Wasted and Castrated: Two Diseases Affecting the Ochre Star, Pisaster ochraceus, in North America

Understanding the diseases that plague marine organisms is essential to the management and conservation of coastal ecosystems, especially in the face of a possible sixth mass extinction. An increase in mass-mortality events, often caused by epizootics, is modifying intertidal ecosystems. When predators that have disproportionately large trophic impacts on their community and maintain community structure (i.e., keystone predators) suffer from widespread population declines it destabilizes population dynamics ecosystem-wide, and can have long-term or sometimes permanent effects. This thesis is comprised of two studies that examined two maladies affecting a keystone predator, the ochre star Pisaster ochraceus, in Eastern Pacific intertidal zones. A recent massive die-off event affecting sea stars, referred to as sea star wasting disease (SSWD), devastated populations of P. ochraceus along the West Coast of North America. Pisaster ochraceous also hosts a ciliate parasite, Orchitophyra stellarum, that partially castrates males and occupies the epidermis of both sexes, presumably in a commensal association with the sea star. I was interested in using whole-arm removal as a tool for diagnosing O. stellarum infections, but whether it had a negative impact on sea star health in concurrence with SSWD needed to be confirmed. I asked whether P. ochraceus subjected to surgical arm removal were more susceptible to SSWD and to death from that disease, and designed a three-part experiment to answer that question. I also wanted to understand the potential impact of O. stellarum on P. ochraceus populations, and needed to start by determining its distribution and prevalence throughout its host range. I surveyed P. ochraceus populations for O. stellarum, both in the gonads and on the epidermis of its host, along the West Coast of the United States. I explored several factors that could explain variation in the prevalence of this parasite. I also attempted to understand if SSWD contributed to a shift in host-parasite dynamics by comparing current prevalence to historical reports. Pisaster ochraceus does not appear to suffer or die from SSWD after having an arm surgically removed, increasing my confidence in the use of this methodology for this and future studies, although due to my small sample size, I conclude that the potential negative effects of surgical arm removal require further examination. Prevalence of O. stellarum infections in the gonads of P. ochraceus was very low, but the ciliate was present on the epidermis of 51% of sea stars. I compared current percent prevalence of epidermal association with O. stellarum with data from Stickle & Kozloff (2008) and found that, at the three sites they surveyed, percent prevalence has not changed significantly. I did find that smaller P. ochraceus populations had a higher prevalence of ciliates. Although this result contradicted my hypothesis and widely accepted epidemiological models, it could be due to parasite-mediated mortality or (more likely) reductions in reproductive output due to castration by O. stellarum. I also found that populations with higher percentages of males had a higher prevalence of ciliate association, which is to be expected because O. stellarum primarily parasitizes testes. There is high geographic variation in epidermal ciliate prevalence, but there is no clear pattern linking variation in prevalence to sampling region or latitude. This agrees with previous research that reported high variability in both space and time for ciliate infections, although this study is the first to provide information on epidermal O. stellarum association at a fine spatial resolution (previous studies focused on infection, or only sampled a few geographically separate sites).