The mystery of the mitten crabs

Wildlife-police raids and customs inspections are trying to halt this highly invasive species

May 22, 2020
Mitten Crab Image
Maybe cute, often delicious, definitely illegal [Credit: Smithsonian Environmental Research Center | licensed by Smithsonian].

Freshly steamed mitten crab meat punctuated with rich, golden fat is so delicious — and so valuable — that every year tens of thousands of the highly invasive crabs are illegally live-shipped from Asia to America as a Chinese New Year delicacy. When some of those crabs escape to the wild, though, their populations can soar so fast that they overrun fragile ecosystems like the San Francisco and Chesapeake bays.

A newly formed team in the U.S. Fish and Wildlife Service recently responded with its first-ever targeted operation aimed at cracking down on the crab trade. Their seizure of 15,000 crabs even had a catchy code name: Operation Hidden Mitten.

Because the crabs have to be shipped alive in order to command market prices, they usually aren’t particularly well hidden, says Barbara Hassan, a supervisory agriculture specialist for U.S. Customs and Border Protection, which works alongside the U.S. Fish and Wildlife Service to stop the illegal crab trade. They are simply tied up so they can’t escape and shipped in boxes to the U.S. Still, Hidden Mitten was a fitting name because the Asian crabs have already invaded coastal waters in the U.S. For the past decade, they’ve been hiding on both the east and west coasts.

The worry with mitten crabs is that — at times — their populations can surge. In the late 1990s, for example, northern San Francisco Bay was invaded by about three-quarters of a million mitten crabs, according to a study published in 2003. While they may seem relatively harmless, the crabs are known to burrow, the study authors point out — and when there are a lot of them digging burrows, they can damage and collapse levees. Additionally over the last century, several reports have shown that mitten crabs can carry a parasitic worm, which can dig into the lungs of humans or animals when they eat the crabs, biologist Andrew N. Cohen pointed out in 2003.

The San Francisco invasion probably started when a few illegal imports escaped into the bay, according to Brian Tsukimura, a biology professor at Fresno State University and former president of The Crustacean Society, an organization of scientists who study crabs, lobsters, and related species. 

The crabs naturally seek out saltwater to breed, so any escapees probably made the journey from city to bay by crawling along river banks, Tsukimura speculates. Farther inland, the crabs have scurried across parking lots and invaded irrigation systems, according to the Los Angeles Times.

“You could see them,” Tsukimura says, “run right past the feet of egrets and cranes.” The birds simply didn’t know the unfamiliar crabs were potential food.

By the mid-2000s, fishermen on the East Coast were also reporting mitten crab sightings in the Chesapeake Bay and the Hudson River, according to the Mitten Crab Watch database, which tracks citizen reports. But there hadn’t been as many as there were in San Francisco Bay.

After that, however, sightings on both coasts decreased drastically. Researchers still aren’t sure why.

The crabs aren’t gone, says Gregory Ruiz, a marine ecologist at the Smithsonian Environmental Research Center’s labs in Chesapeake Bay and San Francisco Bay. Even now, he still hears reports of occasional sightings. But since the population explosion in the late ’90s, Ruiz explains, the crabs haven’t been reproducing much.

“I don’t want to call it dormant,” Ruiz says, but “it’s almost below detection.”

So the crabs arrived, boomed, went bust, and are still here in low numbers. The same “boom-bust” cycle occurred in mitten crab populations in Asia and northern Europe, says Ruiz. “It’s not unique to the geographic region,” he explains, “but it may be driven by some changes in the environment year to year.” There may be a trigger — possibly as simple as the right amount of rainfall and temperature — that causes the crabs to reproduce like crazy, Ruiz says, but so far, no one’s solved that mystery.

“It’s very typical for invasive species to go through a boom and then a bust,” says Tsukimura. The reason for the “bust” in San Francisco Bay could have been a period of cold water temperatures, he says. Adult crabs can survive a range of conditions, but mitten crab larvae can’t survive temperatures below about 60 degrees Fahrenheit, according to a 1991 study. This means warming waters in response to climate change could increase the range of these invasive crabs, Tsukimura explains.

The potential for another population boom — and the damage it could bring to fragile waterways like the Chesapeake and San Francisco bays — is enough to keep government officials on alert, especially when it comes to illegal imports. 

Shipments of mitten crabs are usually seasonal, coinciding with Chinese New Year (in January or February) because the crabs are a holiday delicacy, says Hassan. “You know, people like their food — they like their cultural food,” she says. “This is no different. It’s like hot dogs on the Fourth of July.” 

As Chinese New Year approaches each year, customs officials find crabs during their routine inspections of incoming packages. This year, U.S. Customs, working with the U.S. Fish and Wildlife Service, intercepted 85 shipments of mitten crabs, which totaled over 3,000 kilograms worth of crabs, a U.S. Customs official said, and this is an increase over last year’s haul. Part of the reason they’ve seized so many is because the U.S. Fish and Wildlife Service and U.S. customs have been specifically targeting shipments containing the crabs with Operation Hidden Mitten and by monitoring illegal shipments from China. 

Cincinnati — a major shipping hub — is where a lot of the seizures occur. Since last October, U.S. Customs has captured over 1,500 kg worth of crabs in Cincinnati alone. The crabs are shipped to Cincinnati by air, Hassan explains, and because they are seasonal imports, officials are generally ready and waiting when they arrive. But it’s hard to say how many crabs get through undetected, Hassan says.   

Still, as Ruiz from the Smithsonian points out, the crabs are already here, in Maryland, Virginia, Connecticut, New York, and San Francisco, and there’s no current effort to control these populations that have already become established. The implication, of course, is that we don’t know when or if they’ll have a population boom, like San Francisco saw.

Establishing bounty systems to encourage fishermen to catch the crabs currently living in U.S. waters is probably a bad idea, Tsukimura says, because economically valuable species such as blue crabs might be caught and killed instead. In the Hudson River, “there’s no way to really control them,” agrees Catherine McGlynn, of the New York State Department of Environmental Conservation. Instead, the focus is generally on preventing the crabs from entering new waterways by encouraging people to clean, drain, and dry their boats when they move from one waterway to another, McGlynn says. “Success is containment.”

About the Author

Curtis Segarra

Curtis is a photographer and science journalist who focuses on health, Earth science, and ecology. Growing up in New Mexico, his life was centered around nature—hiking, biking, and exploring. When he wasn’t outdoors, he was reading (he loves travelogues). Later, while studying geology at Trinity University, he realized he could combine these passions by becoming a science journalist. Now, he uses his words and photos to help others see practical beauty in science.


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