Material and Methods
Field tissue sampling
Field surveys aimed at quantifying prevalence of SSWS were conducted between April and July 2016 in the low intertidal zone at five sites in central Oregon: Fogarty Creek (44.8386, - 124.0588), Boiler Bay (44.8303, -124.0608), Yachats Beach (44.3114, -124.1086), Strawberry Hill (44.2492, -124.1154), and Tokatee Klootchman (44.2037, -124.1170). These surveys were conducted using 5m x 2m belt transects (5-10 transects per site). Animals were collected by hand at low tide. Arm length was recorded (center to longest arm) for each animal, and only adults (with >3cm arm length) were scored for disease status (Menge, Cerny-Chipman, Johnson, Sullivan, et al., 2016a). We recorded visual disease symptoms based on the six-level ranking protocol, as per Menge et al. (2016); these included, in order of severity: twisting arms (1), deflated (2), lesions (3), lost arms (4), losing grip on rocks (5), and disintegrating or "melting" (6). Animals were considered apparently normal if none of these symptoms existed.
We returned to each of these sites to collect tissue for genetic analysis (Table S1). We also collected tissue at a sixth site (Smelt Sands, 44.3212, -124.1081, on 16 Aug 2016), but we did not conduct transect surveys there. From each adult individual, tube feet (~5-10) were collected using scrubbed and sterilized forceps, then stored in 1.5-mL microcentrifuge tubes containing 1 mL of 95% ethanol. All samples were stored on ice and then at –20C until ready for DNA isolation. In total, we collected tissue from 410 sea stars, 92 of which were wasting.