Discontinuous distributions of the fish acanthocephalans Pomphorhynchm laevis and Acanthocephalus anguillae in Britain and Ireland: an hypothesis

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Abstract
The current distributions of the freshwater fish acanthocephalans Pomphorhynchus laevis and Acanthocephalus anguillae are described and shown to be discontinuous and mutually exclusive, both regionally and locally, in the British Isles. An hypothesis is erected to account for this pattern. It is suggested that as the continental freshwater cyprinids colonized post‐glacial mainland Britain via the eastward‐flowing rivers and the Thames‐Rhine link, they brought with them both species of acanthocephalan. The present, more extensive distribution of P. laevis in the British Isles and Ireland is explained by (1) early formation of a marine strain that colonized the Baltic and North Sea and estuaries of North Sea rivers, (2) later transfers of infected barbel to other English rivers from the R. Thames by man, and (3) transfers to Ireland of infected cyprinids from England by man. Different and restricted availability of preferred definitive and intermediate hosts subsequently resulted in the formation of distinct strains in England and Ireland. The distribution of A. anguillae can be explained by similar anthropogenic influences, but since its definitive and intermediate hosts are more widely available, strain formation has not yet been detected. Competitive interactions between the two parasites in the intestine of the definitive hosts are thought to be responsible for their mutual exclusiveness.