Some parasites manipulate their host’s phenotype to enhance predation rates by the next host in the parasite’s life cycle. Our understanding of this “parasite-increased trophic-transmission” is often stymied by study design challenges. A recurring difficulty has been obtaining uninfected hosts with a coevolutionary history with the parasites, and conducting experimental infections that mimic natural processes. In 1996, Lafferty and Morris provided what has become a classic example of parasite-increased trophic-transmission; they reported a positive association between the intensity of a brain-infecting trematode (Euhaplorchis californiensis) in naturally infected California killifish (Fundulus parvipinnis) and the frequency of “conspicuous behaviors”, which was thought to explain the documented 10-30x increase in predation by the final host birds. Here, we address the primary gap in that study by using experimental infections to assess causality of E. californiensis infection for increased conspicuous behaviors in F. parvipinnis. We hatched and reared uninfected F. parvipinnis from a population co-occurring with E. californiensis, and infected them 1-2 times/week over half their life span with E. californiensis and a “small cyathocotylid” trematode (SMCY) that targets the host’s muscle tissue. At 3 time-points throughout the hosts’ lives, we quantified several conspicuous behaviors: contorting, darting, scratching, surfacing, and vertical positioning relative to the water’s surface. Euhaplorchis californiensis and SMCY infection caused 1.8 and 2.5-fold overall increases in conspicuous behaviors, respectively. Each parasite was also associated with increases in specific conspicuous behaviors, particularly 1.9 and 1.4-fold more darting. These experimental findings help solidify E. californiensis-F. parvipinnis as a classic example of behavioral manipulation. Yet our findings for E. californiensis infection-induced behavioral change were less consistent and strong than those previously documented. We discuss potential explanations for this discrepancy, particularly the idea that behavioral manipulation may be most apparent when fish are actively attacked by predators. Our findings concerning the other studied trematode species, SMCY, highlight that trophically transmitted parasites infecting various host tissues are known to be associated with conspicuous behaviors, reinforcing calls for research examining how communities of trophically transmitted parasites influence host behavior.

Please read "Experimental infections with Euhaplorchis californiensis and a small cyathocotylid increase conspicuous behaviors in California killifish (Fundulus parvipinnis)" by Weinersmith et al. (2023) in the Journal of Parasitology for details on methods. 

Analysis - Weinersmith et al JP Final.R

This file provides the R code used for all analyses and the creation of graphs for the manuscript. This file also includes the definitions of all variables included in the data files.

Data - Conspicuous Behaviors for 2016 Cohort.csv

This .csv file contains the data used to analyze the discrete conspicuous behaviors described in Table 1 of Weinersmith et al. (in press). 

Data – Proportion Near the Water Surface for 2016 Cohort.csv

This .csv file contains the data used to analyze the impact of infection on the proportion of fish found near the water surface.