Mating of the slugs Arion lusitanicus auct. non Mabille and A. rufus (L.): Different genitalia and mating behaviours are incomplete barriers to interspecific sperm exchange

The large slug known as Arion lusitanicus (or A. vulgaris) is an important pest that is spreading through much of Europe. Arion rufus disappears at sites where A. lusitanicus has established strong populations. The finding of morphological intermediates suggests that A. lusitanicus hybridizes with A. rufus, but interspecific mating had not been proved. Considering the marked differences in their genitalia, it has been hard to envisage how mixed couples might transfer sperm. Arion lusitanicus and A. rufus were collected from pure populations near Görlitz, Germany, and used for laboratory mating trials involving either two individuals of A. rufus (henceforth RR), two of A. lusitanicus (LL) or one of each species (mixed). Matings were video recorded and some couples were killed during or after copulation to study spermatophore transfer and genital anatomy during mating. Three mixed pairs copulated. However, mixed pairs were significantly less likely to copulate than either RR or LL pairs (7% vs 52 and 36%). At each stage of mating, the probability of proceeding further was lower in mixed pairs than predicted from rates in RR and LL pairs, but this effect was strongest for yin-yang formation and initiating copulation. One problem was that A. lusitanicus tried to circle after yin-yang formation, whereas A. rufus remained stationary. In this respect, and in the repositioning of its everted oviduct, it was A. lusitanicus that compromised. LL copulations lasted over twice as long as RR copulations, but spermatophore formation took similar times, permitting reciprocal spermatophore exchange in mixed couples even though their copulations ended much earlier than in LL pairs. Our observations of mating behaviour of intraspecific pairs largely agree with previous descriptions of A. rufus, but we discuss some discrepancies between our findings and the fuller descriptions available for A. lusitanicus.