Morphological variation in Echinorhynchustruttae Schrank, 1788 and the Echinorhynchusbothniensis Zdzitowiecki & Valtonen, 1987 species complex from freshwater fishes of northern Europe.

Echinorhynchustruttae and the Echinorhynchusbothniensis species complex are common parasites of salmoniform and other fishes in northern Europe. Echinorhynchusbothniensis and its sibling species Echinorhynchus 'bothniensis' are thought to be closely related to the Nearctic Echinorhynchusleidyi Van Cleave, 1924 based on morphological similarity and common usage of a mysid intermediate host. This study provides the first analysis of morphological and meristic variation in Echinorhynchustruttae and expands our knowledge of anatomical variability in the Echinorhynchusbothniensis group. Morphological variability in Echinorhynchustruttae was found to be far greater than previously reported, with part of the variance attributable to sexual dimorphism. Echinorhynchustruttae, the two species of the Echinorhynchusbothniensis group and Echinorhynchusleidyi displayed considerable interspecific overlap in the ranges of all conventional morphological characters. However, Proboscis profiler, a tool for detecting acanthocephalan morphotypes using multivariate analysis of hook morphometrics, successfully separated Echinorhynchustruttae from the other taxa. The Echinorhynchusbothniensis species group could not be reliably distinguished from Echinorhynchusleidyi (or each other), providing further evidence of the affinity of these taxa. Observations on the distribution of Echinorhynchustruttae in its definitive host population are also reported.