Darkface Arion

Arion distinctus occurs in a variety of moist habitats, including gardens, waste ground and woodland, but may be absent in harsh upland habitats. In Switzerland it occurs up to 2000 m. It is typically found amongst ground litter or sheltering under wood, stones and soil clods. Studies in agriculture and horticulture have often not distinguished A. distinctus and A. hortensis, but both species are considered to be economically significant pests.

Unfortunately with most relevant studies it is unclear to what extent the authors were dealing with populations of the sibling species A. hortensis or with mixed populations. However, a more recent study of a pure A. distinctus population in southern England reports results comparable with most others (but not with Hunter 1968, so that study might have concerned A. hortensis). The majority of individuals matured in early winter and adults persisted until summer. Eggs laid at the beginning of this period produced animals of adult size already by the start of summer, but these disappeared underground over summer and did not mature reproductively until the end of the year. This is an annual life cycle. The development of slugs hatching from eggs laid later in spring appeared to be delayed by the dry summer conditions, generating a bimodal size distribution in autumn, and these slugs only matured in spring, with possibly some delaying further. Hence, most studies have reported a full range of sizes of this species throughout the year, although reproducing adults are largely absent over summer and autumn. Arion distinctus matures several weeks or more later than the sibling species A. hortensis. In captivity eggs took about 27 days to hatch at 12–15 °C. Genetic evidence implies that A. distinctus is at least predominantly an outbreeder. …