Mating in Ariunculus isselii, an arionid slug without a spermatophore

摘要

Ariunculus is a genus of terrestrial slugs often treated as a subgenus of Arion. A survey of the literature reveals doubts about many of the described species: only A. speziae and A. isselii are now generally recognized as belonging to Ariunculus. Ariunculus isselii is widespread on Sardinia. We describe its external appearance, mating behaviour and genital anatomy, and interpret the functioning of the genitalia based on specimens killed during copulation or shortly afterwards. Instead of the distinct epiphallus of Arion, there is only a small ampulla, because sperm are pumped out piecemeal to the recipient rather than transferred in a spermatophore. We speculate (in a framework of sexual selection theory) that this may explain why mating lasts 12 h after the initial genital eversion, much longer than in most Arion. The absence of a spermatophore provides grounds for keeping Ariunculus separate from Arion. Similarly to some Arion species, the papilla is inserted into the partner's partially everted bursa trunk; we discuss four different hypotheses for the function of this arrangement. Also as in some Arion species, the oviduct is everted during mating so as to apply a ligula. We provide information on egg laying in captivity and report that the species can self-fertilize.