Scientists Discover Growth Limit for North American Crayfish

From Backyard Brag to Scientific Discovery

Caitlin Bloomer thought she had the catch of a lifetime when she pulled an enormous crayfish from Kentucky mud during a biological survey. She nicknamed it “Crawzilla” and was convinced no one had ever documented a larger specimen. That confidence lasted about five minutes.

When she showed her prize to a colleague, his response was predictably skeptical: “No way, I’ve caught bigger.” What started as friendly rivalry between scientists quickly evolved into something far more interesting: a nationwide hunt through museum collections and research institutions to answer a deceptively simple question: just how big can North American crayfish actually get?

“I thought, ‘There’s no doubt I have caught the biggest crayfish anyone’s ever seen!’ But as all biologists and sport fishermen do, he took one look and said no way,” Bloomer, a teaching assistant professor at the University of Illinois Urbana-Champaign, recalled with good humor.

The Great Crayfish Search

What emerged from that modest disagreement was a collaborative effort that spanned the country. Bloomer contacted biologists and museum curators, asking them to examine their preserved collections for unusually large specimens. Using precision calipers, researchers compared sizes across thousands of specimens, ultimately identifying 20 exceptionally large individuals representing 18 different species. Their findings were published in the journal Ecology.

Crawzilla didn’t claim the crown, but Bloomer wasn’t disappointed. Her specimen made the top 20 and was notably the only burrowing crayfish to make the list. “Mine was not the biggest, but it did make the top 20, so I’m super proud of it,” she said.

The actual record-holder was a Tennessee bottlebrush crayfish preserved at West Liberty University in West Virginia. Collected in 2011 by Dr. Zac Loughman and his students, this champion measured 86.6 millimeters (roughly 3.4 inches) excluding its tail, claws, and antennae. To put that in perspective, the carapace alone stretched about the length of a credit card, and with all appendages included, it nearly spanned a sheet of notebook paper.

The Mystery of the 80-Millimeter Ceiling

Here’s where things get genuinely fascinating. Despite examining specimens from different species, geographic locations, and environments, the researchers noticed something striking: North American crayfish seemed to max out around 80 millimeters. Every single species appeared to hit this invisible ceiling.

This raised an uncomfortable scientific question: why? Some crayfish species elsewhere in the world, like the Tasmanian giant freshwater crayfish, grow dramatically larger. So what’s stopping North American crayfish? The team investigated the usual suspects.

They examined latitude, oxygen availability, habitat type, and environmental conditions. They looked for patterns that might explain why certain species or individuals reached larger sizes than others. But nothing clicked. Geographic location didn’t correlate with maximum size. Water oxygen levels showed no meaningful relationship. Habitat type appeared irrelevant.

“We weren’t seeing any patterns,” Bloomer explained. “So we started thinking maybe this isn’t about the environment at all.”

The Biology Behind the Barrier

This realization pointed researchers toward an intriguing hypothesis: the growth limit might be biological rather than environmental. Perhaps the crayfish’s own physiology imposes constraints on how large their bodies can become.

One leading theory involves their exoskeletons, the hard outer coverings that protect them. Differences in shell structure or muscle anatomy could impose physical limits on body size, much like the skeleton of a land animal constrains how large it can grow. But this remains speculative for now.

“This was a case where we saw something unusual in the field and it sparked a future research direction,” Bloomer said. “We think that it could really be worthwhile to look at the physiology of crayfish to see if something there is influencing their body size.”

What makes this discovery even more remarkable is that crayfish are among the most familiar aquatic animals in North America. They’re the gateway creature for countless children exploring streams. Yet despite their ubiquity, fundamental questions about their biology remain unanswered.

This work also highlights the unexpected value of museum collections. By examining preserved specimens accumulated over decades, scientists answered questions that field research alone could never address. The collaboration between institutions, including the Illinois Natural History Survey and the Smithsonian Institution’s National Museum of Natural History, proved essential.

One giant crayfish and a moment of scientific skepticism opened an entirely new research frontier into what controls the size of these familiar creatures.

Source: College of ACES at the University of Illinois Urbana-Champaign