Deroceras laeve, the marsh slug, is a species of small air-breathing land slug, a terrestrial pulmonate gastropod mollusk in the family Agriolimacidae. Distribution The distribution of Deroceras laeve was originally Palearctic, from the subpolar zones to the southern margins. Today this slug species has been introduced worldwide except Antarctica, also on tropical islands such as New Guinea and on Pacific islands. Europe: British Isles: Great Britain and Ireland. The species has shown little evidence for significant decline in Britain. Czech Republic - least concern (LC) Netherlands Poland Slovakia Ukraine Sweden Switzerland - lower concern in Switzerland and others Asia: Pakistan China Taiwan The Americas: California Costa Rica El Hatillo Municipality, Miranda, Venezuela Colombia introduced to Dominica (first report in 2009) introduced to Brazil and others Description The slug is from brown to dark brown, usually with dark and characteristic but not well visible spots arranged in groups. The shape is almost cylindrical, posterior end is abruptly widened. The mantle covers 50% of body length (unusually large). There are wrinkles on skin present (may disappear in preserved slugs). Mucus is thin, colourless. This slug is 15–25 mm long when preserved. The populations of this species that occur in Central Europe are much darker than other Deroceras species in that area. Reproductive system: Penis is often reduced, elongated if present, without proper penial gland but with two or more tiny glandular papillae and its end. Retractor is unforked and attached at half penis length, stimulator small, cone-shaped but looks more like a papilla. Tubular oviductus and atrium are unusually long. There is no rectal caecum. Ecology Habitat Deroceras laeve has high ecological tolerance, but needs permanently wet habitats. It is usually found in lowlands and very humid habitats, swamps, riversides, wetlands, especially alder and oak woods, marshlands and degraded areas, also greenhouses, often near water under wood or detritus. It tolerates subpolar and tropical temperatures. Newly created habitats are often colonized after a few years. In Switzerland in up to 1800 m altitude, but usually below 1000 m, in Bulgaria in up to 2500 m. Deroceras laeve can be a serious pest in greenhouses. On the other hand, the species is threatened by continuous elimination of wet habitats by drainage, construction projects and road construction. It is the only land gastropod that goes deliberately into the water and can survive for days submerged. Because of this unusual behavior, the species can be