Science & Nature

Have You Seen Little ‘Blue Lobsters’ Washed Up on Chicago Beaches? Congrats, You’ve Met One of the Great Lakes Most Successful Invaders

For an invasive species specialist like Reuben Keller, Lake Michigan is teeming with research opportunities.

“Almost everything that we see in the lake that is alive is invasive,” said Keller, a professor at Loyola University Chicago’s School of Environmental Sustainability. “Any rocks are covered in zebra or quagga mussels, both of which are invasive. Most of the time that we dive, the only fish that we see are round gobies, which are an invasive fish."

While some of those invasions are well documented — let’s not forget the dreaded sea lampreys, the stuff of nightmares — other incursions, though every bit as “successful,” have received less attention.

Consider the rusty crayfish (Orconectes rusticus).

Native to a small area of the Ohio River watershed, the rusty was most likely introduced to Lake Michigan as fishing bait, and in just a few decades has achieved utter dominance over the native crayfish that were once found off Chicago’s lakefront but are now nearly nonexistent.

Despite their rapid colonization of the lake, it’s easy to see how the crayfish have largely flown under the radar.

Crayfish are, by their nature, elusive to spot, Keller said. These nocturnal creatures live at the bottom of murky rivers and ponds or, in the case of Lake Michigan, make their homes miles offshore.

Rusty crayfish live well offshore of Chicago’s lakefront, but they tend to wash up on shore in winter for reasons researchers can only theorize. (Patty Wetli / WTTW News)

But they do occasionally turn up on Chicago’s beaches, typically this time of year.

Or at least winter is when Keller most often fields calls to his lab from people reporting evidence of strange creatures washing up on the lakefront, usually in the form of dismembered claws that look like the remains of little blue lobsters.

Those are rustys, Keller said.

He admitted he doesn’t really know why winter is peak season for rusty sightings: Maybe more of them starve and die in the cold and get flushed from their nooks and crannies. Or maybe storms bring them to the surface and toss them ashore. Maybe it’s just easier to spot their carcasses on an empty beach in February.

Whatever the cause, Keller said their appearance is a reminder of an underwater world that has little if anything to do with humans’ perception of the lake.

“Invasive or not, the fact that we see (rusty crayfish) washed up in large numbers on the beach tells us that there’s a lot of them out in the lake,” Keller said. “So I see it as being a sign there’s a whole lot of interesting stuff going on out there.”

Join us on a deep dive with Keller into the world of rusty crayfish.

What’s in a name?

First things first: What’s the difference between crayfish and crawfish?

Nothing. They’re the same creature, just two different names for these freshwater crustaceans.

Keller calls them “crayfish,” which is the preferred choice in the northern part of the U.S., as well as in his native Australia. People in the southern U.S. call them “crawfish.”

“It works either way,” Keller said.

What are they doing in Illinois?

There are hundreds of different species of crayfish spread across the world, but the highest diversity by far is found in the United States, particularly the southeastern U.S., which Keller called the “epicenter” of crayfish biodiversity.

The invasive rusty crayfish, seen here at Montrose beach in February 2025, is larger and more aggressive than native crayfish species. It's become the dominant species found in Lake Michigan by researchers at Loyola University Chicago. (Patty Wetli / WTTW News)

The best available records for Lake Michigan’s southern basin indicate there were two native species — the virile crayfish (Faxonius virilis) and the northern clearwater crayfish (Faxonius propinquus) — that have almost completely been replaced by rusty crayfish over the last 20 years. (Keller is quick to point out that the lake was already heavily altered by human activity before people started paying attention to its crayfish population, so at some point there could have been other species.)

“They’re just outcompeted,” Keller said of the natives. “The rusty crayfish are bigger, they’re more aggressive."

How this competition plays out is a bit of a mystery though.

“We don’t get to see a lot of what really happens … 10, 20, 30 up to 70 feet underwater in Lake Michigan,” Keller said. "What we do know is that in areas where there used to be really good populations of the native species, we can now only find the invasive rusty crayfish.”

