Parasite hijacks mud crabs’ reproduction, behavior

As each day and night draws us closer to Halloween, a real-life invasion of the body snatchers is taking place in the Pamlico and Neuse rivers.

Since 2016, a team of researchers led by Dr. April Blakeslee of East Carolina University’s Department of Biology has monitored mud crab populations in the two rivers to study the prevalence and impact of a parasitic barnacle native to the Gulf of Mexico. The findings were published this summer in the Proceedings of the Royal Society B, a biological research journal.

The parasite, called Loxothylacus panopaei or Loxo for short, is highly damaging to its mud crab hosts, castrating them and replacing the crab’s reproductive system with its own. A series of roots extends throughout the tissues of the crab, including its nerve center, Blakeslee said. The crabs can no longer reproduce but continue to live and feed, in some cases even protecting the parasite’s spawn as their own.

The parasite’s takeover of the bodies and behavior of the still-living crabs led the team, which includes several graduate and undergraduate students, to dub the infected creatures “zombie crabs.”

The research team studied the effects of salinity on both the crabs and the parasite, monitoring populations using crab condos — plastic crates filled with oyster shells to create a natural habitat — at six sites on the Pamlico and five on the Neuse. They found that the parasite is less prevalent as salinity decreases, either by moving upstream or during a flooding event such as a tropical storm or hurricane.

“We saw that essentially at anything below 10 parts per thousand that the parasite just does not exist,” Blakeslee said.

Department of Biology assistant professor April Blakeslee, right, watches as graduate student Haley Hagemeier examines a mud crab.

Unfortunately, the low salinity is less suited for the crabs, too.

“They have evolved to have a range of salinities that they can survive and they tend to do much better when they’re in more moderate salinities,” she said. “We used time-to-right response trials; you put a crab on its back and time how long it takes them to flip back over. It gives you some indication of stress because [if they’re] healthy and happy, they’ll flip back over really fast because it’s a very vulnerable position, but if they’re in any way stressed, they’ll take longer.”

In the moderate, less stressful salinities, the parasite can spread rapidly throughout the population. The team found infection rates at some sites as high as 80%.

Since the parasite is an invasive species in Atlantic waters, probably arriving in the Chesapeake Bay with transplanted oysters in the 1960s, the crabs here have not evolved to deal with it, Blakeslee said, and “they can really get hammered by this parasite.” In the Gulf of Mexico, where the mud crabs have developed some resistance, infection rates are much lower, typically less than 10%.

High infection rates have a negative impact on the crab population since the parasite stops them from reproducing, and there’s a higher rate of predation by larger crab predators. That impact could ripple up and down the food chain, since the mud crabs are part of food webs that include organisms like blue crabs, fish and birds, Blakeslee said.

The research is also important because there’s a similar species of parasite that’s also found in the Gulf of Mexico that can infect blue crabs, a commercially important (and delicious) species.

“We’ve now seen how rapidly this parasite can spread … so we’re hoping our research can help us in understanding what could happen if we don’t try to prevent these invasions from occurring,” she said.

Going forward, Blakeslee and her students will continue to monitor mud crab populations at a smaller number of sites and with less frequency.

“It will be helpful to continue to look at it through time because it’s a story that’s evolving, because this parasite has only been in the area for a few decades. So the changes in this ecosystem are still ongoing,” Blakeslee said. The team is also testing for genetic differences to see whether the crabs evolve to tolerate lower salinities to avoid the parasite, or whether the parasite can evolve to follow its hosts.

ECU students Chris Moore, Timothy Lee, Rebecca Barnard, Kyle Swanson, Laura Lukas and Matthew Ruocchio are co-authors of the publication.

“They learn about field research, they learn about using the equipment that we use out there, and they’ve learned a lot about invasive species and just being able to identify organisms out in the field,” Blakeslee said. “We get lots of other organisms [in the condos] too, so they’ve learned a lot about taxonomic diversity, different types of small critters.”

The North Carolina Estuarium in Washington, which serves as one of the sampling sites, also partnered with Blakeslee’s team and Kayla Clark of ECU’s School of Art and Design to develop an exhibit on invasive species featuring the zombie crabs, which opened in 2017.

A healthy mud crab.

Shell Snatchers
Zombie Crabs