High density and strong aggregation do not increase prevalence of the isopod Hemioniscus balani(Buchholz, 1866), a parasite of the acorn barnacle Chthamalus fissus(Darwin, 1854) in California

Parasitism is a common life history strategy among animals and has been the topic of empirical and theoretical study for decades. Many classic mathematical models assume increased spatial aggregation and density increase prevalence in a host population. Parasites can nevertheless take on a diversity of life history strategies that could result in deviations from these assumptions. I investigated the importance of density and aggregation of hosts on the prevalence, or percent of hosts infected by a parasitic castrator with a complex life cycle. An isopod parasite, Hemioniscus balani(Buchholz, 1866), infects an intertidal barnacle, Chthamalus fissus(Darwin, 1854), and renders the barnacle unable to produce eggs. To test the assumptions that density and aggregation increase parasitism, I conducted a survey of density, aggregation, and parasitism in the rocky intertidal zone in Santa Barbara County, CA, USA. I found spatial patterns of host density and aggregation did not lead to increases in parasitism. Furthermore, there was no evidence of a selfish herd effect, whereby the per capita risk of infection decreases with population size. Understanding patterns and drivers of infection is particularly important in marine systems, which are understudied compared to terrestrial diseases, and work done in terrestrial systems could not correspond to patterns and processes in marine systems.