A giant crayfish nicknamed “Crawzilla” has sparked an unexpected scientific discovery, revealing a possible biological limit to how large North America’s crayfish can grow.
The investigation began when Caitlin Bloomer pulled an enormous Crawzilla crawdad from the mud during a biological survey in Kentucky. Crawzilla is the creature’s actual common name, and Bloomer was convinced she had captured the largest crayfish ever documented. But when she proudly showed her discovery to a colleague, the reaction was not quite what she expected.
“I thought, ‘There’s no doubt I have caught the biggest crayfish anyone’s ever seen!’ I brought it back to campus to show my colleague, naturally bragging about having caught the biggest-ever crayfish. But, as all biologists and sport fishermen do, he took one look and said, ‘No way, I’ve caught bigger,'” said Bloomer, a teaching assistant professor in the Department of Natural Resources and Environmental Sciences within the College of Agricultural, Consumer and Environmental Sciences at the University of Illinois Urbana-Champaign.
A Nationwide Hunt for North America’s Biggest Crayfish
What started as a little friendly rivalry soon turned into a nationwide search for record-breaking crayfish. Bloomer contacted biologists and museum curators across the country, asking them to examine their collections for unusually large specimens.
Using calipers, instruments designed to measure objects with great precision, the researchers compared the sizes of preserved crayfish. Their search produced a list of 20 exceptionally large individuals representing 18 different species. The results were published in the scientific journal Ecology.
Although Bloomer’s original Crawzilla did not claim the top spot, it still earned a place among the largest specimens identified.
“Mine was not the biggest, but it did make the top 20, so I’m super proud of it,” Bloomer said. “And it was also the only burrowing crayfish to make the list, so I’ll take the win.”
Scientists Identify the Largest Known North American Crayfish
The biggest specimen belonged to a Tennessee bottlebrush crayfish preserved in the collection at West Liberty University in West Virginia. It measured an impressive 86.6 millimeters, or approximately 3.4 inches, excluding its tail, claws, and antennae.
The record-holder was not a recent catch. It had been collected in 2011 by Dr. Zac Loughman and students from West Liberty University, making the discovery possible through the examination of existing museum specimens.
“I knew we had some massive crayfish in the West Liberty University Astacology Collection, but it was a surprise to learn we had the two largest specimens known from North America! Our record-holder, a Tennessee bottlebrush crayfish collected by Dr. Zac Loughman and West Liberty students back in 2011, shows just how amazing and diverse crayfish can be,” said Zackary Graham, an assistant professor of biology at West Liberty University. “Its carapace alone is about the length of a credit card, and with the tail and claws, it stretches nearly the width of a sheet of notebook paper. Both of these giants are now proudly displayed in my office, and I show them to just about everyone who walks in.”
Other institutions also contributed impressive specimens. The Illinois Natural History Survey, which belongs to the Prairie Research Institute at the University of Illinois, accounted for six of the 20 largest crayfish. The Smithsonian Institution’s National Museum of Natural History contributed another four.
Why Can’t North American Crayfish Grow Even Bigger?
Beyond settling the competition, the researchers uncovered a more intriguing scientific question. Their measurements provided some of the first estimates of the upper size limits for North American crayfish, and the results suggested that these animals may face a surprisingly consistent growth ceiling.
Despite differences among species and environments, the largest individuals tended to reach roughly 80 millimeters in the body measurement used by the researchers. That raised an obvious question: Why do North American crayfish appear to stop growing at around this size when some species elsewhere in the world, including the Tasmanian giant freshwater crayfish, can become dramatically larger?
“They all seemed to max out around that 80-millimeter mark, but we weren’t sure why, especially given that there are absolutely massive crayfish in other parts of the world,” Bloomer said. “We started looking at things like latitude, oxygen availability, and habitat type as possible correlates to explain why certain species and individuals were larger than others, but we weren’t seeing any patterns.”
The team examined whether geographic location, oxygen levels, or the type of environment where a crayfish lives could explain why some grow larger than others. However, none of these factors showed a clear relationship with maximum body size.
That unexpected result pointed the researchers toward a different possibility. Rather than being primarily controlled by environmental conditions, crayfish growth may be constrained by the animals’ own biology.
One potential explanation involves their exoskeletons, the hard outer coverings that protect their bodies. Differences in shell structure or muscle anatomy might impose physical limits on how large North American crayfish can become.
For now, that remains a hypothesis. Further research will be needed to determine whether these biological features actually restrict their growth.
“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.”
Giant Crayfish Reveal How Much Scientists Still Don’t Know
Crayfish are familiar inhabitants of streams, rivers, and other freshwater environments. Many people encounter them as children, making them among the most recognizable aquatic animals in North America.
Yet even with their widespread presence, scientists are still discovering basic facts about their biology, including how large they can grow and what determines their maximum size.
“Anyone that goes to a stream as a kid knows what crayfish are — they’re almost a universal onboarding for people to care about aquatic life. I still go out with people who are in their 50s or 60s, who recall catching crawfish in the creek when they were seven,” Bloomer said. “It’s just amazing how much we still have to learn about them.”
Bloomer also emphasized that the investigation would not have been possible without museum collections. By comparing specimens preserved over many years, researchers were able to answer a fundamental question that would have been difficult to investigate through fieldwork alone.
What began with one unusually large crayfish and a disagreement between colleagues has now opened a new line of scientific inquiry into the biological factors that shape the size of these familiar freshwater creatures.
