Increased vulnerability of amphipods to predation owing to altered behavior induced by larval acanthocephalans

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Abstract
The amphipods Gammarus lacustris and Hyalella azteca, when infected with the acanthocephalan larvae Polymorphus paradoxus, P. marilis or Corynosoma constrictum, show altered evasive behavior and (or) responses to light. The behavioral alterations may be manifestations of an evolutionary strategy adopted by the parasites for enhancing transmission to their respective definitive hosts. The differences in behavior, and consequent microdistributions of infected amphipods, may be tactical and specific to the different feeding habits of the definitive hosts. This hypothesis was tested, in part, by exposing equal numbers of infected and uninfected amphipods to 2 of the definitive hosts of P. paradoxus: mallard ducks and muskrats. Gammarids infected with cystacanths of P. paradoxus were significantly more vulnerable to predation by mallards and to accidental ingestion by muskrats. Hyalellids harboring cystacanths of C. constrictum, which use several species of waterfowl, including mallards, for definitive hosts, were more vulnerable than uninfected hyalellids, but less vulnerable than gammarids infected with P. paradoxus. No gammarids infected with cystacanths of P. marilis were eaten by mallards or muskrats; P. marilis is not infective to either of these hosts.

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