British distribution: Throughout Britain, generally common.
Arion ater is a very large, herbivorous slug, extended length 10-15 cm when mature, sometimes more, and it is a common sight in grassy places on damp evenings, especially after rain. It is a member of the family Arionidae, with a vestigial internal shell and lacking a dorsal keel (compare Limax maximus).
It is notable for its wide range of colour variation, often black or dark brown, but with grey, orange-brown or almost white variants. Orange-brown individuals have been regarded as a separate species, Arion rufus, (or as a subspecies, Arion ater rufus) and indeed some internal differences in the genitalia may be observed after dissection. However, the correlation of colour with internal characters is unreliable and common practice is to recognise just the one species.
The black morph is generally common, while the orange-brown morph is regarded as being more common in the south – though note that the orange-brown individuals shown here are from Scotland. It would be expected that black individuals would be favoured in cooler areas (thermal melanism), since they would be more efficient at absorbing solar radiation, whereas the orange-brown individuals would reflect more, a likely greater advantage in the south (though neither morph is likely to be active on sunny days). Dark coloured morphs are also characteristic of higher altitudes, correlated with lower temperatures and with humidity (Albonico, 1948, cited from Backeljau et al., 2001).
Not a lot seems to be known on genetic control of colour variation, though Williamson (cited in Backeljau et al., 2001) reported that body pigmentation is determined by three loci, one with alleles for black melanin and brown melanin (black dominant), one controlling presence (dominant) or absence of dark lateral bands in young individuals, and one controlling the spread of melanin over the adult body. Three possible alleles were recognised at this last locus, one coding for an even spread (dominant), one for melanin confined to the mediodorsal area, and the last (recessive to the other two) coding for a white body, melanin present only in the tentacles and the fringe of the foot.
Photographs to show colour variation:
Most Arion species contract into a hemispherical bodyshape when disturbed. A. ater also may rock from side to side while in this contracted shape - a behaviour which separates it from related species.