Focusing on naturalized terrestrial gastropods, this is the 38th volume of a series on the fauna of New Zealand. Gastropods (slugs and snails) are a very diverse group of molluscs: most are marine, but many occur in freshwater and terrestrial habitats, and it is estimated that there are about 35,000 terrestrial species world-wide. New Zealand's indigenous terrestrial gastropod fauna is among the richest in the world, with an estimated 1350 species, but only 11 of the estimated global diversity of 71-92 families are represented in the New Zealand indigenous fauna. New Zealand's naturalized terrestrial gastropod fauna currently comprises 29 species, representing 15 families that were not part of the country's original fauna. These species originate in Europe, North America or the Pacific; some may have been introduced secondarily from populations first naturalized elsewhere. The species established in New Zealand are associated with humans and their crops in their native range, with great propensity for passive dispersal, and have been widely distributed through human commerce.
Several species became established in New Zealand so early during the period of European settlement that zoologists of that time thought them to be members of the native mollusc fauna. There are 15 species of snail and 14 species of slug in the New Zealand naturalized fauna.