Feature Story                                     September 2013

Teams of volunteers track parasitic barnacles in Chesapeake Bay

By Monaca Noble (SERC)

Volunteer Eun Joo Chung and researcher Monaca Noble pull up a mud crab collector in Centerville, MD. Photo by Nelson Demby
Volunteers Kristen Minogue, Christine Garrahan, and Sabrina Wilson work with researcher Eric Bah to collect the mud crabs from the oyster shell and look for parasites in Harrington Harbor. Photo courtesy of Christine Garrahan.
Mud crabs, especially the young of the year, are hard to see and sometimes hard to grab without forceps. These volunteers are giving these shells a close inspection. Photo by Monaca Noble
Not every team was able to see the introduced parasite. Loxo was only found at two sites this year, Oxford and Broomes Island, MD. This crab collected in Oxford has four Loxo sacs attached to her abdomen. We aren't certain whether this crab has been infected with four parasites or if a single parasite produced four sacs, genetic analysis is needed. Photo by Monaca Noble.
Crab collectors that were sorted in June were redeployed at the site to be sorted again in August. Here Monaca Noble shows high school student Hyunsoo Chung how to tie a bowline knot, the knot used to tie the collectors to the dock. Photo by Nelson Demby
John Ault redeploys a crab collector off the dock in Oxford. Photo by Monaca Noble
American Eels like this one often hide beneath the oyster shell in the collectors. Volunteers had fun trying to get them out of the sorting tray without injuring them; they are slippery. Photo by Monaca Noble

For ten years researchers from the Smithsonian Environmental Research Center’s Marine Invasions Lab have been collecting mud crabs from 10 sites in Maryland in order to monitor and understand what drives the introduced barnacle parasite Loxothylacus panopaei (Loxo for short) that infects them. This summer we tried a new approach for completing this survey: We enlisted the help of citizen scientists. A total of 40 volunteers signed up, 22 people in June and 18 in August, to collect crabs. Our volunteers ranged in age from 14-year-olds to retirees. We were very lucky to have several high school students and families join in the fun. Volunteers were divided into two teams, each team visiting 1 or 2 sites per day. Volunteers learned to identify the crabs and collect them for further analyses and, of course, learned how to identify the presence of the Loxo parasite. Once the crabs were collected, volunteers also learned to collect physical data such as salinity and temperature, using a YSI probe.

“My husband and I were looking for a chance for our daughter to learn how scientific experiments are designed, and how the results collected from experiments are interpreted, as she has been interested in biological science. Participation in this project provided my family the perfect opportunity to think about scientific research and environmental science.”
olunteer Eun Joo Chung

White-fingered Mud Crabs (Rhithropanopeus harrisii) are native to the East and Gulf Coast. Loxo is native to the Gulf of Mexico, parts of Florida, and the Caribbean, and parasitize several mud crab species throughout this range. The parasite was introduced to the Chesapeake Bay in 1964, probably through the import of Gulf Coast oysters and associated infected mud crabs, as mud crabs live in oyster beds and are easily transported with oysters. The parasite is common in much of the Bay, but its population and abundance fluctuates annually.

The Loxo parasite looks like a barnacle in its larval form, but that is where the similarity ends. In a scenario straight out of a horror novel, a female larva infects a recently molted crab by burying into the carapace and growing to adulthood. Once inside she assumes control over the host crab, forcing it to do her bidding. She tricks both males and female crabs into tending and raising her larvae by controlling the host’s molting and reproductive functions and behaviors. The Loxo larvae are housed in a sac that the parasite forms on abdomen of the crab, where a female crab normally would have her eggs. When the sac is formed, a free-swimming male Loxo will enter the sac, attach to the sac wall, and spend the rest of his life fertilizing the thousands of Loxo eggs that will be produced. The crab will protect this reproductive sac for the parasite just as it would its own young: He or she will aerate it and keep it free of fouling, becoming the perfect guardian to the young Loxo larvae. When the larvae are ready, they are released into the water to seek out a new crab host. This process eliminates the crab’s ability to reproduce, so high rates of parasitism could have negative consequences for local crab populations. 

“This isn't exactly what I expected when SERC said I was going to pick crabs!
Crabs....Mud.....Good Times”
olunteer John “Jack” Ault

We monitor the mud crabs and their parasites by collecting crabs from the habitat collectors that we place at each of our 10 sites. Sites are spread between the East and West side of the Bay from Centerville, MD, our most northern site, to Aqualand Marina on the Potomac River in Newburg, MD. At each site we set out four habitat collectors, small plastic crates filled with dead oyster shells that sit on the mud. These crates stay in the water for two months, during which time mud crabs take up residence between the oyster shells. At two-month intervals (June and August), each crate is pulled up and the crabs are hand-collected from the oyster shell habitat for analysis in the lab. Collected crabs are measured, sexed, and examined for outward signs of the Loxo parasite back in the lab. More detailed information on the parasites and the survey can be found in the September 2012 Feature Story.

This was one of the best years on this project for me. I thoroughly enjoyed meeting new people every day and seeing the enthusiasm and care given to the project. Highlights included finding large rates of parasites at the site in Oxford, MD; while not good for the crabs, it was good for the volunteers to get a close-up look at the introduced parasite. Other exciting moments involved catching American eels and a blue crab in the crab collectors.

Citizen scientists can make a difference in our research, and can also learn about other organisms along the way by getting hands-on experience with American Eels, fish, blue crabs, barnacles, and worms collected in the crab collectors with the mud crabs. Funding is often limited to maintain data collection on long-term projects such as this one, making citizen involvement even more important. But beyond that, the Smithsonian Environmental Research Center is committed to offering informal science education opportunities such as this, because we are committed to increasing science literacy and stewardship in our community.

"Collecting mud crabs was an amazing experience! I learned about the dangers of introducing non-native species to our ecosystem, and the trouble that the Chesapeake mud crabs are in because of the Loxo parasite that came with the mud crabs from the Gulf of Mexico. I hope to collect mud crabs again next summer."
                                                                                         Volunteer Hyunsoo Chung

"I found the experience to be fun and very informative. I also learned a lot of different things about mud crabs that I did not know previously."
                                                                                          Volunteer Angela Trenkle

We’d like to thank all the volunteers who took part in this project as well as all the marina owners who’ve helped make this this project possible over the years. We couldn’t do it without you. We hope you’ll join us next summer for another round of surveys.