College of Liberal Arts & Social Sciences Theses and Dissertations

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isopod, parasite, manipulation, behavior, neurobiology


Parasites with indirect life cycles often facilitate changes in their intermediate hosts in ways that increase the likelihood of transmission to their definitive hosts. Acanthocephalan infections typically correlate with altered pigmentation, antipredatory behavior, and changes in mating behavior in arthropod intermediate hosts that increase risks of predation by definitive vertebrate hosts. Additionally, these changes have been shown to associate with the developmental stage of the parasite which facilitates the likelihood of survival in the final host. These changes have been proposed to due to direct manipulation by the parasite, host counteradaptation to minimize the costs of infection, or are an indirect byproduct of pathology. The acanthocephalan parasite, Acanthocephalus dirus, infects the stream-dwelling isopod Caecidotea intermedius as an intermediate host and one of several freshwater fishes as a definitive host. Inside the isopod, A. dirus develops from the early non-infective acanthor and acanthella (immature) stages to the late infective cystacanth stage (mature, capable of transmission to the final host). Developmental stage of A. dirus also correlates with changes in isopod color, antipredatory behavior, and mating dynamics. C. intermedius infected with late-stage parasites have been shown to have reduced pairing success in nature. Additionally, it has been shown that male mating responsiveness (e.g. willingness to mate) is reversible (from no mating attempts to positive mating attempts). However, little is known about the potential ultimate and proximate mechanisms underlying these relationships. Additionally, the potential role of host counteradaptation (compensation) during early stages of infection has not been examined. To examine isopod mating behavior in early-stage infections, I used field-based experiments to assess if host compensation was occurring in male C. intermedius. I found that infected isopods did not increase their mating effort compared to uninfected males. Thus, I concluded that male isopods do not compensate for a future reproductive loss. To assess factors that influence male mating responsiveness in late stages of parasite development, I used a combination of field and lab-based experiments. Since chemical cues have been shown to be important in aquatic environments and because predation is necessary for completion of the parasite life cycle, I examined if predator cues could influence male mating responsiveness using a lab-based experiment. I found that predator cues alone do not appear to be influencing mating response. However, I did find that reversibility of mating response can be maintained in a laboratory-setting. I also examined if mating responsiveness is flexible and reversible in nature using a field-based experiment. I found that male mating responsiveness is flexible in nature towards the end of C. intermedius life cycle. I also found that reversibility of mating response occurs within 200 minutes of removal from a natural setting. Thus, it is unlikely that mating responsiveness could be due to an indirect effect of pathology. The ultimate mechanisms I have studied indicate that parasite manipulation is the most likely cause of mating behavior in C. intermedius. Early-stage parasites can not survive transmission to the definitive host. Thus, manipulation of male mating behavior is not beneficial to the parasite at this life stage. Additionally, since male mating behavior is flexible and reversible in nature, it is plausible that parasites can manipulate this behavior to conserve energy (absence of predators or towards end of the breeding season) and increase the likelihood of survival into the definitive host. I examined if neuromodulation could be a proximate mechanism controlling mating behavior. Dopamine and serotonin levels were assayed for infected and uninfected isopods with suppressed mate guarding behavior. I found no difference between infection status and either dopamine or serotonin levels. Thus, these neurohormone levels did not appear to be influencing mating behavior in male C. intermedius.