The following first appeared in the Bay Journal.
One creature invades the body of another. It snakes through the tissue and takes root, changing the behavior and appearance of its host. And then, a reproductive victory: The host must raise the invader's young in place of its own.
This sounds like science fiction, but it's not. It's a real world biological process taking place largely unnoticed in portions of the Chesapeake Bay.
The players in this drama are the small, white-clawed mud crab and an even smaller parasite called the Loxothylacus panopaei or Loxo for short.
Mud crabs infected by Loxo have been dubbed "zombie crabs" by scientists and volunteers who are working to understand the process and its impact on the crab's population. The work is led by biologist Monaca Noble and biodiversity genomics fellow Carolyn Tepolt of the Smithsonian Environmental Research Center in Edgewater, MD.
"It's kind of an amazing story," Tepolt said.
The Loxo parasite is native to the Gulf of Mexico, Caribbean, and parts of Florida. Its presence as an invasive species in the Chesapeake region was first recorded in Virginia's York River in the 1960s, and researchers suspect it arrived on the shells of Gulf oysters that were imported to seed commercial oyster reefs.
The Loxo infects at least nine species of mud crabs throughout its range. In the Chesapeake, its target is the Rhithropanopeus harrisii, a brown crab with white claws that typically grows no larger than a human thumbnail.
In its larvae stage, the Loxo resembles a microscopic barnacle that floats freely through the water. Within days, it transforms to a more shield-like shape and seeks a host. Timing is critical. The Loxo can only infect a mud crab for about 24 hours after the crab molts, when its exoskeleton is soft and unprotected. Only females invade a host.
"They travel through the crab tissue and form rootlets, little tendrils throughout the tissue of the crab," Tepolt said.
When the process is complete, the union is disturbingly thorough. "You can't find a distinct individual parasite inside the crab, so you can't dissect it out," Tepolt said. "It's a body-snatcher."
When the crab molts again, the Loxo prepares to reproduce. A sac forms under the abdomen or apron of the crab, and a male Loxo attaches itself long enough to provide sperm. The next generation begins to form inside. The sac enlarges, pushes out the crab's abdomen, and becomes visible — the only sign of infection that can be observed without a microscope.
Thousands of larval parasites emerge from a pore in the sac about every five to 10 days. The process repeats several times before the sac is exhausted and a fresh one takes its place.
During this process, the Loxo shuts down the crab's ability to produce its own young.
"The body snatcher aspect of this is that it essentially castrates the crab, and all of its energy goes to support the parasite's reproduction," Tepolt said. "It changes the crab's behavior so that it takes care of the larvae as if they were its own eggs. They put their legs and claws around the sac in a defensive posture and attempt to protect it. Even the males do it."
Males transform physically, too. "Male crabs are feminized," Tepolt said. "The shape of their abdomen changes and gets rounder, like the females." This makes it easier for male crabs to hold, aerate, and protect the Loxo's eggs.
Tepolt and Noble are in the midst of both short-term and long-term studies that are investigating the impact on mud crab populations and the ecological conditions that might help them resist the parasite.
Smithsonian biologists began investigating the parasite 12 years ago, but the work was not always well-funded. Preserved crabs, infected and not, often sat in the lab awaiting time-intensive analysis.
In recent years, a robust volunteer program has boosted the research effort. This summer, 87 volunteers helped to collect mud crabs from the research center's dock and other sites across southern Maryland. A smaller team of regular volunteers helps in the lab year-round.
"We have information on crab size, distribution, sex ratio and whether they have the parasite or not," Noble said. "But now, going forward, the project has a second goal. How can we engage volunteers, teach them about the biology of these parasites, and teach them invasion ecology? It's a great opportunity."
As a result of volunteer support, the effort now includes 12 long-term sites and 10 sites that were added this year to support a more comprehensive analysis of the Rhode River, a small tributary south of Annapolis where the Smithsonian lab is located.
The number of mud crabs found at the sites has varied greatly from one year to the next, making it difficult to assess the larger population. In general, fewer mud crabs are found in places with more parasites, and this could be related to the shutdown of the mud crab reproductive system. Still, researchers say this particular type of mud crab is likely abundant in the Chesapeake Bay and, according to Noble, that's good.
"There are lots of animals that we don't eat that are still important to the Bay," Noble said. "Mud crabs are one of them. They are important predators. They eat a lot of things that live on oyster shells. They are also important prey for other crabs, fish and birds — a tasty treat for many things."
The presence of Loxo at study sites has varied too. "Some places don't have parasites at all and others have high abundance. Trying to tease apart the reasons is more problematic," Noble said.
Researchers speculate that low salinity and cool temperatures help to reduce infections, and preliminary data suggest mud crabs in the parasite's native range are much more resistant to attack.
But more research is needed. It could yield important information for mud crabs, as well as any potential situation in which related parasites invade the Bay and affect other species. The Loxo has not attacked blue crabs in the Chesapeake Bay, but a related species of parasite does impact blue crabs in the Gulf of Mexico.
"If it's not a problem now, could it be, if conditions change?" Noble asked.