The identity of Deroceras klemmi

The original description of Deroceras klemmi Grossu, 1972 was based on material in the Museum of Natural History Vienna (NHMW). Our examination of the types has resolved earlier uncertainties about the anatomy caused by some paratypes being other species and by a mix-up of another paratype with the holotype followed by an undocumented swap of specimens between the NHMW and Grossu’s collection to correct this. We describe the anatomy and mating behaviour of D. klemmi, comparing it particularly with D. reticulatum, with which it has been widely confused. Deroceras klemmi always has a single long penial gland, but more diagnostic is the sarcobelum with the shape of a blunt, roughly equilateral triangle, laterally flattened. The configuration of the penes at copulation is particularly similar to that of D. reticulatum, but there are significant differences in mating behaviour, and our observations of a mixed pair suggest reproductive isolation. We compare also the anatomy of Deroceras lothari Giusti, 1973, with which D. klemmi has been incorrectly synonymised; even juveniles are distinguishable by the presence of a proper rectal caecum only in D. klemmi. Mitochondrial COI barcoding sequences confirm a close relationship of D. klemmi, D. reticulatum, D. agreste, and D. turcicum. Reliable evidence of the occurrence of D. klemmi exists only for Slovenia and western Croatia, adjacent parts of Italy and Austria, and as a probable introduction in Hungary. It lives in open habitats from sea level to at least 1500 m, occurring also synanthropically. We designate D. dalmatinum Grossu, 1972 as a junior synonym of D. klemmi.