Host manipulation by a trematode parasite (Microphallus sp.) favors invasive rusty crayfish (Orconectes rusticus) in competition with congeners

Background/Question/Methods

Parasites can alter host abundance, but few studies have shown how host manipulation by parasites alters competitive interactions between species.  Recently, a trematode parasite, Microphallus sp., was identified in crayfish in northern Wisconsin and Michigan lakes.  This trematode infects the three main crayfish species that occupy these lakes, Orconectes rusticus, O. propinquus, and O. virilis.  Crayfish are an intermediate host for Microphallus sp. and must be consumed by a predator for the parasite to complete its lifecycle.  Some trematodes are able to manipulate behavior of intermediate hosts to increase transmission.  If Microphallus sp. manipulates crayfish behavior, we expect it to have different effects on each species due to the specificity of host manipulation.  Crayfish avoid predation largely by utilizing shelter.  Therefore, we quantified shelter use as a measure of vulnerability to predators.  We collected infected and uninfected crayfish of all three species from the field and examined their shelter use in laboratory experiments in the presence and absence of a conspecific.  Microphallus sp. may impact the ability of crayfish to compete with conspecifics due to reduced vigor of infected individuals; however, we expect differences in shelter use in the absence of competitors to be indicative of behavioral manipulation.

Results/Conclusions

Infection with Microphallus sp. had different behavioral effects on different crayfish species.  In the absence of a competitor, infected O. propinquus spent significantly less time in shelter than uninfected O. propinquus (P < .001).   In addition, infected O. propinquus were more likely to lose shelter competition to uninfected individuals (P = 0.001).  There was no difference in shelter use between infected and uninfected O. rusticus (P = 0.546), but there was a trend suggesting O. rusticus that were infected at low levels (< 50 parasites) were more likely to win shelter competition compared to uninfected individuals (P = 0.052), and infected O. rusticus displayed aggressive behavior more often than uninfected individuals (P = 0.047).  There was no difference in shelter use between infected and uninfected O. virilis (P = 0.18).  These results suggest that behavioral manipulation by Microphallus sp. makes O. rusticus more aggressive and less vulnerable to predation and O. propinquus more vulnerable to predation.  Because relative susceptibility to predation is important for crayfish species replacements in northern Wisconsin and Michigan lakes, behavioral manipulation by Microphallus sp. may have important implications for community composition in this area.