Parasite-altered behaviors in acanthocephalan/amphipod systems

Abstract

Acanthocephalan parasites are known for inducing behavioral alterations in their intermediate hosts. Infection with Polymorphus paradoxus causes amphipods, commonly known as scuds, to display altered photic and escape responses. The neurochemical serotonin has been previously implicated in these behavioral responses. This dissertation presents research investigating changes in serotonin correlated with P. paradoxus infection. The pattern of serotonin-like immunoreactivity differed with infection: ventral nerve cords from P. paradoxus-infected amphipods contained more visible serotonergic varicosities than did nerve cords from non-infected amphipods. Polymorphus marilis, a related parasite which induces only slight changes in photic response and no change in escape behavior, was not associated with any change in serotonergic immunoreactivity. High performance liquid chromatography failed to detect any difference in the amount of serotonin in ventral nerve cords from infected and non-infected amphipods. The absence of a difference, however, could be because the local strain of the parasite does not induce altered behaviors, as evidenced by subsequent behavioral assays. Another acanthocephalan/amphipod system exists in Italy, where Pomphorhynchus laevis infects Echinogammarus stammeri. I examined the behavior of infected amphipods in field and laboratory studies. Pomphorhynchus laevis-infected amphipods drifted more, were more attracted to light, and were more active than noninfected amphipods. Non-infected amphipods reduced their activity in the presence of fish odors, whereas infected amphipods remained active. Pomphorhynchus laevis-infected amphipods of E. stammeri are abundant year round, and would be a suitable system for future investigations of the mechanisms of parasite-altered behavior.