Flatworms play canaries role in New England's estuaries

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Parasites that are sensitive to their surroundings are helping scientists learn more about the health of Northern New England marshes and estuaries, including N.H.’s Great Bay.

Research funded by N.H. Sea Grant indicates that parasitic flatworms called trematodes provide a snapshot of the human-influenced factors affecting marshes, as their populations are impacted by the number of roads near a marsh and the amount of nitrogen in the mud. 

Think of trematodes as the marsh equivalent of the canary in the coal mine — their absence or abundance in an ecosystem provide scientists with important insights about the biological, chemical and physical processes taking place in and around the marsh. The trematodes’ reliance on three different hosts ranging from mud snails and crabs to fish and marsh birds to complete their life cycle is the key to their indicator status.

“We as humans tend to have this notion that parasites are the cause of disease and they’re bad, but they’re actually a very abundant and natural part of most healthy functioning ecosystems,” said Jeb Byers, professor of ecology at the University of Georgia.  “In the case of these trematodes, they’re very dependent on having these hosts in good health and good abundance. They tend to be one of the most sensitive species and one of the first species to disappear when a system starts to go downhill,” he added.

Trematode under a microscope

Above: Parasitic flatworms called trematodes are helping researchers determine the health of marshes in Northern New England. Photo by: Irit Altman

The study, “Large-Scale Spatial Variation in Parasite Communities Influenced by Anthropogenic Factors,” was recently published in the journal Ecology. Byers was an UNH associate professor of zoology during the data collection process.

Byers and lead author Irit Altman (UNH ’00) selected 15 sites ranging from Duxbury, Mass., to Harpswell, Maine — with four sites located on the shores of Great Bay — at which they collected hundreds of eastern mud snails, Ilyanassa obsoleta. The snails are conspicuous inhabitants of estuaries ranging from Florida to southern Canada, dotting the mudflats during low tide. In New England, eight species of the trematode parasites begin their lives in mud snails, setting up shop and turning the snail’s reproductive system into a larval trematode production factory. The parasites are shed from the snail and then go on to infect other host species.

Mud snails on the marsh mud flats

Above: Eastern mud snails Ilyanassa obsoleta dot the surface of marsh mud during low tide. The mud snails are the first host in the life cycle of parasitic flatworms called trematodes. Photo by: Irit Altman

Researchers counted the number of trematode-infected mud snails to estimate the parasite prevalence at each location. They also inventoried fish species, sampled the marsh sediments for nitrogen and used GIS maps to inventory the number of roads within a one-kilometer halo around each of the 15 sites.

Researcher walks through marsh mud for parasite research.

Above: Irit Altman (UNH '00) walks through the mud at low tide in N.H.'s Great Bay to collect mud snails. Photo by: Sarah Mikulak

They found that as the number of roads within one kilometer of the sampling site increases, both the number and diversity of trematode larvae in mud snails significantly decreases. Roads and their associated culverts may negatively impact trematode populations in a variety of ways: they might create fish barriers, discourage birds from searching the area for food, or influence the water chemistry through enhanced runoff from the road surface in a way that is harmful to one or more of these host species or trematodes themselves, Byers explained.

Altman and Byers also found that sites containing slightly higher levels of nitrogen had more trematodes, including one location near a golf course by Great Bay. Byers explained that nitrogen is a fertilizer that creates more microalgae on which the mud snails feast, thus bolstering the snail population and allowing trematodes populations to thrive. However, an excess of nitrogen could tip the balance and end up negatively impacting the hosts and the trematodes. 

“This study contributes to the growing body of knowledge recognizing that human activities have pervasive influence on natural systems, including on parasite communities,” Altman said.  

The abundance of fish species that serve as definitive hosts — those species inside which the trematodes will finish their life cycle — also plays a role in the number of trematodes found in a site. The four sites they sampled on Great Bay had an abundance of these species, including striped bass, white perch and American eel, indicating good habitat availability to support the fish populations.

“All of the physical, biological and chemical aspects are very interrelated in a marsh,” Byers said. “These parasites provide a nicely integrated and holistic way in which you can see what’s going on in an estuarine system.”

Byers and Altman admit that identifying trematode species and population numbers may seem too technical to gain widespread favor among resource managers, but in some cases this process might be less time-consuming and less expensive than collecting data on all the host species, the water chemistry, and the physical environment surrounding a marsh. Byers’ cited a previous study he and Altman conducted involving trematode populations and diamondback terrapin turtles that illustrates this point.

“Our most recent trematode research provides an initial step for understanding how parasite communities can be used for applied questions to assess estuarine health and function,” Altman added.

For more information, please contact Irit Altman at altmani@bu.edu or Jeb Byers at jebyers@uga.edu.  

Rebecca Zeiber, N.H. Sea Grant Science Writer