Richness and diversity of intestinal metazoan communities in brown trout Salmo trutta compared to those of eels Anguilla anguilla in their European heartlands

The hypothesis that intestinal helminth communities in freshwater brown tro ut are dissimilar in composition and structure to those in the European eel was tested by an analysis of component communities from 72 localities and of infracommunities from 34 localities in the British Isles and Norway. Der ived indices were then compared with published data from eels. Composition of helminth communities differed considerably between the two hosts as a gr oup of 4 species occurred commonly in trout and so gave greater predictabil ity to the community composition. These 4 species were trout specialists an d in 97 % of the localities a trout specialist dominated the community rath er than a generalist acanthocephalan as is typical for eels. By contrast al l measures of community structure and indices of richness and diversity ind icated that helminth communities in trout were isolationist in character, s pecies poor and exhibited low diversity at both component and infracommunit y levels. All values of indices for trout helminth communities were strikin gly similar to those obtained from eels. Evidence of interspecific interact ions within the trout helminth communities and a limit of 4 to infracommuni ty species richness further enhanced the similarities and suggested a commo n determinant of community structure. The hypothesis was thus supported in respect of species composition but refuted in respect of community structur e.