Attack of the clones: the mysterious mutant threatening to invade Canada’s waterways

An alien crayfish has touched down in North America. Can Dr. Crayfish save us?

This marble crayfish, the only known decapod with the ability to clone itself, was photographed in Ontario with a permit. Once sold freely in the pet trade, this arthropod is now prohibited throughout Ontario. If spotted, marbled crayfish should be reported to the Invading Species Hotline (1-800-563-7711) or online at www.eddmaps.org.
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There’s something strange maneuvering its way into our ponds, rivers and lakes. 

It’s a mutant. It’s a strong, independent, all-female species — with an extra set of chromosomes — that literally don’t need no man. It has murky origins in post-cold war Germany’s aquarium trade. It’s already been unleashed on central Europe, Madagascar and Japan to devastating effect. It’s self-cloning, spawning hundreds of identical progeny at a time. It eats anything, from rotten leaves and snails to fish broods, small fish and insects. It’s a five-metre-long sabre-toothed reptile with a taste for human flesh. 

Okay… that last sentence is a lie. But the rest really is true.

The mutant-in-question is the 10-centimetre marbled crayfish, Canada’s latest invasive species. And while it might not be the pet-of-a-Bond-villain you were imagining, don’t let that fool you; the threat Procambarus virginalis poses is very real. 

Its October 2021 discovery in City View Park in Burlington, Ont. — the first time one has been seen in the wild in North America — and subsequent formal identification last summer has biologists and environmental officials scrambling. How do you stop such a tiny creature that’s able to hide away in murky ponds, and doesn’t need a mate to produce a seafood boil’s worth of baby crayfish? 

This is a question for Dr. Crayfish.

Also known as Dr. Premek Hamr — a retired schoolteacher originally from the Czech Republic and current resident of the Kawartha Lakes, Ont. — is the authority on crayfish in Canada. Hamr, who goes by the moniker @DrCrayfish on social media, was researching crayfish long before other invasive crayfish, like the rusty crayfish, made crayfish research cool. What started with an off-chance encounter during an undergrad field project on Montréal’s Nun’s Island in the ‘80s ultimately became a lifelong passion.

“The coolest things in the water that day were crayfish,” laughs Hamr. “So I did a little project. Then I did my Honours the following year on crayfish… then got a full scholarship to Trent University in Peterborough, Ontario, to study my master’s degree on crayfish.”

Premek Hamr, AKA Dr. Crayfish, pictured through an aquarium in his Kawartha Lakes home.
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Back then, there were no invasive crayfish to worry about. Hamr was focused on native species like the Appalachian brook crayfish, which lives in waterfalls (“it used to be my favourite crayfish, but the Americans changed its name to the common crayfish, which I don’t agree with because it’s not common,” he says) before his work eventually took him to Tasmania for a PhD on the world’s biggest crayfish, the Tasmanian giant freshwater crayfish. 

After seven years in Tasmania, Hamr returned to Canada where he lectured at Trent University in Peterborough, before taking up a highschool science teaching position in Toronto. For 20 years, Hamr continued his research during summers, regularly publishing papers on crayfish. 

“That way, I became kind of ‘the guy’ for crayfish in Canada,” he says. “Now, I’m retired — and all of a sudden we have all these invasive crayfish entering the country.”

Hamr was the one who first identified the Burlington marbled crayfish in 2021. While it isn’t the first non-native crayfish species to invade Canada, marbled crayfish — or “marbles,” as Hamr calls them — put the “cray” in crayfish. 

Marbles are the result of another species — the slough crayfish — mutating in an aquarium in Germany in 1995 after being imported from Florida by shady means. They are asexual and triploid, meaning they have three sets of chromosomes rather than the usual two. While triploidy is occasionally observed in wild slough crayfish, the offspring of triploid crayfish don’t normally survive. But somehow, a triploid slough crayfish mutated in a way that meant it could not only survive its genetic condition, but also reproduce asexually, i.e. without a mate. Thus, the marbled crayfish was born.

Hamr says it’s possible that, en route to Germany in a cargo hold, the slough crayfish was exposed to cold temperatures — used in laboratories to induce mutations — leading to the creation of the first marbled crayfish. Many aquarium fish are predatory and need live food, meaning these fast-multiplying crayfish make the ideal self-sustaining food source. So Hamr’s hypothesis is that pet stores likely started culturing them soon after their miraculous conception in 1995. 

