are shedding a lot and there’s a strong signal at any time. Which is great for detecting. There’s less chance that you’d miss it.”

Before the team rolls out the green crab tool for network-wide use, they need to finalize part two of the initiative: data accessibility. It may sound less exciting, but making the results publicly accessible, and quickly, is just as vital as the eDNA sampling and sequencing. 

While other groups are also implementing eDNA testing to monitor for European green crabs, to Lemay’s knowledge this coordinated, coast-wide, and open-access approach to tracking the invasion is unique. 

“It’s going to be amazing. It’s essentially a real-time data visualization of detections in BC at these partner locations. I don’t think there’s any existing tool or network that’s doing that.”

These detections could, in turn, warn others to be on the lookout—and potentially entice more groups to join the ICO network.  

To learn more about the green crab invasion in the northeast Pacific, check out this Hakai Institute “Nature Files” video, The Case of the Colonizing Crabs, and the Hakai Institute Long Story Shorts video How Do Invasive Species Take Over?