Abstract

Acanthocephalans are parasites with complex life cycles. The parasitic larval stages are found mostly within crustaceans, while the adults live in the digestive tract of vertebrates that prey upon the intermediate hosts. Transition of the cystacanth larvae to the definitive host is mediated by a trophic interaction. To ensure transmission to the final vertebrate host, these parasites are known to induce behavioral changes in the intermediate host, increasing its vulnerability to predation. The acanthocephalan, Profilicollis antarcticus (Zdzitowiecki), has as intermediate host the estuarine crab, Hemigrapsus crenulatus (Milne-Edwards), and as definitive host, the gull, Larus dominicanus. We hypothesized that persistent behavioral alterations observed in crabs infected with acanthocephalans are determined by physiological changes, which are expressed in alterations of the metabolic rate of the intermediate host. Determinations of oxygen consumption were carried out for parasitized and control (non-parasitized) crabs, and metabolic rates were calculated based on oxygen consumption. The crabs infected with cystacanths of P. antarcticus had higher metabolic rates than the control crabs. Patterns of activity of the parasitized and control crabs were recorded according to seven behavioral displays arbitrarily defined. Parasitized crabs were much more active-and excited-than control crabs. These results suggest that the cystacanth larvae of the acanthocephalan, P. antarcticus, induces a phenotypic change in the crab, H. crenulatus, both in the metabolic rate and in activity patterns. This study represents the first experimental demonstration that altered behavior induced by acanthocephalan parasites on their hosts has a physiological basis. The physiological manipulation of the parasite alters the metabolic rate of its host which is expressed in behavioral changes that increase its vulnerability to predators which are the definitive hosts of the parasite. (C) 1998 Elsevier Science B.V. All rights reserved.