Freshwater Gammarus species from Europe, North Africa and adjacent regions of Asia (Crustacea-Amphipoda). Part I. Gammarus pilex-group and related species

2) These freshwater species can be classified in three artificial groups: (a) the G. pulex-group (species without dorsal carina and with dense setation on pereiopods 3 and 4 and uropod 3), (b) the G. balcanicus-group (species without dorsal carina and poorly setose pereiopods 3 and 4 and uropod 3) and (c) the G. roeseli-group (species with dorsal carina). These groups are merely artificial ones since transitive (intermediate) species do exist. Moreover, the origin of the species is not known, so that polyphyly is not excluded. 3) In the present work the Gammarus pulex-group is revised, based on rich material from Europe, North Africa, Asia minor and adjacent areas. 4) The taxonomic differences between the various species are usually small but distinct and stable. This is especially evident in mixed populations of two or more species. In those populations no intermediates between the taxa have been observed. 5) In some cases, morphological differences between two species are hardly discernible although reproductive isolation is present (e.g. G. fossarum and G. wautieri). 6) Hybridization experiments can solve taxonomic problems and test the taxonomic value of morphological differences between populations. Such experiments between many Asiatic and European populations might clarify their taxonomic status. 7) The taxonomic characters may largely be variable within one population as well as in different populations of the same species. 8) Characters that are very stable in one species can be largely variable in other species (e.g. presence of calceoli, length of rami of uropod 3). Nevertheless we can distinguish certain stable characters (e.g. the structure of the mandibular palp), but also instable ones (e.g. the number of dorsal and lateral spines on the urosomites) in all species. 9) Gammarus pulex has given rise to several isolated populations, adapted to subterranean life (being blind or having enlarged eyes). These populations are considered distinct subspecies. 10) Within some taxa (e.g. G. fossarum and G. p. pulex) morphologically aberrant populations can be found. Since these populations do successfully cross-breed and are sympatric they must be considered mere variations. 11) We had serious problems to determine the identity of several Gammarus species, especially from Asia Minor, because of the impossibility to obtain literature and type material of some Russian authors. (So we cannot exclude the possibility that our species described from Asia might be identical with a species formerly described by a Russian author). 12) For all species, except the most common ones, complete lists of all localities studied are given. Moreover, in 3 maps the distribution of the various species and subspecies is illustrated. 13) It was not possible to illustrate all morphological details of every taxon mentioned in the present work. Only G. pulex, the type species of the genus Gammarus and the nominal form of the entire group, is figured completely. For the other taxa, only those parts are illustrated that are fundamentally different from those of G. p. pulex.