sperm transfer), and may occur because the amputee's penis retraction ability has been inhibited by the partner by transfer of a secretion. As a result, these individuals can no longer mate, and are only able to reproduce uniparentally. Aphallic individuals of Ariolimax species are still able to mate, but only as females, as they have intromission and internal sperm exchange. Freshwater snail sexual selection The freshwater snail Physa acuta is a self-fertile organism that can be exposed either to strong sexual selection or to self-fertilization depending on its mode of reproduction. Noel et al. used Physa acuta to experimentally determine whether accumulation of deleterious mutations is avoided by inbreeding populations of the snail, as well as by outbreeding populations undergoing sexual reproduction. Inbreeding promotes the homozygous expression of deleterious recessive mutations in progeny, thus exposing these mutations to selective elimination because of their deleterious affect on the progeny. Outbreeding sexual reproduction allows females to select male mating partners with smaller mutation loads thus also leading to a reduction of deleterious mutations in progeny. Noel et al. concluded that both outbred and inbred populations of Physa acuta can efficiently purge deleterious mutations. Mating of marine gastropods For example, in mating of sea slugs Siphopteron quadrispinosum there occur traumatic mating. During mating of Aplysiidae chains of mating slugs may occur. See also Reproductive system of gastropods References External links Gastropod anatomy Gastropods