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

1) This part of the revision of the freshwater Gammarus species deals with the G. roeseli-group. Members of this artificial group are characterized by the presence of dorsal processes (carinae) on the metasome segments. 2) A historical review is given from the first description of G. roeseli and the taxonomic confusion that followed, to the start of this study when 3 species and at least 15 subspecies were known. 3) Where possible, the type material of all these taxa is re-examined and compared with hundreds of newly collected samples and material from all major museums in Europe. 4) In this study the same morphological characters are used as in the revision of the G. pulex-group (Karaman & Pinkster, 1977). The variability of the various characters is discussed. 5) Complete descriptions and many illustrations are given of the males of all species, along with a key for their identification. Where necessary, figures are given of those details in which females differ from the males. 6) It is proved that G. triacanthus and its many subspecies must be considered synonymous with G. roeseli. G. argaeus stojicevici is raised to specific rank. Four new species (G. goedmakersae, G. mladeni, G. obnixus and G. pavo) are described, resulting in a total number of seven species within this group. 7) In many localities members of the G. roeseli-group have been found together with members of the G. pulexor G. balcanicus-group. Experimental work on G. roeseli showed that in such cases G. roeseli is more eurybiont than the members of the G. pulexor G. balcanicus-group (higher resistance against low oxygen concentrations and elevated temperatures). Subterranean members of this group have not been found so far. 8) From zoogeographical data it can be concluded that this group originates from southeastern Europe and Asia Minor. Only in Recent times G. roeseli has penetrated inland waters in western Europe. Members of this group are lacking in northern Europe, the British Isles, Italy and the Iberian Peninsula. 9) For all species complete lists of all studied localities are given (except for G. roeseli, for economical reasons), together with two maps in which the distribution is illustrated.