Biologists battle over North America’s largest crayfish

When Caitlin Bloomer wrestled the massive Crawzilla crawdad—yes, that’s its actual common name—out of the muck during a biological survey in Kentucky, she was convinced it was the largest crayfish ever recorded.

“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, part of the College of Agricultural, Consumer and Environmental Sciences at the University of Illinois Urbana-Champaign.

The friendly competition between colleagues quickly accelerated. Bloomer enlisted biologists and museum curators around the country to inventory their collections for the largest specimens. Armed with precise measuring tools called calipers, the crayfish crew identified 20 individuals across 18 species that were larger than the rest. They published the rankings in Ecology.

“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.”

A Tennessee crayfish takes the crown

The true winner was a Tennessee bottlebrush crayfish—part of the museum collection at West Liberty University in West Virginia—that measured 86.6 millimeters (3.4 inches), not including the tail, claws or antennae.

“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, 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.”

Additional bragging rights go to the collection at the Illinois Natural History Survey, part of the Prairie Research Institute at U. of I., with six of the top 20, alongside the Smithsonian Institution’s National Museum of Natural History, with four.

Size limits point to physiology

More importantly, the competition established some of the first upper size thresholds for North American crayfish and sparked new questions about what determines body size.

“They all seemed to max out around that 80-millimeter (3.1-inch) 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 absence of a relationship with environmental factors suggested North American crayfish may be limited by their physiology instead. The researchers think their maximum size could be tied to the makeup of their exoskeleton or aspects of their musculature.

“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.”

Familiar creatures, unanswered questions

Crayfish are among the most common and familiar animals, so it’s surprising that biologists are still learning some of the basics.

“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.”

She added that the team was able to address this fundamental question only by mining museum collections, highlighting the importance of these public resources for science.

Who’s behind this story?

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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