Gyrodactylus teuchis is a widespread parasite of wild and farmed salmonids throughout Europe. It has been frequently confused with the notifiable pathogen G. salaris, to which it bears a striking morphological similarity. The species is frequently referred to as 'cryptic', and diagnoses are primarily based on molecular evidence. We provide the first comprehensive re-description of G. teuchis from a natural wild brown trout population in the Danube watershed, based on the state of the art morphometrics in addition to standard molecular markers. We demonstrate that despite the lack of uni-variate diagnostic character measurements, G. teuchis can be reliably distinguished from G. salaris using multivariate morphological approaches such as Principal Component Analysis or Canonical Variate Analysis, suggesting that automated diagnostic approaches for G. salaris can be modified to take account of potential G. teuchis in samples. This is the first record of G. teuchis from a host population unlikely to have been modified by human stocking efforts. The morphological variability observed in the samples collected from one site on 1 day reflects the overall level of variation reported for European G. teuchis. We also report new sequence variants of the internal transcribed spacer (ITS-1) of the nuclear ribosomal gene cluster with evidence for intra-individual heterogeneity of ITS-1 within this population of G. teuchis.
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