The status of brown and rainbow trout, Salmo trutta and S. gairdneri as hosts of the acanthocephalan, Pomphorhynchus laevis

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Abstract
The status of brown and rainbow trout as hosts ofPomphorhynchus laevis was studied in the field and by means of laboratory investigations. Field data indicated that rainbow trout might belong to the group of preferred hosts of P. laevis, whereas brown trout belonged to the group in which the parasite achieved less than optimal growth and maturation. This was confirmed by laboratory infections. In rainbow trout P. laevis attained up to three times the growth rate in brown trout and maturation occurred whereas in brown trout establishment was lower, growth slower and no parasites matured. Changes in the behaviour of infected Gammarus pulex induced by the presence of P. laevis cystacanths were such as to render the shrimps more vulnerable to predation by trout and other surface and mid‐water feeding fish, and selective predation upon infeged G. pulex by fish was demonstrated. nvestigations into the stimuli necessary for eversion of cystacanths of P. laevis revealed that the most important factor was a non‐specific component of bile, and it was concluded that cystacanths were likely to evert in any species of fish. Recognition of the different status of brown and rainbow trout as hosts of P. laevis still fails to explain some peculiarities in the distribution of the parasite in the British Isles, where in Britain it occurs in trout in only one river but in Ireland in all rivers throughout the country. It is suggested that the Irish parasites may constitute a different strain of P. laevis, since they use a different species of intermediate host and are better able to survive in brown trout.

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