Parasites disrupt a keystone mutualism that underpins the structure, functioning, and resilience of a coastal ecosystem

Parasites can alter the traits or densities of mutualistic partners, potentially destabilizing mutualistic associations that underpin the structure, functioning, and stability of entire ecosystems. Despite the potentially wide-ranging consequences of such disruptions, no studies have directly manipulated parasite prevalence and/or intensity in a mutualistic partner, nor quantified the resulting community-level effects. Here, we investigated the effects of a common trematode parasite (Cercaria opaca) on the strength of a keystone facultative mutualism in western Atlantic salt marshes between the foundational marsh cordgrass, Spartina alterniflora, and the ribbed mussel, Geukensia demissa. Cordgrass increases mussel survivorship and growth through shading, while mussels enhance cordgrass growth by producing nutrient-rich biodeposits. This mutualistic association also creates conditions that enhance biodiversity and ecosystem functioning, and mediates the ability of foundational plants to ..., Experimental manipulation of parasite infection intensity in the field
Acquiring infected marsh periwinkles
 From June to July 2017, adult Littoraria irrorata were collected at low tide from Gunning Hammock Island, NC (34.672454 N, -76.501743 W), where infection with Cercaria opaca was known to be common. Collected periwinkles were gently washed in filtered seawater and placed in small petri dishes. Each dish was filled entirely with filtered seawater (~30 mL) and sealed with a plastic lid such that the periwinkle was completely immersed and unable to escape. Petri dishes with periwinkles were allowed to sit at room temperature for 24 h. After this period, the contents of each dish were inspected under a microscope to determine the presence of shed cercariae (Morton 2018). Average incidence of trematode infection from five collections (4501 snails in total) was 39.4% ± 1.8 (mean ± SE). For each infected individual, cercariae were identified to species (Stunkard & Cable 1932, Hollima..., , # Parasites disrupt a keystone mutualism that underpins the structure, functioning, and resilience of a coastal ecosystem

[https://doi.org/10.5061/dryad.fbg79cp4c](https://doi.org/10.5061/dryad.fbg79cp4c)

**Field Experiment** – Contains data related to our experimental field manipulation of trematode infection in ribbed mussels.

| plot | Individual experimental plot ID number. |
| :---------------------------- | :-------------------------------------------------------------------------------------------------------------------------------------- |
| treatment | Plot infection intensity treatment designation. |
| elevation.change | Change in plot elevation (cm). ...