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Invasive crayfish are dying in the Midwest. Could a fungus be the cause?

Most mornings when Eric Larson hauls a wire-mesh trap into his fishing boat on northern Wisconsin’s Trout Lake, he finds dozens of coffee-colored crayfish wielding their oversized pinchers in a frenzy, trying to fight their way out.

Larson, an ecologist at the University of Illinois Urbana-Champaign, records each animal’s shell length and sex. Then he places them into a bucket of water laced with fish toxicant, an ethically approved form of euthanasia for this invasive species. At Wisconsin’s Trout Lake, and other lakes nearby, Larson and his team can catch, study, and euthanize a couple thousand crayfish in just a few hours. 

Native to the Ohio River and named for the rust-colored splotches adorning their shell, rusty crayfish (Faxonius rusticus) arrived in northern Wisconsin in the 1960s when anglers released them as bait. The four-inch-long crustacean had few native competitors, and populations exploded: Within four decades, rusty crayfish inhabited an estimated 86 percent of Wisconsin’s watersheds.

The problem isn’t unique to Wisconsin: As of 2021, nonnative populations of rusty crayfish have appeared in 28 U.S. states, from Maine to Oregon. The opportunistic omnivores devour aquatic insects and fish eggs and bully local crayfish out of their habitat, throwing the entire ecosystem into disarray. (Read how antidepressants in waterways could make crayfish bolder.)

Since the crayfish arrived in Trout Lake in 1979, for instance, aquatic snails and small fish such as bluegill have declined dramatically. Native crayfish species have fared even worse: A 2011 study found that native virile crayfish (Faxonius virilis) disappeared from more than three-fourths of Wisconsin lakes infested with rusty crayfish.

Crayfish in general are crafty at adapting to new environments. Consider the  signal crayfish: It’s native to parts of western North America but has wreaked havoc when it has spread to England, Sweden, and Oregon’s Crater Lake, where the crustaceans have pushed the endemic Mazama newt to the brink of extinction. Invasive crayfish prevail because, as expert burrowers, they can patiently wait out human attempts to trap or chemically exterminate them. 

“You do everything you can to not allow them to be introduced,” says John Umek, an ecologist at Nevada’s Desert Research Institute who has documented the signal crayfish’s spread throughout Crater Lake. “Once introduced, complete eradication is nearly impossible.”

However, about seven years ago, Larson began noticing something odd at Trout Lake: He was catching fewer and fewer crayfish. What he and others ultimately found could provide scientists with a rare—and effective—tool in fighting back against this tricky invader.

Are crayfish wrecking their own habitat?

To investigate the crayfish’s decline in Wisconsin, Larson and his colleagues pored over decades of trapping data recorded from 17 lakes throughout northern Wisconsin. The data showed that eight of the lakes experienced rusty crayfish declines between 1975 and 2017—and when Larson and his team looked for possible reasons, they discovered that the water bodies all had muddy or sandy bottoms.

As rusty crayfish forage, they uproot aquatic plants: Surveys of Trout Lake have found that aquatic plant diversity in some areas plummeted by as much as 80 percent after the arrival of rusty crayfish. But the crustaceans also need places to hide from hungry fish and other rusty crayfish that would prey on them. In lakes with plenty of rocks, the crevices between boulders offer the crayfish shelter—but in sandy-bottomed lakes, aquatic plants provide the only cover. It’s possible, Larson theorizes, that in places such as Trout Lake, crayfish have damaged aquatic vegetation so thoroughly that they had nowhere left to hide.

But that can’t be the whole story, Larson says, because even in some rocky lakes, rusty crayfish numbers are falling.

Lindsey Reisinger may have found another explanation. A freshwater ecologist at the University of Florida, Reisinger is compiling evidence that a previously undescribed fungal parasite has infected and weakened the crayfish, making them more vulnerable to predators such as great blue herons, perch, and bass. A member of the microsporidian genus Nosema, this fungus is related to the deadly Nosema apis, the parasite that is devastating honeybee colonies around the world.

During the past three years, Reisinger has been monitoring an outbreak of this new Nosema parasite in Wisconsin water bodies where rusty crayfish have declined, including Trout Lake.

Like other microsporidian parasites, the Nosema species appears to wreak havoc on the crayfish’s muscular system. In Reisinger’s lab, many of the infected rusty crayfish are more sluggish than their healthy counterparts. They also struggle to flip themselves over once capsized—a vital maneuver for life in turbulent waters. (Read how national parks are becoming overrun with invasive species.)

This fungus uses microscopic spores to infect new hosts. It’s unclear whether rusty crayfish are first infected by the spores themselves or by eating aquatic insects infected with the pathogen. Reisinger theorizes that the crayfish’s penchant for cannibalism may spread the parasite throughout the population.

 Though Reisinger can’t say definitively that the parasite is what’s driving down the crayfish numbers, she notes that “we’ve seen a really dramatic decline of the [crayfish] population at the same time that we’ve seen this outbreak.” Since native crayfish and other species have evolved in the presence of this fungus, they likely have built up immunity to it, she says.

A natural weapon against invasive crayfish

If crayfish continue to decline in Trout Lake, Reisinger believes wildlife officials could release additional parasitic fungi into other lakes with crayfish problems, to act as a natural biocontrol.

However, Reisinger stresses there is still much to be learned about the fungi before it can be harnessed to halt crayfish invasions. It remains to be seen whether this parasite, like many other microsporidians, is target-specific—that is, it only infects crustaceans like crayfish—or would be deadly to a broader range of hosts. Additionally, Reisinger is still unsure about how long outbreaks of these fungal parasites last. (Learn how rivers and lakes are the most degraded ecosystems on Earth.)

That said, as a pre-existing, native fungus, it has the appealing value of being a natural solution to an unnatural problem. “It’s not like we’re worried about bringing it there,” Reisinger says. “It’s already there.”

Other scientists are investigating how fungi can be weaponized against invasive species. For example, researchers would use a fungus that kills tree of heaven—a fast-spreading tree native to China that has invaded the United States—to make a commercial herbicidal product that’s injected directly into the plant.

Killer parasites and the rusty crayfish’s own self-destructive habits offer hope for restoring local ecosystems—but Larson stresses that the best way to control the crustaceans is to prevent their heedless introduction in the first place. For instance, he says, if organizations need to dispose of crayfish obtained for use as seafood or in research, the crayfish should be humanely euthanized instead of released into non-native environments.

Larson feels encouraged by the dwindling number of rusty crayfish tussling in his traps. “I keep thinking, Surely, they’re going to bounce back up,” Larson says. “But we haven’t seen that yet.”

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