Like real-life characters from a honor film, some parasites can manipulate their host's behavior in dramatic ways that often benefit the parasite. These parasites usually lead complex life cycles, requiring an exact sequence of hosts in order to complete their life cycles. As a result, the host's behavioral changes often increase transmission of the parasite from one host to the next. The California killifish (Fundulus parvipinnis) lives in estuaries along Southern California and Baja California. Infected individuals display conspicuous swimming behaviors four times more often than do uninfected ones, rendering them 30 times more likely to be eaten by avian final hosts. Despite the presence of several hundred cysts packed around their brains, infected killifish appear otherwise healthy, growing and reproducing normally. This suggests that E. californiensis uses highly specific mechanisms to influence killifish behavior. Here, I investigated the distribution and abundance of the F. parvipinnis-E. californiensis system as well as the underlying physiological mechanisms of behavior modification.

We initially developed a procedure that decreased overall processing time per host by up to 91% while obtaining accurate (R ²>0.8) estimates for both parasite prevalence and intensity. An assessment of host-parasite ecology revealed the ubiquity of killifish and E. californiensis in one Southern California and two Baja California estuaries. Killifish were the most common fish at two of these estuaries and almost 100% of the killifish populations were infected with E. californiensis. A histological comparison of neuroanatomy showed that infected brain tissue appeared compressed in regions with heavy intensities of metacercariae. High E. californiensis density was associated with increased dopaminergic activity in three distinct brain regions. Euhaplorchis californiensis inhibited a normally occurring, stress-induced elevation in serotonergic metabolism in the brain stem. The density-dependence of these monoaminergic changes strongly suggests that they were parasite-induced. Altered monoaminergic activities may explain behavioral differences leading to increased predation of infected killifish by their final host predators. The potential influence of E. californiensis on the interaction strength of trophic linkages in these food webs suggests that it is a key species in the estuarine food webs of Southern California and Baja California.