A map of the Lake Muskoka and Moon River area of Ontario where other invasive crayfish species have already run riot, spread through connected fresh waterways
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A collection of crayfish species that can be found in Ontario and Quebec, including: Northern clearwater, rusty, big water, virile, and Appalachian brook crayfish
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But, if you start with one, within a couple of years you could have hundreds in your tank. “Unless you have a lot of fish to feed,” says Hamr, “what are you going to do? Sell them or dump them.”

In the decades following their creation, “marbled crayfish for sale” became a common listing in Canada and the U.S. on sites like Kijiji, eBay, and Facebook Marketplace, and in many states and provinces they are still legal to trade. Despite this, they were never found in the wild in North America. 

Until, one October day in 2021, a curious photo was uploaded to species identification site iNaturalist.

As well as interacting with his X followers, Hamr is a big presence on the crayfish pages of iNaturalist. Amateurs and curious citizens often upload geo-tagged photos of a crayfish they’ve found with a label like ‘unidentified North American crayfish.’ Hamr makes regular forays into iNaturalist, combing through vaguely-defined photos and using his expertise to correctly identify any mislabelled species. In late 2021, during one of these forays, he came across a photo of an unidentified crayfish walking on a path in a park in Burlington. 

“My first reaction would have been ‘oh crap,’ or something ruder than that!” says Hamr. “…it’s finally here.”

Hamr was fairly sure the photo was of the infamous marbled crayfish and reported it straight away. He then sent the photo to some friends of his researching marbled crayfish at the University of Southern Bohemia in the Czech Republic. They confirmed his fears to the extent that was possible based on the dorsal view shown in the photograph.

After years of unrestricted online trade, the inevitable had happened. “I’m still amazed that they hadn’t been found anywhere else,” says Hamr. “So now we in Ontario have the dubious honour of having the first wild population of marbled crayfish in North America.”

The Marbled Crayfish Working Group was swiftly assembled, consisting of officials from the City of Burlington, Fisheries and Oceans Canada, local conservation group Conservation Halton, Trent University, the Ontario Federation of Anglers and Hunters, and Ontario’s Ministry of Natural Resources and Forests. “And, of course, me, because I’m the crayfish mercenary,” adds Hamr.

The underside of a marbled crayfish, with hundreds of eggs stored beneath its abdomen. Because marbled crayfish can clone themselves, only one egg is needed to establish a new population.
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Brook Schryer, assistant coordinator of the Invading Species Awareness Program — was part of that team. When Hamr notified Schryer about the suspected marbled crayfish sighting in the Burlington park, Schryer wanted to get out and do some surveillance as quickly as possible. 

“It was late in the year, well beyond good monitoring time for crayfish,” says Schryer. “But this is such a high profile crayfish that I didn’t really care. I was ready to dive into the cold water and take a look.” 

Schryer’s urge to act was understandable. On top of the damage that invasive species inflict on native wildlife populations, an intergovernmental report released last year found that the global economic cost of invasive species exceeds US$423 billion per year. And, according to Environment Canada, Canada alone loses up to $35 billion a year thanks to only 16 invasive species — just 1.1 per cent of the estimated 1,442 invasive species currently in Canada.

Past invasions from other crayfish also point to why it’s important to take early action. Likely introduced to Ontario by anglers in the ‘60s, the rusty crayfish hails from the Ohio River Basin in the U.S. ‘Rusties’ are large, fearless and aggressive; since their introduction, they’ve spread across the province and in many areas nearly eliminated the native crayfish Hamr has long held dear.

“In Southern Ontario, if you catch a crayfish, you have at least a 50 per cent chance of catching a rusty,” says Hamr. In his home in the Peterborough region, native crayfish are pretty much gone. He’s forced to search out remote places — uncommon in Southern Ontario — and get into headwaters. 

“If you’re lucky,” says Hamr, “and if there’s some sort of a natural barrier, only then might you find native crayfish.”

Canada has already lost many native crayfish populations thanks to competition from rusties, and with them now spreading into Quebec and most recently Manitoba (where they are prohibited federally, the only federal-level crayfish ban in Canada), more could follow. 

Still, cooler heads prevailed back in 2021 — even if they had identified the presence of at least one marbled crayfish, there was nothing they could do at that time of year — and the search was delayed until spring 2022. 

On their first visit to City View Park in spring 2022, Schryer and Hamr drove around fruitlessly, even entering a barricaded area, before they were approached by the park’s groundskeeper. Luckily for them, the groundskeeper was a member of the Ontario Federation of Anglers and Hunter, and when he found out who they worked for, he was keen to help out.

