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Ecology of a Host-Parasite System in the Rocky Intertidal Zone


The field of ecology seeks to understand and predict the patterns and processes shaping the distribution and abundance of populations, the structure and organization of communities, and the dynamics and energetics of ecosystems structure. Parasitism is understudied as an interaction driving these patterns and processes in spite of research demonstrating that parasitism is the most common type of consumer interaction and numerous case studies clearly documenting that parasitism impacts populations, communities, and has a role in ecosystem energetics.

Ecologists have used the rocky intertidal zone as a model for understanding basic ecological processes and theories; thus, elucidating the role of infectious processes in this system will provide researchers with novel insights that may translate into broad structuring principles in the field. However, there is very little research on infectious processes in the rocky intertidal zone. Thus, in spite of decades of ecological research in this system, parasitic interactions remain unexplored and research may yet reveal a strong structuring role of infections in this system.

Barnacles are infected by a protandrous, semelparous, castrator that affects only female function; this parasite is largely unexplored but potentially strongly impacts barnacle ecology. In this thesis, I addressed basic questions concerning the ecology of this host-parasite system.

First, I explored behavioral defenses of the barnacle host, Chthamalus fissus, versus its isopod parasite, Hemioniscus balani. I found when infectious stages were present, barnacles decreased filtration rate by ~ 50% when they were at high risk of infection. Barnacles not at risk did not respond to the presence of an infectious stage. The difference in response based on barnacle condition implies a high cost of reduced feeding as a parasite avoidance behavior.

Second, I explored how barnacle sex allocation varied with size, and how this differential allocation affected patterns of parasitism. The barnacle host is hermaphroditic, where gender is not fixed and individuals allocate variable energy to male or female functions. Since the parasite requires ovarian fluid, only barnacles with female reproductive function should be appropriate hosts. We documented a unimodal relationship between barnacle size and female reproductive function. This female function-size relationship was mirrored by patterns of parasitism. Further, we found within the subset of suitable hosts, parasitism increased with size.

Third, I explored spatial patterns of parasitism in the field as mediated by parasite predators. I investigated whether a sea anemone, protects an associated barnacles from parasitism. Barnacles associated with anemones had reduced parasitism and higher reproductive productivity than did barnacles remote from sea anemones. In the laboratory, anemones readily consumed the transmission stage cryptoniscus larvae of the parasite. Hence, anemone consumption of parasite transmission stages may provide a mechanism by which community context regulates, and in this case reduces, parasitism at a local scale.

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