Emily Grason/Washington Sea Grant
Researcher Mary Fisher holds a European green crab captured in Willapa Bay, Washington.

Over the past several years, shellfish growers in Washington’s Willapa Bay have raised the alarm that Manila clams, one of the region’s most important commercially grown shellfish, were inexplicably declining. At the same time, these aquaculturists have witnessed growing populations of one of North America’s most destructive invaders: the European green crab.

But determining if the crabs were directly responsible for the declining clam populations — that is, whether they were eating them — was trickier than it sounds. Marine ecosystems are complicated, and as it turns out, green crabs don’t give up their secrets easily.

“I’ve spent more than 20 years trying to figure out what crabs eat,” said P. Sean McDonald, associate teaching professor in the UW Program on the Environment and School of Aquatic and Fishery Sciences. “It is an incredibly difficult proposition. Crabs are like little self-contained blenders, and anything that goes into their mouths is immediately turned into a slurry. It’s like trying to identify the individual parts of a smoothie.”

That’s where lead researcher Mary Fisher and the Washington Sea Grant Crab Team entered the picture.

“You can try to identify a crab’s stomach contents visually — dissecting the crab, opening up the stomach, and sifting through the contents — but that’s extremely difficult and usually not very precise,” said Fisher, who conducted this research while she was a doctoral student in the UW School of Environmental and Forest Sciences. “That’s where DNA-based analysis comes in, which can be a lot more effective.”

Their methods, described in a new paper published May 31 in the journal PLOS ONE, would prove to be an important tool in investigating this marine mystery.

P. Sean McDonald/Washington Sea Grant
A minnow trap containing several European green crabs.

Coastal invaders

European green crabs first arrived on the East Coast of North America in the 1800s, likely carried by merchant ships from Europe. But they didn’t appear in coastal Washington until the late 1990s.

“European green crabs are globally damaging invaders,” said Emily Grason, program manager of Washington Sea Grant’s Crab Team. “They’ve got a pretty bad reputation for changing ecosystems where they show up and build large populations.”

They have a number of traits that make them successful invaders: a broad diet, a high tolerance for variations in water temperature and salinity, and a high reproductive rate compared to other crabs of their size.

Because of that reputation, wildlife managers acted quickly to intervene and control the green crab population. As a result, their numbers have remained relatively low, but they have nonetheless managed to establish a foothold in the region.

“At the root of this research were concerns from the shellfish growing community in Willapa Bay about failed Manila clam harvest,” said Alex Stote, the Crab Team’s coastal specialist, who has since moved into a role with the Washington Department of Natural Resources. “They rely on natural-set clams — that means they don’t purchase seed from elsewhere, they rely on the natural life cycle of the clams — but starting in 2015 they were seeing significant population declines.”

This phenomenon coincided with the same years in which shellfish growers started observing large numbers of green crabs in and around their shellfish gear for the first time. Given the suspicious timing, the Crab Team got to work investigating whether the two trends were linked.

Alex Stote/Washington Sea Grant
Professor P. Sean McDonald sets crab traps in Long Beach, Washington, at sunrise.

A genetic mystery

The team trapped and harvested green crabs during the warmest months, when the crabs’ metabolic demands are highest. They sourced 60 large, adult male crabs captured in Willapa Bay, hypothesizing that these would be the most likely crabs to be able to crack the Manila clams’ hard shells. Then, they analyzed the crabs’ stomach contents using a relatively novel DNA analysis technique.

“There are two main kinds of DNA analysis you can do in this situation,” Fisher said. “There’s qPCR, which you can think of like a carefully made fly-fishing fly, designed to specifically target one species in their stomach contents. Then there’s the one we used, known as DNA metabarcoding. This works more like a fishing net — it’s somewhat selective, but you’re still picking up a bunch of different species at the same time.”

The analysis revealed a complicated picture.

P. Sean McDonald/Washington Sea Grant
Researcher Alex Stote sets traps in a channel in Nahcotta, Washington.

The big reveal

Unfortunately for the Manila clam growers in Willapa Bay, this research may have raised more questions than it answered.

“Only one of the 60 crabs we looked at had recently consumed Manila clams,” Fisher said. “That was a really surprising finding, because most of these crabs were trapped right on the clam beds. It tells us that larger male crabs do sometimes eat Manila clams, but we didn’t observe that consumption frequently enough for us to be able to point a finger at green crab yet as the only culprit in clam harvest declines.”

Because the number of crabs they studied was relatively limited, other possibilities remain. For example, smaller green crabs could be the ones responsible for eating Manila clams, or the large adult crabs could be eating them more at different times of year. Large enough green crab populations could even be causing physical disruptions to the clams’ habitat that make it harder for them to survive.

The researchers were quick to point out that these possibilities are simply speculation, and a definitive answer would require more research.

P. Sean McDonald/Washington Sea Grant
Researcher Mary Fisher dissects a European green crab for diet analysis in a UW laboratory.

Impacts across the ecosystem

Even though the solution to the mystery remains elusive, the study also revealed quite a bit about what green crabs eat in general — and where they may already be having impacts on Washington’s coastal ecosystems.

“One of the species they seemed to eat quite a lot of was our native hairy shore crab, which, as we’ve seen in California, is a population that can really be impacted by green crab invasion,” Grason said. “We’re working on further research now to investigate how significant those impacts might be.”

One concern is that the green crabs’ favorite foods might overlap with the diets of native species. For example, large enough green crab populations could potentially outcompete Dungeness crabs for prey species like Crangon shrimp, indirectly impacting Dungeness populations as a result.

The one thing researchers can agree on is that green crabs pose a substantial threat to Washington’s native species and aquaculture industry alike. But Fisher’s DNA techniques represent a new tool in the toolkit of those working to keep them in check, so a solution to the mystery may be just around the corner.

Story by Will Shenton.