Arion is a genus of air-breathing land slugs in the family Arionidae, the roundback slugs.

Most species of this Palearctic genus are native to the Iberian Peninsula.

Species can be difficult to distinguish from one another upon cursory examination, because individuals of a species can vary in color and there are few obvious differences between taxa. The color of an individual can be influenced by its diet.

Some Arion are known as pests, such as A. lusitanicus auct. non Mabille (= A. vulgaris), which damages agricultural crops and ornamental plants, and A. rufus, a familiar garden pest. Arion slugs are often transported internationally in shipments of plant products and mushrooms. Arion slugs have been identified in North America and Australia as invasive species, altering the plants of ecosystems through seed predation and competing with native slugs.

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