Attack of the zombie crabs

Mud crabs have been infected by a parasite that turns them into ZOMBIE crabs.

Infected Zombie Crabs

SWAN QUARTER—Something is very wrong with this mud crab. The little sack on the crab’s abdomen looks like an egg sack, but it’s actually a parasite. This parasite, Loxothylacus panopaei, has infected mud crabs all along the East Coast, turning them into zombie crabs. The parasite castrates the crabs and replaces the crab’s reproductive system with its own, making them reproductively dead.

The mud crab, though small (about 2 centimeters wide) is an important part of marine ecosystems. The mud crab scavenges for smaller organisms and is eaten in turn by commercial fish, crabs and birds.

Investigating the Parasite's Spread

Supported by funding from North Carolina Sea Grant, Dr. April Blakeslee and a team from East Carolina University are investigating what impact the parasite is having on these important crabs. They drop a series of crab traps (what they like to call crab condos) along the Neuse and the Pamlico rivers, then pull them out again after a few weeks and count the number of infected mud crabs.

“So we call it a body-snatching parasite,” said Dr. Blakeslee, “Because once it gets into a crab it produces a series of what looks like roots all throughout the inside crab, and goes through what we would call the brain area of the crab as well.”

Not only does the parasite castrate the crab AND hack its’ brain, the crab hasn’t evolved to fight the parasite. That’s because the parasite is an invasive species from the Gulf of Mexico that the crab has never seen before.

“So there’s currently an evolutionary arms race going on between the two of them and right now the parasite is winning,” said Christofer Brothers, a student in Blakeslee’s lab.

But how did the parasite get here? Dr. April Blakeslee says it probably hitched a ride on a shipment of oysters from the Gulf of Mexico 20 years ago.

“One of the big issues is because we are so global and so connected and ships go to all different locations,” said Blakeslee. “So trying to deal with this issue of invasive species has been a difficult one for a long time.”

Climate change isn’t helping. Because the parasite is originally from the Gulf of Mexico, it thrives in salty, warm water. As sea level rise pushes warm salt water further up coastal rivers, the parasite has more places to invade. And unfortunately, once an invasive species has taken root, it’s hard to get rid of it.

Preventing Further Spread

But Dr. Blakeslee said it’s important to understand as much as possible about the parasite.

“One of the ways that we always try to think about is preventing further spread,” she said. “So if we can figure out how it’s moving from place to place we might be able to then try to prevent it from spreading to new locations.”

For example, cargo ships often use ballast water—large tanks of water—to balance the ship during a rough passage. When ships are unloaded, the ballast water is discharged and sometimes invasive stowaways get released too. Cleaning ballast water, boat hulls and gear could prevent invasive parasites from spreading to new location. Which might give the mud crab a break. 

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