Surveying for Terrapins

Monitoring mudsnails to assess diamondback terrapin populations

Jason Williams & Russell Burke

Department of Biology, Hofstra University, Hempstead, NY 11552

Parasites can be used the monitor the health of ecosystems and the ecology of their hosts. For example, along the east coast of the United States, the trematode Pleurogonius malaclemys infects the eastern mudsnail Ilyanassa obsoleta as its intermediate host and the diamondback terrapin Malaclemys terrapin as its definitive host. The eastern mudsnail Ilyanassa obsoleta is a common marine invertebrates in soft-sediment habitats along the east coast of the US. It is a grazer of microorganisms on the surface of sediments. Mudsnails act as intermediate hosts for at least 9 species of parasitic trematode flatworms (e.g., Curtis, 2009; Blakeslee et al., 2012). Trematodes, commonly known as flukes, invade the snails and undergo asexual reproduction in their bodies. The trematodes emerge from the snails as a swimming (cercarial) stage that either infects another intermediate host, settles on a substrate (as metacercarial cysts) or directly infects a vertebrate definitive host.

P. malaclemys is unique among the trematodes of east coast in that it produces cercaria that settle preferentially on the mudsnail intermediate hosts (see Fig.1).  After settling on the shells and operculum (protective structure on their foot) of the mudsnails the trematode infects the terrapins when terrapins eat mudsnails. In the body of the terrapins the cysts give rise to adult flukes that reside in the intestine. Here the adult flukes produce eggs that are shed with the feces of the terrapins and go on to infect new mudsnails, completing the life cycle (Fig. 1).

The amazing thing about this system is that because the trematode P. malaclemys apparently uses only terrapins as their adult hosts, and because this trematode is the only species that forms external cysts on eastern mudsnails, the number of cysts on mudsnails can be used as an index of the number of terrapins locally (Byers et al. 2011).  Many questions remain, such as how large an area is represented by the cysts on a particular population of snails, but nevertheless it can be valuable to collect cyst data in a standardized way.


Important Links:

Surveying for Terrapins Methods

Surveying for Terrapins Example Data sheet

View Pleurogonius Life Cycle

Watch Diamondback Terrapin Survey using Trematode Cyst Counts





Literature Cited

Byers, J. E., I. Altman, A. M. Grosse, T. C. Huspeni, and J. C. Maerz. 2011. Using parasitic trematode larvae to quantify an elusive vertebrate host. Conservation Biology 25: 85-93.

Curtis, L. A. 2009. The Gastropod Ilyanassa obsoleta as a Resource: Utilization by Larval Trematodes in a Low-Prevalence System. Journal of Parasitology 95:799-807.

Chodkowski, N., Burke, R. L., and Williams, J. D. 2016.  Field surveys and experimental transmission of Pleurogonius malaclemys (Trematoda: Digenea), an intestinal parasite of diamondback terrapins Malaclemys terrapin). Journal of Parasitology.  doi:


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