Feature Story                                      September 2014

Parasites, Crabs, and Slippery Eels – Highlights from the 2014 Chesapeake Bay Parasite Project

By Monaca Noble

Researcher Tim Mullady works with volunteer John Hagens at our Corn Island site August 2014. Photo by Monaca Noble 

2014 marks the eleventh year of the Chesapeake Bay Parasite Project and its second year as a citizen science project. Including five staff and an intern, 50 people participated in this year’s sampling events. The two events, one in June and one in August, were very successful. We collected hundreds of mud crabs, found parasites, worked in the rain, sunshine, and even enjoyed music from a live band at Harrington Harbor. We saw eels, many species of fish, blue crabs, large jellyfish, and at one site witnessed an osprey catch a very large fish that he/she could just barely lift from the water. We had great volunteers who worked really hard, some putting in 10-hour days! Thank you to everyone who came out.

The Invasion – An Unwelcome Hitchhiker
The parasitic barnacle Loxothylacus panopaei (Loxo for short) is native to the Gulf of Mexico, the Caribbean, and parts of Florida and parasitizes several species of mud crab throughout this range. In 1964 they were discovered in the Chesapeake Bay on the native White-fingered Mud Crab (Rhithropanopeus harrisii) in the York River. Because mud crabs are easily transported with oysters, researchers believe that mud crabs infected with Loxo 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.


Through participation in the SERC mud crab survey project, I was able to learn firsthand about the presence of the Loxo parasite in the Bay watershed area. Before learning about this project, I wasn't aware of this issue." Volunteer Samantha Watterson, age 23


The Parasite – Demonstrating an Impressive Level of Host Control
Loxothylacus panopaei females burrow into recently molted crabs and take control over major functions like molting and reproduction. Once inside, they produce sacs filled with larvae that are tended by the host crab until the larva are old enough to be released into the water where they will seek out a new crab host. 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
Volunteers Nicole Meister, age 13, and Samuel Enriquez, age 16, work together at the Centerville site in August. Photo by Monaca Noble 
most striking. She changes the crab’s behavior, especially males, in order to trick them into mothering her young. Female crabs have the physical attributes and behaviors need for mothering, such as a wide apron for holding and aerating eggs, so taking care of Loxo’s eggs rather than her own isn’t a major behavioral shift. But to coerce a male crab to mother Loxo larvae is a major feat. In the male crabs, Loxo causes the apron of the male crabs to widen, making it easier for him to hold, aerate and protect her young. Loxo then causes a big change in behavior, giving the male crab the mothering instinct to care for her young, in essence “feminizing” the males. This level of host manipulation speaks to the power of Loxo and other parasites that assume complete control over their hosts.

The Project – Lofty Goals and a Long-Term Commitment
Dr. Gregory Ruiz and others at the Smithsonian Environmental Research Center (SERC) began to study Loxo in populations of the White-fingered Mud Crab in the Rhode River in the early 1990s, looking at the influence of genetics and the uneven distribution of the crabs in relation to infection rates. In 2003, Dr. Ruiz began this 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.
 


 

“This was my first time volunteering with SERC and the experience was incredible. I was placed into a team and together we searched for White- fingered Mud Crabs in the condos. The collection of these crabs was to provide scientists with information on how many mud crabs were infected by the Loxo parasite. Although searching the condos for mud crabs was tedious at times, working in such a nice environment made it a great experience. Not only did I learn about the White-fingered Mud Crab and the Loxo parasite, I learned about other organisms located in the Chesapeake Bay. I learned how the parasite affects the host and even reprograms the male crab to produce offspring. Thanks, Monaca, for giving me this experience!” Volunteer Nicole Meister, age 13


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
Methods – Crab Condos, the Habitat Trap

Mud crabs are probably the most abundant crab in the upper Chesapeake Bay. They live along the shore in oyster shells, small rocks, and small woody debris. To catch the crabs we made them an oyster-shell home – small crates filled with oyster shells that we call “crab condos.” Over the two months that our crab condos are in the water, the crabs move in. After two months our teams of volunteers pull up the condos, dump the shells on a sieve and hand-collect the crabs. Volunteers look for parasites and ovigerous (egg-bearing) females, but do not sort them into groups. All crabs from a single condo go into a single bottle. At each of the 10 sites there are four crab condos. The volunteers spend about an hour sorting through each condo. The crabs are taken back to SERC where we measure, sex, and examine the crabs for outward signs of the Loxo parasite.


“I find all parasitology interesting....But in this specific case, I find the parasite, Loxothylacus panopaei, to be interesting in and of itself: it replaces a crab’s – male or female – reproductive system so that it has baby parasites rather than larval crabs. How cool is that? It’s better than the isopod that replaces a fish’s tongue (Cymothoa exigua – check it out!). I’m thrilled to be involved in studying these beautifully weird organisms, and getting experience doing actual field work.

And as an added bonus, the sorting is fun in and of itself: turning over an oyster shell only to have half a dozen crabs of varying sizes scuttle out, the mad dash to catch any eels or fish that got into the traps and return them to the water, and the competition to find more crabs than the other volunteers.”
Volunteer Samuel Enriquez, age 16


Citizen Science – Yes, We Can Collect Quality Data
Careful data collection is critical in science. In order to make inferences about the crab and parasite populations at a particular site, we need to make sure that we collect all the crabs in the crab condo, which is a standardized sampling unit. In order to assess data quality, we added a verification step to this year’s sampling protocol to measure how well citizen scientists performed. At each site we divided our sorting teams into two groups. Each team was responsible for collecting all of the crabs from two of the crab condos and conducting a verification search on the remaining two crab condos. For example, team A collected all the crabs from crab condo number one, then team B checked through all the clean shells looking for crabs that team A missed. Any crabs that were missed during the verification check were placed in a separate bottle so we can record the number and size of the missed crabs back in the lab.

So how did our citizen scientists perform? They were awesome. In nearly all cases, very few crabs were found during the verification sort.

What’s Next for 2015?
We plan to continue the survey in 2015 and may expand the survey to collect more targeted data if we are successful in getting additional support. We would like to thank all the marina owners for their support of this project over the years. Without them this project would not be possible.
 

Researcher Darrick Sparks guides volunteers through the sampling process at a site in Southern Maryland. Photo by Alison Cawood Visiting Scientists Flavio and Heloisa Fernandes work with Samuel Enriquez in Centerville in June 2014. Photo by Monaca Noble