The Search for Zombie Crabs: The 2015 Chesapeake Bay Parasite Project

By September 2nd, 2015 at 6:00 am | Comment

This is a guest post by Monaca Noble, a biologist at the Smithsonian Environmental Research Center’s Marina Invasions Laboratory. For the last 10 years, Ms. Noble has worked on issues related to the transport of marine species in ballast water and the introduced parasite Loxothylacus panopaei.
Some young volunteers help measure fish and eels. Photo by Monaca Noble.

Some young volunteers help measure fish and eels. Photo by Monaca Noble.

This June, 49 enthusiastic volunteers came out to search for zombie crabs in Maryland’s Chesapeake Bay. Together they searched through shells from 52 crab collectors distributed throughout the Chesapeake Bay’s tributaries. Volunteers found thousands of White-fingered Mud Crabs (Rhithropanopeus harrisii), hundreds of fish (Naked Gobies, American Eels, and others), and several parasitized zombie crabs at our site on Broomes Island, MD.

What are zombie crabs? Zombie crabs are mud crabs that have been parasitized with the introduced parasitic barnacle, Loxothylacus panopaei (Loxo for short). Loxo is a parasite native to the Gulf of Mexico, the Caribbean, and parts of Florida. It parasitizes at least nine species of mud crabs (xanthid crabs) throughout this range.

Loxo females inject themselves into the shell of recently molted crabs and take control of major functions like molting and reproduction. Loxo’s control over the crab is so complete that the crab essentially becomes a zombie working to raise the parasite’s young before dying an untimely death.

An ovigerous crab (left) and a parasitized crab (right), notice the smooth sac of the parasite compared to the gritty-looking mass of eggs. Photo by Monaca Noble

An ovigerous crab (left) and a parasitized crab (right), notice the smooth sac of the parasite compared to the gritty-looking mass of eggs. Photo by Monaca Noble

Though Loxo is an equal opportunity parasite, infecting both female and male crabs, but it is in the male crabs that her power to transform her host is most striking. The parasitic Loxo female changes the male crab’s behavior, training him to mother her young ,and his anatomy, widening his narrow apron making it easier for him to hold, aerate and protect her young.

Loxo was discovered in the Chesapeake Bay in 1964 on the native White-fingered Mud Crab in Virginia’s York River. Mud crabs are easily transported with oysters. Researchers believe Loxo-infected mud crabs were introduced to the Chesapeake when oysters from the Gulf of Mexico were transplanted to seed commercial oyster reefs. The parasite is now common in much of the bay, but the population and abundance varies greatly among years.

Dr. Gregory Ruiz and others at the Smithsonian Environmental Research Center (SERC) began studying Loxo in populations of the White-fingered Mud Crab in the Rhode River in the early 1990s, looking at the uneven distribution of the crabs in relation to infection rates. In 2003, Dr. Ruiz began the Chesapeake Bay Parasite Project, a large scale survey with Mark Torchin, now at the Smithsonian Tropical Research Laboratory, to look at the effect and prevalence of Loxo on mud-crab populations over time.

Each summer for the last 11 years, crabs and their parasites have been collected from 10 sites in Maryland including sites near Queenstown, Centerville, Oxford, Aqualand (near the Governor Harry W. Nice Memorial Bridge), Combs Creek, Broomes Island, Harrington Harbor, Deale, Corn Island and SERC.

Mud crabs live along the shore in oyster shells, small rocks, and small woody debris. To catch the crabs we made them “crab condo”– small milk crates filled with oyster shells. After two months our volunteers pull up the condos, and hand-collect the crabs. Volunteers look for parasites and ovigerous (egg-bearing) females. The crabs are taken back to SERC where we measure, sex, and examine the crabs for outward signs of the Loxo parasite.

Volunteers having fun hand sorting through shell at Combs Creek marina. Photo by Monaca Noble.

Volunteers having fun hand sorting through shell at Combs Creek marina. Photo by Monaca Noble.

In 1991, seventy percent of the White-fingered Mud Crabs in the Rhode River were parasitize. In recent years, that percentage has dropped to near zero. Is this dip indicative of a real tread or a result of limited sampling effort? To being to address this question, this year we doubled our survey effort, adding 10 additional Rhode River sites to our 10 sites cardinal sites scattered throughout the Bay. We are currently processing these new samples in the laboratory and we hope to have some preliminary data in the spring before we set out the 2016 crab condos.

To learn more about this project and how you can join our next sampling day visit us here.

Leave a Reply