It’s hard not to be impressed with the rusty’s power play, Keller said.

“I admire what they can do. Just about anywhere that we find crayfish, most of the individual organisms that we find are rusty crayfish. And that’s whether we’re sampling in a shallow mucky pond in a Chicago park, or if we’re 50 feet underwater in Lake Michigan, and everywhere in between. They’re just a highly successful, highly adaptable species.”

How destructive are they?

A larger mussel shell on Montrose beach is evidence of native species now dominated by zebra and quagga invasives. Researcher Reuben Keller said in all his dives in Lake Michigan, he's never come across a live native mussel, only dead ones. (Patty Wetli / WTTW News)

The rusty crayfish arrived on the scene at roughly the same time as the infamous zebra mussel.

“Zebra mussels have fundamentally rearranged the Great Lakes ecosystems. There have been these massive changes, the system is in huge flux,” Keller said. “It’s difficult to then point to anything that’s going on and say, ‘I know the crayfish must have done this.’”

But in systems outside Lake Michigan, where rusty crayfish are the primary invader, their impact can be easier to observe and measure.

“They’re the largest invertebrate in freshwater ecosystems, so just by their size they have the ability to do things that a lot of other organisms don’t,” Keller said.

When their population reaches high densities, rustys, which are omnivores, can exhaust their sources of protein-rich food, like dead fish or snails. So they’ll start eating the aquatic plants, and because they live on the bottom of a water body, that’s the part of the plant they eat.

“They get a tiny little piece of food, but they’ve killed the whole plant that lives up above,” he said. “The aquatic plants, they can just disappear, and that in turn leads to declines in populations of fish because the fish need the plants for shelter.”

Do native crayfish stand a chance?

Maybe.

Over the course of several years, Keller and his research team have methodically worked their way up the entire Lake Michigan shoreline of Illinois, starting in the south and gradually moving north.

“For a long, long time, we’d been doing dive after dive and it’s rustys, rustys, rustys,” he said.

Until one day, on a dive about four years ago, things changed.

Keller pulled out a crayfish. “‘Wait, that’s not a rusty. What’s that?’ I was super excited finding the natives.”

At roughly Wilmette, the team found an area where rusty crayfish and natives overlap. North from there, it’s only the natives.

There’s no obvious environmental change that would explain why the natives are hanging on along Illinois’ northernmost shore, Keller said, while hastening to add he’s not a crayfish and maybe there’s an environmental factor they perceive that he can’t.

A rusty crayfish claw, seen at Montrose beach in February 2025. Crayfish use their claws (called chelipeds) both to catch prey and to defend themselves. (Patty Wetli / WTTW News)

“My suspicion is that the rustys are moving north and if we go back, we’ll find that they’re another half-mile, mile further north,” he said. “I hope that I’ll be able to get some funding to go back in another three or four years and see what’s going on. It’ll be interesting to go back.”

Is controlling the rusty crayfish a lost cause?

“That’s a question I struggle with,” Keller said.

The genie may be out of the bottle in Lake Michigan, but unlike terrestrial ecosystems, where invasive species can spread across connected landscapes, freshwater habitats are often self-contained.

“If you have two ponds that are a mile apart, or two lakes, there’s no way for a freshwater species to move from one to the other,” Keller said. “So that means that preventing the spread of freshwater invaders is something that we can do.” (Keller’s team is actively working to confine an invasion of red swamp crayfish in the Chicago River.)

Regulations have caught up and have teeth, he said, with it now being illegal in Illinois to use rusty crayfish as bait.

Every few months, Keller said, conservation police officers with the Illinois Department of Natural Resources send him photos saying, “‘We just grabbed this bucket of crayfish from so and so, or we found it at a live food market.’ And they want my help identifying the species. So they’re active and doing good work.”

Contact Patty Wetli: [email protected]

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