Arionidae

Arionidae is a family of terrestrial, air-breathing slugs within the class Gastropoda and the order Stylommatophora. They are commonly known as roundback slugs due to their smoothly curved dorsal profile. The family includes several genera, with Arion as the best-known. Arionidae species occur in the Holarctic region, native to Europe and western Asia, with many species introduced elsewhere, including North America and parts of Asia and Oceania. They inhabit moist environments such as leaf litter, soil, under logs, rocks, and in gardens and agricultural areas.

Anatomy and reproduction: Members lack an external shell and have a mantle that covers much of the body. They possess a pneumostome on the right side of the mantle for respiration. They are simultaneous hermaphrodites, typically mating with another individual and laying eggs in damp soil or leaf litter.

Ecology and behavior: Most feed on decaying plant matter, fungi, algae, and fresh vegetation; some species are omnivorous scavengers. They are generally nocturnal or crepuscular to avoid desiccation and seek shelter in cool, damp microhabitats during the day. Predators include small mammals, birds, and invertebrates; slime secretions can deter some threats.

Taxonomy and notable species: Arionidae belongs to the superfamily Arionoidea within Stylommatophora. The group is diagnosed by particular anatomical features of the mantle and reproductive system, with molecular studies refining relationships within the group. Notable species include Arion ater (black slug), Arion rufus (red slug), Arion hortensis (garden slug), and Arion vulgaris (an invasive species in many regions).