Genital morphology differentiates three subspecies of the terrestrial slug Arion ater (Linnæus, 1758) s.l. and reveals a continuum of intermediates with the invasive A. vulgaris Moquin-Tandon, 1855

Abstract

The terrestrial slug Arion vulgaris Moquin-Tandon (= A. lusitanicus auct. non Mabille) is an important agricultural pest that has invaded much of Europe. Previous work has demonstrated hybridisation with A. ater (Linnaeus) s.l. We describe the genital anatomy of morphological intermediates found in eastern Saxony (Germany), comparing them with the parent species. We recommend a standard method of genital dissection and consider a set of five genital characters. The intermediates demonstrate the homology of the ligula-bearing organs of the two species; the ancestral position of the ligula in the dilated part of the oviduct has moved to the upper atrium in A. ater s.l. Furthermore we differentiate three morphotypes of A. ater s.l. in eastern Saxony, associated with different mitochondrial DNA sequences. One is A. ater ater, previously unrecognised from this part of Germany and the Czech Republic. The two other morphotypes correspond to the predominantly British and the Continental subspecies previously recognised on genetic and morphological grounds. We designate as the lectotype of Limax rufus Linnaeus, 1758 a non-surviving specimen from Almondbury, England, described by Lister; thus the predominantly British subspecies becomes A. ater rufus. The appropriate name for the Continental form is A. ater ruber