The Crab-Walking Dead

A parasitic barnacle turns crabs into mindless zombies.

Published October 20, 2015

At the docks near Edgewater, Maryland, along the banks of the Chesapeake, Monaca Noble leads a group of 49 volunteers on a sunny Saturday afternoon in August while they hunt for zombies. No one in the group is an extra for The Walking Dead. Instead, these citizen scientists are looking for an invasive parasitic barnacle called Loxothylacus panopaei.

Loxo, as it’s known, takes over the brain and body of both flat-backed and white-fingered mud crabs, turning them into mindless factories for producing more barnacles. Noble, a researcher at the Smithsonian Environmental Research Center (SERC), led this group of volunteers to figure out just how badly local crabs were affected.

“Because infected crabs can’t reproduce, Loxo can have a major impact on the population of these crabs,” Noble said.

Ever since Loxo was first discovered in Virginia’s York River, which empties into the Chesapeake, in 1964, scientists at SERC have been trying to understand the spread of Loxo, and how it turns tiny crabs into zombies. Researchers believe that the barnacle first arrived in the Chesapeake with some infected crabs hiding in a shipment of Gulf of Mexico oysters that were sent to boost the bay’s dwindling beds of shellfish.

Loxo might be a barnacle, but it looks and acts nothing like the kinds that attach to rocks on the shore or the hulls of ships. Rather than building a conical shell, free-swimming female larvae inject themselves into the bodies of newly molted mud crabs.

“Once they’ve molted, they’re really soft and vulnerable. You have to be careful if you pick one up because they’re so squishy,” said Carolyn Tepolt, a postdoc at SERC who works on Loxo.

After Loxo injects itself into the crab’s soft body, it begins to grow around the crab’s organs. “If you dissected it, there’s no way you could see a different organism in there because it’s so intimately entwined with the crab,” Tepolt said.

In a male crab, Loxo extends tentacles into the crab’s genitals, effectively castrating it. In both sexes, the parasite takes over the crab’s brain—short-circuiting its desire to mate or do anything else but provide support for Loxo—and triggers changes that widen the crab’s outer shell to provide more room for future Loxo larvae. Lastly, a tiny sac emerges from the back of the crab. After being fertilized by male Loxo larvae, the sac swells and pumps out new broods of larvae every few days.

Adult mud crabs molt every 30 to 45 days, but younger crabs molt much more frequently, so the crabs are fairly susceptible to Loxo infections. The parasite—and the zombie crabs it creates—have spread throughout the Chesapeake and down the Eastern Seaboard, but there seems to be no pattern to when or where the infestation hits hardest.

“There’s been a wide variation in the number of parasitized crabs between different years, and we have no idea what might be causing this,” Noble said. In past years, more than three-quarters of mud crabs in the Rhode River have been infected, while in other years, there have been none.

To help gather data on the invasion Noble recruits volunteers. Twice each summer, these citizen scientists sort through mud crabs that have been trapped by SERC in the Rhode River to measure and sex the crabs, and to identify any outward signs of infection.

Tepolt is beginning to understand why the infestation can be so uneven. She believes that variations in salinity affect the survival of Loxo larvae. The mud crabs can survive in both freshwater and ocean water, but Tepolt’s experiments in the lab show that Loxo needs an environment with mild salinity. This information doesn’t help researchers reduce Loxo numbers, but it does help them prioritize where to look.

But both Noble and Tepolt say that eradicating Loxo from the Chesapeake will be impossible. “The best we can do is try and keep it from spreading any further,” Noble said.

Their work, though, might help protect the Chesapeake from Loxo’s more worrying relative, Loxothylacus texanus, which turns the much more economically important blue crab into zombies. L. texanus hasn’t arrived in the Chesapeake, but scientists are worried that the Bay’s luck might not hold. This second species of zombie crab could be financially and ecologically devastating to the fragile Chesapeake—turning the region into a marine spin-off of The Walking Dead.