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Mudbug menace: how crayfish are taking over the world, and how to stop them

An invasive Louisiana red swamp crayfish standing in the grass, spreading its claws toward the camera in a threat display.
Andrew Cannizzaro/Flickr
A Louisiana red swamp crayfish spreads its claws in a threat display. This crayfish was found in Fairfield, Ohio, hundreds of miles from its native range.

A lazy summer day is a perfect time to head out on the water with rod, reel, and a bait bucket full of crayfish. But not all crayfish are the same, and there’s decent odds that that bucket contains the ecological equivalent of a hand grenade – an invasive species that is quietly displacing native species and transforming ecosystems across the state.

There are three non-native crayfish species of concern in North Carolina, all of which are fast breeders, voracious omnivores, and tough survivalists that have usurped waterways across the country. But two species, rusty and virile crayfish, are only established in a few spots. Sightings of the third species, the Louisiana red swamp crayfish, dot the entire state map like a bad case of chicken pox.

Red swamp crayfish are here to stay, but there are easy ways to limit their spread.
Michael Perkins, aquatic wildlife diversity biologist with the Inland Fisheries Division of the North Carolina Wildlife Resources Commission

Michael Perkins, aquatic wildlife diversity biologist with the Inland Fisheries Division of the North Carolina Wildlife Resources Commission, thinks that Louisiana red swamp crayfishes’ success boils down to their use as a food source.

“If most people go to a low country boil, a crayfish boil, it’s red swamp crayfish,” Perkins explains.

While crayfish can walk on land for a surprisingly long distance, colonizing new waterways usually requires human help. And the red swamp crayfish is farmed both locally and out of state, making it easy for humans to find. The other two invasive crayfish species are illegal to own or transport in North Carolina, but no such restrictions limit Louisiana reds. Deliberately stocking them requires a permit, but acquiring them as bait – and releasing them if the day’s fishing doesn’t work out – remains all too easy.

And once established in a body of water, invasive crayfish are there to stay. Louisiana reds in particular are incredible survivalists. They can deal with fresh water, slightly salty water, extended periods of drought, low oxygen levels, and temperatures ranging from Gulf Coast summer to winter in Washington State. They’re found across the country and have established populations on every continent except Australia and Antarctica.

Behavioral differences between invasive and native crayfish might also contribute to the newcomers’ success, but not necessarily in the way you’d expect. To understand why virile crayfish are invasive in Alabama, current North Carolina State graduate student Alex Rocco set up cage matches between virile crayfish and native crayfish. He expected the invasive crayfish would win fights by being more aggressive. Instead, he found that matches between two invasive crayfish were much less likely to become violent than any other species combination.

Rocco speculates that virile crayfishes’ lower territoriality allows them to live at high density. “If they’re completely crowding out the other crayfish who are less tolerant of sharing space with other individuals, then that’s going to basically create a big advantage for them.”

Close-up photo of a virile crayfish on a dark blue background.
Alex Rocco/Jacksonville State
A virile crayfish. This species’ tolerance of crowding may help it displace native crayfish species.

Living close together seems to present no problem for the Louisiana red swamp crayfish, either.

Bronwyn Williams, research curator for non-molluscan invertebrates at the NC Museum of Natural Sciences, describes a research trip to the coastal plains as, “you basically take a dip net in there and you just start kind of scooting it through and oftentimes you’re pulling out dozens of red swamp crayfish in a single netful. I mean they are thick in the swamps.”

This is bad news for freshwater ecosystems. Perkins describes the Louisiana red swamp crayfish’s impact on the ecosystem as “mass devastation at every level.” Crayfish are what ecologists call “keystone species”: organisms that are critical for the structure of entire ecosystems. They eat a wide variety of foods, from leaf litter to insect larvae, and are in turn eaten by fish, amphibians, and birds. Replace a native crayfish (or, more likely, a community of several more-specialized native crayfish) with a different species, and the whole ecosystem becomes unstable.

“Think of it as Jenga,” says Williams. When invasive crayfish take over, “you won’t see anything for a while, and then maybe they just knock out just one thing that you’re not expecting. And next thing you know, you don’t have the insect diversity. Now, maybe that means that the amphibian diversity is dropping because they no longer have a food source. And same thing with the fishes, now you lose kind of that diversity of fishes.”

This paints a pretty dire picture for anywhere the invasive crayfish are introduced, but not all hope is lost.

“Red swamp crayfish are here to stay,” Perkins says, “but there are easy ways to limit their spread.”

Invasive crayfish get to new waterways largely by being released from bait buckets. Anglers can stop their spread by using the crayfish that are already in the waterway and not moving bait from place to place. Human actions have allowed these unassuming mudbugs to colonize the state and the globe, and human actions can still protect the waterways where invasive crayfish have not yet gained a clawhold.

Want to learn more about invasive species? CREEP is a podcast about creatures invading our space, and changing the world around us, presented by WUNC and the North Carolina Museum of Natural Sciences. Listen and subscribe wherever you get your podcasts.

Editor's Note: An earlier version of this story incorrectly identified the organization Bronwyn Williams is affiliated with.

Sophia Friesen is a science writer and WUNC’s 2022 AAAS Mass Media Fellow. Before working with WUNC, they wrote for science news outlets including Massive Science, preLights, and the Berkeley Science Review, covering everything from wildfire mitigation to pterosaur flight abilities.