Trematode parasites have minimal effect on the behavior of invasive green crabs

Abstract

Trophically-transmitted endoparasites can manipulate the behavior of intermediate hosts to increase transmission to definitive hosts. Less clear, however, is whether these relationships exist when parasites and hosts have limited coevolutionary history, e.g., a native parasite infecting an invasive host. We investigated infection by the northeastern North American trematode Microphallus similis in non-native green crabs ( Carcinus maenas ) to assess whether infection by M. similis influenced feeding behaviors in C. maenas and if this changed with time post-infection. We manipulated infection by randomly assigning crabs to parasite exposure and control groups. We then measured individual crab behavior at five time points (pre-infection, 72 h, 1 week, 2 week, and 3 week post-infection) with an established ethogram that recorded multiple behavioral types. We also conducted righting response trials at each time point and additionally at 4 and 5 weeks post-exposure. Compared to controls, infected crabs showed little difference in recorded behaviors, and burden of infection (i.e., number of trematode cysts) was not correlated to any behavioral metric. This lack of behavioral impact occurred at all stages of infection. Active parasite penetration/establishment early in the infection process did not provoke greater behavioral response than later stages, when the encysted parasite becomes relatively dormant. Although M. similis is capable of infecting non-native C. maenas , our results suggest that it does not manipulate host behavior, possibly because of limited coevolutionary history with C. maenas in the region, or because host manipulation is unnecessary for successful completion of its life cycle. In nature, this may have implications for the crab’s invasion success in the western Atlantic, where it has escaped much of its native parasite diversity.