When he heard they were looking for crayfish, the groundskeeper (who has asked to stay anonymous) pulled out his phone and showed them a photo of a crayfish he’d taken on the park’s soccer field the previous fall. Sure enough, it was a marbled crayfish.

This was important; the original crayfish photo submitted to iNaturalist was not conclusively a marbled crayfish. It was a dorsal view, meaning certain key identifying features weren’t visible. But the groundskeeper’s image was side-on, and left Hamr in no doubt.

“You could clearly see the marbling on the carapace,” says Hamr. “That picture actually proved that they were marbles. And, to give credit to the groundskeeper, it was taken before the iNaturalist photo, so his photo was actually the first record of a marbled crayfish in North America.”

Premek Hamr collects data on the diversity and abundance of crayfish species in Ontario. Working alongside the OFAH, a combination of surveying and eDNA samples will be essential in tracking the spread of marbled crayfish.
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Spurred on by the groundskeeper’s photo, Schryer and Hamr tried numerous monitoring techniques — baited minnow traps, sweeps with dip nets and the collection of environmental DNA (eDNA) — to hone in on what was now Canada’s most wanted crustacean. But everything came back negative — even the eDNA samples, used to determine if a scrap of marbled crayfish DNA was hanging around in the park’s ponds. 

Still, something didn’t sit right. Two positively identified photos coupled with the marbled crayfish’s ability to procreate asexually meant further checks were needed. Later that summer, Hamr and Schryer returned to the park, focussing primarily on eDNA sampling. The samples were sent off to the lab of Chris Wilson, adjunct professor at Trent University. This time, the results came back positive. 

“But you don’t know if eDNA comes from a carcass, or from droppings from a bird, or whatever,” says Hamr. “It can be from anywhere.” 

They needed to catch a crayfish red-handed. It was time for a more proactive approach. In an attempt to freeze the crayfish out, the Working Group and the City of Burlington decided to dewater several of the park’s ponds. 

The ponds were drained over the winter of 2022-2023, ensuring that crayfish-blocking filters were used and that the water was pumped to a location far from any other bodies of water. Expert advice from Hamr’s colleagues in the Czech Republic said that the best method for eradicating the species is exposing them to cold temperatures, and by dewatering the ponds, the crayfish would have nowhere to go.

The marbled crayfish is nothing if not resilient, however. Burrowed into the substrate, a marbled crayfish needs exposure to temperatures of around 1 to 2 degrees Celsius for at least two weeks. And in a winter like we had in 2022, in a part of Canada like Southern Ontario, those temperatures just aren’t guaranteed. The mild winter coupled with what Schryer suggests was suboptimal draining meant that the crayfish, if they were there, still hadn’t shown their faces. 

The response team continued to plot. Ongoing eDNA sampling continued to come back positive.

Then, in July of 2023, Schryer received a text from the same Burlington park groundskeeper who took that first photograph of a marble. Once again, he’d seen a crayfish — and this time, he knew to hold onto it.

“It was actually crawling on land like the original crayfish,” says Schryer. “And sure enough, it was a marble.”

The floodgates opened. Soon, two, three, four crayfish were captured in passive traps around the park. “It just started growing exponentially,” says Schryer. 

The fear now was that the crayfish, though relatively unlikely due to the geography of the location, could crawl across land into other bodies of water. Another sampling blitzkrieg ensued, this time in water bodies near the park. Chris Wilson, who could sample and process water for eDNA at a faster rate, was brought in. The DFO even deployed electrofishing gear, passing current through the water to zap unsuspecting crayfish. While some marbled crayfish eDNA did show up, no physical specimens were found. The Working Group theorized that, rather than crayfish being present, eDNA-bearing water was likely trickling through underground connections from the original pond. They moved on.

Members of the Marbled Crayfish Working Group search for crayfish in a City VIew Park pond in Burlington, Ontario. (Photo: courtesy of Premek Hamr)
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Confident that the population was contained to the Burlington park, another winter of pond draining went ahead — this time, ensuring the ponds were drained to as shallow a level as possible. While this past winter was once again mild, the de-watering was far more effective — or catastrophic, if you’re a marbled crayfish.

When one official visited the ponds towards the end of winter, he found a crayfish graveyard.

“He was able to hand collect 54 marbled crayfish that were just laying in the muck,” says Schryer. “Not burrowed, not dormant. Just dead.”

