Size does matter-a closer look on Anguillicola morphology.

The present study deals with morphological differences between two closely related parasitic nematode species (Anguillicola crassus Kuwahara et al., 1974 and Anguillicola novaezelandiae Moravec & Taraschewski, 1988) in two different experimentally infected eel species (Anguilla anguilla Linnaeus, 1758 and Anguilla japonica Temminck & Schlegel, 1847). Furthermore, it considers the question whether size differences between those two species are ontogenetically determined or host species-dependent. In order to analyse these questions, experimental infections with the four possible host-parasite systems have been performed, followed by precise morphometric measurements related to body size and head structures of all resulting nematodes 120 days post infection. Body size measurements (length and width) of A. crassus generally exceeded those of A. novaezelandiae, while both Anguillicola species turned out to be smaller in Japanese eels than in European eels. Comparative measurements of neck width, maximum oesophagus width, and posterior head end width were found to be highly significant with regard to the different host-parasite systems. Shape and width of neck have been identified as reliable discriminating factors for species distinction. Generally, the relation of anterior head end width and neck width proved to be distinctly species-specific and can thus serve as a decisive and easily measureable distinguishing feature.