A friendly rivalry pushed researchers to measure specimens from museum collections across the country. The record holder came from West Virginia, and the data point to a mysterious biological ceiling on crayfish size.

When Caitlin Bloomer pulled a hefty crayfish out of the mud during a biological survey in Kentucky, she was certain she had a record. "There's no doubt I have caught the biggest crayfish anyone's ever seen!" she recalled thinking, according to Nautilus.

A colleague disagreed and claimed he had caught bigger ones — and that argument grew into a continent-wide search through museum collections, and the result, published in the journal Ecology, revealed something researchers still cannot explain: North American crayfish seem to stop growing at about the same size, no matter where they live.

For anyone who has turned over creek rocks or fished with crayfish as bait, the finding is a reminder that these familiar animals still hold basic unanswered questions, and that museum drawers can settle arguments field notes cannot.

Crawzilla Meets Its Match

Bloomer's prize was a crawzilla crawdad, Lacunicambarus chimera, a burrowing species. It measured 77.5 millimeters (about 3 inches) from the tip of the head to the bottom of the thorax, Nautilus reported. Bloomer is now a teaching assistant professor in the University of Illinois Urbana-Champaign's Department of Natural Resources and Environmental Sciences.

To settle the dispute, she enlisted biologists and curators across the country to search their collections for giants. Using calipers, the team identified 20 individuals from 18 species that stood above the rest, the University of Illinois College of ACES reported.

Bloomer took the result in stride. Her catch did not take the top spot, but she told the university she was proud it made the list at all.

Crawzilla made the top 20 and was the only burrowing crayfish on the list. Burrowing crayfish spend much of their lives in underground tunnels in wet soil, often far from open water, which makes large individuals harder to find and collect. The winner was a Tennessee bottlebrush crayfish collected in 2011 by Zac Loughman and West Liberty University students and kept in the university's collection in West Virginia. It measured 86.6 millimeters (about 3.4 inches), not counting its tail, claws, or antennae.

Zackary Graham, an assistant professor of biology who leads the West Liberty collection, offered a simple comparison: "Its carapace alone is about the length of a credit card," according to ScienceDaily.

An 80-Millimeter Ceiling

The more surprising result was how tightly the giants clustered. Across species and locations, the biggest specimens topped out near the same mark. "They all seemed to max out around that 80-millimeter mark," Bloomer said in the university's release.

The team tested whether environment could explain the limit, comparing latitude, oxygen availability and habitat type. None produced a clear pattern. That points away from local conditions and toward the animals themselves.

Crayfish grow in steps. Each time an animal molts, it sheds its hard outer shell, swells while the new shell is still soft, and then waits for that shell to harden. Every molt is a period of vulnerability, and the process may become harder to complete as body size increases.

The researchers suggest the answer may lie in physiology. The makeup of the exoskeleton, which crayfish must shed and regrow to get larger, or the capacity of their muscles may set an upper bound on body size. Bloomer said studying crayfish physiology could be a worthwhile next step.

Not every crayfish obeys that ceiling — Tasmania's giant freshwater crayfish can exceed 4 kilograms (about 8.8 pounds) and approach 1 meter (about 3.3 feet) in length, according to the University of Tasmania, which describes it as the largest invertebrate living outside the sea. Why North American species stay so much smaller is part of the puzzle.

Collections Hold the Answers

The study leaned heavily on specimens already preserved in research collections. The Illinois Natural History Survey supplied six of the top 20, and the Smithsonian's National Museum of Natural History supplied four. That makes the project a case for funding and maintaining collections that can answer questions decades after animals are caught. The record-holding Tennessee bottlebrush crayfish, for example, sat in a university collection for more than a decade before this study put it at the top of the list.

The evidence has limits. The ranking reflects the biggest crayfish that happened to be collected and kept, not the biggest that exist in the wild. Measurements focused on the main body, and sampling varied by region and species. The size ceiling is an observed pattern, and the physiological explanation remains a hypothesis that has not been tested.

That matters because crayfish are more than a curiosity. They recycle nutrients, feed fish, birds, and mammals, and burrowing species reshape soil and wetlands. Knowing how big they can grow helps biologists understand how they fit into freshwater food webs and how populations respond to change.

Anglers, hikers and students can still contribute. Anyone who catches an unusually large crayfish can photograph it next to a ruler and share it with a state natural resources agency or a university crayfish researcher, since good records help scientists track where big individuals turn up.

The bottom line: a friendly bet produced the best-documented list yet of North America's largest crayfish and a clear question for future research. Anyone who catches a monster crawdad now has a benchmark of about 86.6 millimeters to beat, but the data suggest few will get there.

What Readers Want to Know

What is Crawzilla?

It is a crawzilla crawdad, Lacunicambarus chimera, caught in Kentucky by biologist Caitlin Bloomer. It measured 77.5 millimeters from head to the bottom of the thorax.

What is the largest crayfish in North America?

The study's top specimen was a Tennessee bottlebrush crayfish in West Liberty University's collection, measuring 86.6 millimeters without its tail, claws or antennae.

How did researchers find the biggest crayfish?

Biologists and curators searched museum and university collections and measured the largest specimens with calipers.

Why do North American crayfish stop growing around 80 millimeters?

The cause is unknown. Researchers found no link to latitude, oxygen, or habitat and suspect limits in the exoskeleton or muscles.

Are there bigger crayfish elsewhere?

Yes. Tasmania's giant freshwater crayfish can exceed 4 kilograms and approach 1 meter long.

Where was the study published?

In the journal Ecology.