DISTRIBUTION OF ACANTHOCEPHALANS WITHIN HOSTS
Distribution in individual hosts
Although adult acanthocephalans are found in the alimentary tract of their vertebrate definitive hosts, they do, in common with other groups of intestinal parasites, exhibit preferences for particular regions of that tract. Crompton (1973) has reviewed the sites occupied by parasitic helminths in the alimentary tract of vertebrates and concluded that the majority of species show a clear preference for a particular region of the alimentary canal even if their distribution extends both anterior and posterior to this region. The preferred site may relate to the physico-chemical conditions in the alimentary tract (Crompton, 1970, 1973; Taraschewski, 2000), to particular stimuli required for eversion of the larval stage (Kennedy et al., 1976), to specific nutritional requirements (of which we know little or nothing) or to interspecific interactions (Holmes, 1973). The site may extend in response to increasing parasite density, i.e. crowding (Kennedy & Lord, 1982: Kennedy, 1984a), or decrease in response to competition from other species (Holmes, 1973). It may also differ in different host species (Kennedy, 1985b), or even strains. The marine strain of Pomphorhynchus laevis, for example, shows a clear preference (Kennedy, 1984a) for the rectum in its preferred host, the flounder Platichthys flesus, whereas both English and Irish strains prefer the posterior intestine in their principal hosts and in flounders (Fitzgerald & Mulcahy, 1983; Kennedy et al., 1976, 1978). Furthermore, the English and Irish strains show small differences in their preferred site in trout Salmo trutta (Molloy et al. 1993, 1995).