ABSTRACT
INTRODUCTION Acanthocephalans, also known as thorny or spiny-headed worms, comprise at least 1000 species. More than half of these tubiform, gutless worms, which reach a length of up to a few centimetres, pass through fish during their life cycle; the others use birds, mammals or other vertebrates as final hosts and do not occur in fish functioning as paratenic hosts. Various fish species serve as final hosts, harbouring the adult parasites in their gut or as paratenic hosts with immature stages of the worms in their body cavity. In addition, intra-intestinal acanthocephalans, immature or reproducing, which parasitize small fishes, may end up in the intestines of larger, predatory fishes via post-cyclic transmission. Thus far, ancient marine fish such as elasmobranches (sharks, rays) have not been found to harbour gravid acanthocephalans in their gut, suggesting that the Acanthocephala did not evolve in the sea but in freshwater where the majority of species occur. Interestingly, they are highly abundant in fish of
the deep sea and of Antarctic waters (Crompton and Nickol, 1985; Taraschewski, 2000, 2005; Zditowiecki et aí., 2004).
