GENERAL STRATEGY

The passage of an acanthocephalan through its life cycle involves ingestion by a host at each stage, from egg to intermediate host, and from intermediate host to paratenic or definitive host. The parasite moves up the trophic levels in a series of stages (and down in one by eggs) and through a food web (Marcogliese, 2002). Although the choice of a particular host will tend to canalise the direction of movement, it is highly improbable that an egg or intermediate host will be eaten only by a species that will serve as the preferred intermediate or definitive host: both eggs and intermediate hosts will be eaten by a number of other species. Some of these will be the preferred host in which the parasite will grow and develop and with which it will form a stable, balanced system. In other species the acanthocephalan may be able to develop, but less successfully and/or more slowly, and these species can be considered to be suitable but secondary hosts. In a third group of hosts the acanthocephalan may be able to survive, but be unable to grow or develop (or only very occasionally) and these can be considered to be accidental or unsuitable hosts. Finally, in many species it will be unable to survive at all and will die rapidly, for example if Macracanthorhynchus ingens, a parasite of racoons Procyon lotor, accidently infects pigs it survives for only two weeks (Nelson & Nickol, 1986).