Distribution of sea star wasting disease symptoms in Pisaster ochraceus in the rocky intertidal zone

Beginning in 2013, many species of sea stars (phylum Echinodermata) along the Pacific coast experienced severe mortality due to sea star wasting disease (SSWD). The ochre sea star, Pisaster ochraceus, experienced one of the highest mortality rates during this outbreak. To test the hypothesis that the intertidal distribution of ochre sea stars influences the incidence and progression of SSWD symptoms, I documented the occurrence of symptoms and survivorship in adult and juvenile stars in the upper and lower portions of the mid-intertidal zone. I also chronicled the progression of SSWD symptoms among individually tagged adult stars to assess changes in symptoms relative to intertidal location and the extent and direction of movement. I predicted that because the higher intertidal zone is more physiologically stressful, there would be a higher proportion of symptomatic stars at higher tidal elevations relative to lower elevations. Because symptoms of SSWD include loss of turgor and individual rays, I predicted that stars in higher tidal elevations would have decreased rates of movement relative to those lower in the intertidal zone. I also predicted that juveniles would have a lower incidence of disease compared to adults. During the spring and summer of 2015-16, I surveyed ochre sea star population at False Klamath Cove in Del Norte County, CA using a 360 m2 permanent grid and recorded the presence of symptoms, tidal elevation, size, location within the grid, and microhabitat location over seven surveys. I used a logistic regression model to test associations between intertidal elevation, microhabitat, life stage, and disease symptom expression over time. I found no significant effects of elevation or microhabitat; however, time and size were significant predictors of disease symptoms. I also conducted a transplant experiment at Palmer’s Point Beach in Humboldt County, CA where I recorded the presence of symptoms, tidal elevation, and movement patterns of individually tagged adult stars in an 8,568 m2 permanent grid. I found no significant differences among the main effects of time or treatment or their interactions. I also assessed whether the predictors of time and treatment were associated with the presence of disease symptoms in tagged stars and found no significant effect. Individually tagged ochre sea stars observed over time in the field exhibited both symptom progression and remission and showed greater movement rates compared to previous studies. In summary, life stage was a significant predictor of symptom presence with adults showing symptoms more frequently than juveniles; however, symptom expression showed no relationships to vertical distribution, microhabitat, or movement rates, suggesting that stress associated with vertical gradients in the intertidal zone might not influence the expression of SSWD disease symptoms