TEMPORAL TRENDS IN THE ASSOCIATION BETWEEN A TREMATODE PARASITE AND BIVALVE HOST IN THE NORTHEASTERN PACIFIC FROM THE PLEISTOCENE TO THE RECENT

The theory of escalation predicts that enemies are significant agents of selection and therefore have the ability to shape evolutionary trajectories of interacting species. While this theory has been supported with evidence from a number of predator-prey systems, and implicitly includes parasites, the potential for antagonistic symbiotic partners to engage in escalation over evolutionary time has rarely been tested. If these antagonistic symbioses are transient in space and time, we would not expect escalation to take place. However, if these assocations persist in single species over evolutionarily significant periods of time, we might expect to see adaptation in response to these interactions.

Living, surficial and fossil bivalves (Leukoma staminea) from central California were examined for digenean trematode parasites, or evidence of parasitic infection in the form of pits on the internal surface of the host's shell. These samples were compared in order to determine if either prevalence, the number of individuals infected, or intensity, the number of parasites in infected individuals, has changed over time. Previous work has indicated that this parasite has been present in the region since the Pliocene, and has infected L. staminea since the Pleistocene. The present study found variation in both prevalence and intensity over the time of introduction to the Recent. Prevalence is highest in the Pleistocene samples and declines through the Holocene. Furthermore, there is a significant change in the intensity of parasitic infection in hosts from the Pleistocene to the Recent, reflected in a decline in the mean number of parasites per infected host in each time period. Pleistocene samples have unusual parasite distributions, with many hosts experiencing heavy infections. Holocene samples have a more typical long-tailed distribution of parasites. Combined, these changes in prevalence and intensity suggest host adaptation over the time period examined.