It was finally a sign that their efforts to slow and eradicate North America’s first wild population of marbled crayfish were harbouring results. 

Earlier this spring, the working group went back to City View Park. Things were much the same, though it appears the population of marbled crayfish has diminished. But Schryer and the rest of the team know they can’t afford to be complacent in their defence of Ontario’s waterways. “Ideally, we would eradicate them,” he says. “Unfortunately, they’re a very resilient species. All you need is one to survive, or one person to release another, and then we’re back to where we started.” 

The group will continue to monitor, and are hoping next winter brings a cold spell capable of ridding Burlington of the marbled crayfish once and for all.

The odds may be stacked in the marbled crayfish’s favour, but there’s a continued will to fight this latest crayfish invasion across local, provincial and federal levels. The eDNA blitz started by Hamr, Schryer and their colleagues will soon cover much of southern Ontario. And, upon Hamr’s recommendation — “he’s the genius behind all this,” says Schryer — the entire Procambarus genus, comprising of the previously-banned marbled crayfish along with another 160 species of crayfish, was prohibited in the province of Ontario as of January this year.

The marbled crayfish is not the only invader competing with native species. Also present in Ontario are rusty crayfish, two of which are pictured here. They were introduced from the U.S. and can be identified by the red spot on the sides of their thorax.
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There’s still the mystery of how those original marbled crayfish ended up in that Burlington park. Schryer points to the seemingly uncoincidental timing of its appearance. In January, 2022, just a couple of months after its discovery, the species was outlawed under the Invasive Species Act, prohibiting marbles from being bought, sold, traded, released, or possessed in Ontario. He suspects that somebody with marbled crayfish in their aquarium tank heard of the impending ban, got spooked and, out of guilt, dumped them. 

“I would always encourage people to come forward and say, ‘I apologize, I have this and I didn’t realize what it was,’” says Schryer. “There is a high probability that we will just come and collect that specimen or show them how to euthanize it.” The Ontario Ministry of Natural Resources and Forests will always take an education-first approach, he says — unless you’re among those brazen enough to list marbled crayfish on Kijiji as “prohibited crayfish for sale.”

“Then, you’ll likely be charged under the Invasive Species Act,” says Schryer.

Hamr believes that to prevent marbled crayfish from following in the footsteps of other invasive crayfish, they need to be completely outlawed across Canada, not just Ontario. 

A “spirit crayfish” — the white morph of the Northern clearwater crayfish found only in Ontario’s Lake Simcoe. (Photo: courtesy of Premek Hamr)
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A scientific illustration of the Northern clearwater crayfish made by Premek Hamr (Illustration: Premek Hamr)
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In October 2023, not long after Burlington’s marbled crayfish issue hit the news, Nova Scotia became the second province in Canada to have a wild marbled crayfish reported. Sarah Kingsbury, a senior aquatic invasive species biologist with Fisheries and Oceans Canada, told CBC’s Information Morning that a potential marbled crayfish had been identified in Yarmouth County. They set their own traps and succeeded in capturing a marble. 

“We — that is, the provincial and federal governments — have to do something about it,” says Hamr. 

Hamr will do his part, “retired” or otherwise — he is Dr. Crayfish after all. He’s giving workshops to federal and provincial employees, and he’ll be patrolling iNaturalist for any sign of these mysterious mutants in Canada’s waterways (or soccer fields). He’s the hero Ontario’s native crayfish deserve, not that Hamr sees himself that way.

What truly motivates him is the preservation of the native crayfish species he loves. He’s just published a new guidebook to Ontario’s crayfish (with help from Schryer, DFO, and the OFAH), and recently decided on a new favourite crayfish: a unique white morph of the Northern clearwater crayfish that lives only in Ontario’s Lake Simcoe. “It’s not an albino,” he profuses. “It’s kind of like the white bears in British Columbia. It’s a spirit crayfish.” He’s worried they’re going to be wiped out by invasives. 

In a world where biodiversity is vanishing at an alarming rate, it’s easy to dismiss the fates of creepier, crawl-ier creatures in favour of their cuddlier kin. But in a place like Southern Ontario, where highways and highrises rule, the idea that there are still these unique, translucent-white gems glimmering below the clear headwaters of a mountain stream, or shimmering as they catch a ray of sunlight in a waterfall, seems a magic worth preserving. And the idea that those lights could now be extinguished one by one, fallen victim to the mutant clones of a man-made species that should never have been, seems like such an avoidable shame.


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