Of mudsnails, terrapins, and flukes: Use of trematodes as a field-based project in parasitology research

We present an exercise for counting trematode cysts on mudsnails that can be implemented as a field-based project in course work by students or by volunteers. The exercise involves the digenetic trematode Pleurogonius malaclemys, which infects diamondback terrapins (Malaclemys terrapin) as its definitive host, and eastern mudsnails (Tritia obsoleta) as its intermediate host. The trematode forms macroscopic metacercarial cysts on the shells and opercula of the mudsnails, and the life cycle is completed when terrapins ingest the intermediate hosts with cysts. Previous research has suggested that quantifying cysts on mudsnails can be a relatively simple method to estimate terrapin population sizes where terrapins co-occur with mudsnails. Three non-professional scientist groups (including students and volunteers) were tested in their estimates of prevalence and intensity data compared to verified cyst counts. Prevalence data were relatively accurate (~76–78%) and reliable (68–89%) across the groups. The intensity of cysts (on shells and opercula of mudsnails) were mostly within 12% of the verified counts (total mean intensity of 1.81–2.97 cysts/parasitized mudsnail) but varied significantly in two cases due to false positives (likely due to sand grains or debris scored as cysts) or negatives (likely due to missed cysts behind the operculum). The exercise can be completed as a lab or series of labs in courses, and data from such projects could be used in terrapin conservation efforts, particularly in helping scientists determine where terrapins occur. We discuss modification of the exercise for other trematode species in regions around the world and hope this expands recognition by the public of the important role of parasites in ecosystems.