The status of flounders, Platichthys flesus L., as hosts of the Acanthocephalan Pomphorhynchus laevis (Müller) and its survival in marine conditions

The status of flounders as hosts of Pomphorhynchus laevis and its survival in both freshwater and marine habitats was studied in the wild and by means of laboratory investigations. Natural infections were found in flounders in the euryhaline regions of the River Avon but not in adjacent or distant regions of the English Channel. Pomphorhynchus laevis of freshwater origin survived and grew but did not become sexually mature in natural infections of flounders in the Avon or in flounders maintained in fresh water and sea water in the laboratory. The position of the parasite in the alimentary canal of its host changed in flounders but not in eels in response to the changing conditions within the flounder with increasing salinity. In the Baltic Sea P. laevis grew to a larger size, became sexually mature and occupied a different position, the rectum, in both flounders and plaice. It is suggested that marine and Baltic parasites constitute a different strain of P. laevis , one which completes its entire life cycle in the marine habitat and for which flounder is the preferred host. By contrast, flounders are not even a suitable host for the English freshwater strain of P. laevis although individual parasites may be carried to sea in this and other migratory fish and survive in this medium for varying periods. The sea appears to be a major barrier to dispersal of this freshwater strain, and this largely explains the restricted and discontinuous distribution of the parasite in Britain.