CONTEXT-DEPENDENT EFFECTS OF TREMATODE PARASITES ON SNAIL HOST FEEDING, MOVEMENT, AND BEHAVIOR.

Parasites can alter their hosts in a myriad of ways, including their physiology, movement, and behavior. In this study, we investigated how trematode parasites impact the movement and behavior of their first intermediate host, the keystone grazer Pleurocera proxima, in both the lab and the field. Trematode parasites frequently castrate their snail first intermediate hosts and can increase metabolic needs. Therefore, we predicted that infected snails would have increased feeding rates, prefer eating food over spending time with conspecifics, and move significantly less than uninfected snails. To test this, we conducted a series of experiments. First, we conducted a field experiment to quantify feeding rates of infected and uninfected P. proxima snails. We found that infected snails consumed 14% more than did uninfected snails. Next, we conducted a laboratory preference experiment to quantify the amount of time spent with either food or a conspecific. We found that snails, regardless of infection status, spent more time with conspecifics than with food. We then investigated how infection influenced movement in both a social context and an isolated context under laboratory conditions. We found that infected snails moved less than did uninfected snails in a social context but more than did uninfected snails in an isolated context. We also investigated whether these differences in preference and movement would translate into altered dispersal in the field. We found no effect of infection status on the distance or direction traveled in the field. This work demonstrates that parasites can influence host movement and behavior in substantial ways and, importantly, that these effects can be dependent on social context. Future work on parasite-mediated behavior should be conducted to investigate these effects under different biotic (as we have done) and abiotic contexts to establish a robust understanding of the role parasites play in shaping our ecosystems.