Ambigolimax parvipenis

The native distribution is unknown, but proposed to be North Africa.^[6] Indeed in Algeria the species is widespread in natural as well as ruderal habitats.^[7] However, the first definite records were from several sites in Cornwall and Devon (SW England) in 1999–2000; a 1987 record from Suffolk might also be this species. Since then it has spread widely in Britain and Ireland and by the end of 2021 was reported from 25 vice-counties in Britain and 3 in Ireland. It is known also from France, Hungary,^[8] Slovakia,^[9] Serbia,^[10] Greece, and Spain, including the Canary Islands and Chafarinas Islands.^[1][6] It is widespread in California^[11] (earliest record = 2005), with one record from Arizona.^[1]

In Europe, the species is typically found in gardens and similar disturbed habitats.^[4] It is often under stones and plant debris.^[7]

Historical (pre-2022) records of Limax nyctelius, Lehmannia nyctelia or Ambigolimax nyctelius from South Africa, Australia, New Zealand, Edinburgh and Elba are now known to be of A. waterstoni, and those from the Carpathian Mountains south to Bulgaria are of Lehmannia carpatica.^[1]

Adult slugs are up to 80 mm long when crawling but often mature at a much smaller size.^[4] Like other limacids, they are slim with a pointed tail, and the pneumostome lies in the posterior half of the mantle. The mucus is transparent and colourless. The background colour is yellowish-grey to various shades of brown, with the flanks paler and the sole pale cream. Usually there are at least traces of a pair of dark lines running either side of the midline of the back, and another pair of dark lines on either side of the mantle. Often there are additional such longitudinal lines and spotting. None of these external characters reliably distinguish the species from Ambigolimax valentianus (which often co-occurs with A. parvipenis) or other species of Ambigolimax.^[1][4][11]

The penis is distinctive by its small size (roughly half the length of the bursa copulatrix and duct) and lack of a penial appendage (although vestigial knobs are occasionally present). However, the penial appendage of A. valentianus may sometimes be inverted inside the penis, so slitting open the penis is necessary to confirm its absence.^[1][4]