DNA Detectives
As European green crabs invade Puget Sound, experts are considering a new method for detecting the sneaky crustaceans: eDNA.
Allie Simpson holds a frozen European green crab in Blaine, Washington. European green crabs are an invasive species to the Pacific Northwest. They are identifiable by the five spines found on either side of the eye stalks. // Kieran Bresnahan
Story by Emily Stevenson // Photos by Kieran Bresnahan
From catching criminals to mapping a family tree, DNA is used to uncover the mysteries of life. It’s also a way to track organisms terrorizing ecosystems around the world like the European green crabs currently invading the Puget Sound.
To catch these invaders, a new method called environmental DNA (eDNA) is being used. Similar to catching killers with traces of DNA scattered throughout a crime scene, eDNA is scattered in the ocean, air and sediment. It’s the accumulation of skin, fur, hair or scales that is shed into the environment. Abby Keller, a graduate student at the University of Washington, is creating an eDNA coding system to locate green crabs in Puget Sound. As she embarks on her journey, some people are skeptical of the results they cannot see, but Keller is optimistic.
“I'm confident that eDNA works because we tend to know how many errors arise,” Keller said in an email.
Abby Keller (left) and Ryan Kelly (right) collect water samples from a stream. The samples are taken back to a lab where they are tested for DNA. // Photo courtesy of Emily Grayson
The first step in collecting a species’ eDNA is collecting water which is then poured through a filter as fine as a sheet of newspaper to catch the cells 25 times smaller than the human eye can see. Scientists use a pump to remove everything but the treasures they call cells.
“The vacuum pump filter is SUPER loud,” Keller said in an email. “I would get some very strange looks from people passing by.”
To preserve the collected DNA, the filter is placed in a tube no bigger than a pinky finger which is filled with a special chemical solution. The filtered tube is brought to the genetics lab where Keller and her team use harsh chemicals to extract the DNA.
After this intense process, a tube full of clear liquid remains. It holds the DNA of all organisms, living or dead, that were once swimming in the body of water it was collected from. To single out the invasive green crab DNA, a special chemical reaction is required.
“This chemical reaction essentially puts a megaphone up to any green crab DNA in the sample so that we are able to detect it,” Keller said in an email.
If a positive test result comes back, Keller can notify invasive species management officials about the possible presence of green crabs. However, some managing officials have historically had a difficult time trusting newly-emerging eDNA methods. It’s easier to believe in what you can see in traps rather than putting faith in a chemical reaction.
A genetic lab at the University of Washington where Abby Keller works. Keller uses pipettes and chemicals/enzymes to test for eDNA in water samples. // Photo courtesy of Abby Keller
eDNA detection, which has existed for a little over a decade, was first used to protect the Great Lakes from invasive carp in 2009. After scientists found traces of the carp, they contacted monitoring officials and the search began. Monitoring teams became skeptical of the positive eDNA results when no carp were initially found – until eight months later when they caught one.
eDNA methods have yet to be incorporated in monitoring plans because management teams are struggling to find the best approach while still learning about eDNA processes, according to Keller’s thesis.
“I think eDNA will be extremely important for early detection, but I also want to make it clear that it will likely not replace existing methods,” Keller said in an email. “But rather complement or supplement.”
Traditional green crab trapping is time-consuming, tedious and tide-dependent. Trapping sites can be as large as Willapa Bay, a whopping 670 square kilometers of mudflats that can hold hundreds of traps, or a small lagoon that only holds four traps.
“When we try and find a new location to put traps, it’s a lot of effort. You have to try and get a hold of land owners and bring out a ton of traps to see what’s out there,” said Allie Simpson, a biologist for Northwest Straits.
Simpson is no stranger to the mudflats. As low tide approaches, she steps into her thick boots and waterproof overalls and dons a windbreaker as she begins her work.
Allie Simpson carries a crab trap across a beach in Blaine, Washington. Simpson, a biologist for Northwest Straits, works to trap invasive European green crabs. // Kieran Brenahan
As the coastal wind blows, the salty scent of decaying seaweed fills the air. Simpson sinks ankle-deep into the mud, trudges along the flats and wades into the water, crossing the Dakota Creek to reach two traps. More than 30 uninspected traps span the low tide. Simpson records and releases every species caught, ensuring no green crabs are detected.
Two hours later, she finishes observing traps. No green crabs this time. Simpson moves to the next trapping site, in a race against the incoming tide.
“The potential elegance and power of eDNA makes it extremely appealing as a management tool for invasions, and indeed it has great potential to increase the scope of early detection,” wrote Emily Grason, a marine ecologist, in an eDNA blog post for Washington Sea Grant.
In just a couple of years, Keller’s eDNA process has captured the attention of local green crab monitoring teams. Karl Mueller, an aquatic invasive species coordinator at Lummi Nation, plays a key role in managing invasive green crabs at Lummi Sea Pond. Last year, Mueller was trapping 10,000 green crabs per week. As eDNA continues to improve, he’s willing to give it a shot.
“Eventually, we’ll be working with outside agency partners on testing its efficacy in the Lummi Sea Pond,” Mueller wrote in an email. “I think it has promise, and it could even replace some aspects of [crab] trapping.”
The next step would be to create a website so non-DNA experts can review the trapping vs. eDNA results on their own, Keller said.
This upcoming eDNA technology will not only help to detect invasive species sooner but will give scientists a better understanding of endangered species populations.
Although eDNA is smaller than the human eye can see, it has the potential to create a big impact.
Emily Stevenson is a fourth-year environmental marine science and Salish Sea studies student, who uses journalism to spread knowledge about environmental issues.
