eDNA enables early detection of invasive green crabs
[By Michelle Ma]
European green crabs feast on shells, destroy swamp habitats by digging in mud, and wipe out valuable seagrass beds. The invasive species also reproduces quickly, making it a nightmare for wildlife managers looking to control its spread in Washington’s marine waters.
Last month, Governor Jay Inslee issued an emergency order in response to more than 70,000 crabs caught on Lummi Nation lands as well as the dramatic increase in crab populations on Washington’s outer coast and at other Puget Sound locations in recent years. As the green crab invasion in the state worsens, a new testing method developed by University of Washington and Washington Sea Grant scientists could help contain future invasions and prevent new outbreaks in using water analyzes and genetic analyses. The results, published online Feb. 6 in the journal Ecological Applications, show that the DNA-based technique works both for detecting the presence of green crabs and for setting traps to catch the animals alive, which is a process more laborious. The results suggest that these two methods could complement each other as approaches to find out where the species’ range extends.
The new method relies on environmental genetic material, known as eDNA, which is found in the water after organisms have passed through. Scientists can collect a bottle of water from a location, extract DNA from the water, and discern which species were present in that area recently.
“We have limited resources to be able to tackle this issue, and it’s important to think about how to allocate those resources effectively and efficiently,” said lead author Abigail Keller, who completed the work as as a master’s student at the UW 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 attempted to address.”
The research team relied on data collected over three months in 2020 from green crab traps at 20 locations in Puget Sound and the Outer Coast. Trapping at these locations has been carried out by a large number of partners participating in statewide efforts to monitor and control the European green crab, including several tribes, the Washington Department of Fish and Wildlife – the lead of the State for Green Crab Management – Washington Sea Grant’s Crab Team, and other state and federal agencies.
For this study, researchers visited each site and collected water samples, then performed genetic analyzes to detect both the presence and quantity of European green crab at each site. This way, they could validate the eDNA data with the actual presence and number of crabs. They found that using eDNA to detect the presence and abundance of the species was as sensitive as trapping and counting live crabs.
This is important, the researchers said, because eDNA as a detection method is new and it has not always been clear how to interpret eDNA detections in past scenarios. This study shows how conventional surveillance methods – in this case, crab trapping and counting – can be combined with eDNA techniques to more effectively find 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,” said co-author Ryan Kelly, UW associate professor at the School. Marine and Environmental Affairs “There are a lot of invasive species and a lot of endangered and endangered species that are difficult to monitor, so this is an important way forward on all of those fronts.”
The study also assesses when eDNA would add value to monitoring invasive crabs, and when conventional trapping and counting still makes the most sense. For example, taking water samples and testing green crab DNA in remote locations – or in areas where outbreaks have not yet been identified – could save time and resources instead of deploying traps. . Alternatively, eDNA would likely not be useful in places where large numbers of green crabs already live and where scientists and community managers are already trapping and controlling these populations, the researchers explained.
“From a management perspective, the value of this tool really comes alive in places that are more remote or have a lot of shoreline to cover, like Alaska, where green crabs have yet to be detected,” said said co-author Emily. Grason, a marine ecologist who leads the Washington Sea Grant Crab team. “I see eDNA as another tool in the toolbox, and we can imagine scenarios where it can be used alongside trapping, especially as an early detection method.”
Finding these crabs soon after they have occupied a new location is important for controlling the population and protecting native habitats. Managers could anticipate new invasions by testing the water at multiple locations, then follow up with more water testing, field monitoring, and trapping if green crab DNA is detected.
The paper identified green crab DNA at a location where the species has yet to be caught, near Vashon Island. The research team followed up a year later with intensive trapping and retesting the water; no green crabs or additional green crab DNA were found. Researchers believe the previous positive sample likely picked up green crab larvae, which weren’t present there a year later. Notably, the effort represented an important test case of how eDNA and traditional trapping can be implemented together for green crab management.
“The reason we pursued this project in the beginning is that early detection of green crabs is difficult – it’s like finding a needle in a haystack,” said co-author P. Sean McDonald, Associate Professor at UW in Environmental and Aquatic Studies and Fisheries Science and the UW Principal Investigator for Crab Team Research. “So if adding eDNA to our toolkit helps us detect these needles, then that’s great to have at our disposal.”
Michelle Ma is associate director of UW News. This article is reproduced here with permission and can be found in its original form here.
The opinions expressed here are those of the author and not necessarily those of The Maritime Executive.