Parasite Population Regulation: Lethal and Sublethal Effects of Leptorhynchoides thecatus (Acanthocephala: Rhadinorhynchidae) on Hyalella azteca (Amphipoda)

Abstract

The hypothesis that Leptorhynchoides thecatus populations are regulated by mortality of heavily infected intermediate hosts, Hyalella azteca, was examined experimentally. Mortality related to the infection process could not be demonstrated because no survivorship difference occurred between amphipod groups exposed to L. thecatus and unexposed control groups after 24 hr. Likewise, amphipod mortality could not be associated with infection intensity during this period. Amphipod mortality, growth, and infection intensity were monitored for 25 days after a 24-hr exposure period. Cystacanths were infec- tive to fish after 25 days. Survivorship of exposed amphipods was at least as great as that of unexposed controls. Infection intensity in exposed male survivors did not differ significantly from that in males that died during the study. Infection intensities in females and survivors of undetermined sex were signifi- cantly higher than in Hyallela that died. The effect of time on amphipod size (as indicated by head length) was significant for male survivors and all females, but not for males that died. No negative effect of infection intensity on growth was demonstrated, nor was there significant correlation between amphipod size and infection intensity for either sex of amphipod at any time during the 25-day experiment. No evidence of lethal effects or sublethal effects on growth was detected.