An ecological validation of a taxonomic distinction: the ecology of Acanthocyclops vernalis and A. robustus (Crustacea: Gopepoda)

Long-standing taxonomic problems involving the vernalis complex of freshwater cyclopoid copepods of the genus Acanthocyclops have now been largely resolved, at least in Europe. Two species, A. vernalis and A. robustus , are recognizable on unambiguous morphological criteria, but each shows variation of a simular kind. Ecological data confirm this distinction. Ecological preferences are themselves reflected by the distribution patterns displayed in a restricted, but not small, area encompassing a diversity of environments. A. vernalis is eurytopic but displays a preference for acidic, calcium-poor waters, low in total ions, and is exclusively benthic. By contrast A. robustus does not colonize strongly acidic waters (min. pH 6.42) and is restricted to alkaline, or weakly acidic, nutrient-rich habitats. In some situations it becomes pelagic. While A. vernalis extends throughout the area frequented by A. robustus , the latter fails to colonize large tracts of upland country for which A. vernalis shows a definite preference and where it is common and widespread.