FNZ 38 - Barker, G. M. 1999 Naturalised terrestrial Stylommatophora (Mollusca: Gastropoda) - Popular Summary
Barker, GM 1999. Naturalised terrestrial Stylommatophora (Mollusca: Gastropoda). Fauna of New Zealand 38, 253 pages.
( ISSN 0111-5383 (print), ; no. 38. ISBN 0-478-09322-5 (print) ). Published 25 Jan 1999
Gastropods, or slugs and snails, are a very diverse group of molluscs. Most are marine, but many occur in freshwater and terrestrial habitats. Worldwide, terrestrial gastropods have been estimated to number about 35 000 species. New Zealands indigenous terrestrial gastropod fauna is among the richest in the world, with an estimated 1350 species. However, of the estimated global diversity of 71-92 families only 11 are represented in New Zealands indigenous fauna.
Through human activities, many species of terrestrial gastropod have been spread to and naturalised in areas outside their original range. Often these species have become pests in their new homes due to their damage to cultivated crops, their role in the transmission of parasitic diseases that affect humans and livestock, or their adverse effects on indigenous flora and fauna.
Our naturalised terrestrial gastropod fauna currently comprises 29 species, representing 15 families that were not part of the original New Zealand fauna. These species originate in Europe, North America, or the Pacific; some may have been introduced secondarily from populations first naturalised elsewhere. The species established in New Zealand are associated with man and his crops in their native range, with great propensity for passive dispersal, and have been widely distributed through human commerce. Several species 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.
The general body plan in most terrestrial gastropods comprises the conical visceral mass coiled into a spiral within a single shell, and the head-foot which outwardly comprises a ventral muscular pad for locomotion and anteriorly a mouth and two pairs of tentacles. These molluscs, commonly known as snails, are able to completely retract their head-foot into the shell for protection from desiccation and from predators. Features of the shell, and of the external morphology and internal anatomy of the head-foot, are important in the identification of snails. There are 15 species of snail in our naturalised fauna.
In the moist and humid conditions that prevail in many regions of the world, the ability to retract into a protective shell has been less important, and many groups of terrestrial gastropods have evolved forms with a reduced shell. In the initial stages of this evolution the shell is carried on the animals back, but is no longer able to fully protect the entire animal. Animals at this stage of evolution, commonly known as semi-slugs, are represented in our naturalised fauna by a single species.
In the more advanced stages of this evolution the shell is reduced to a small plate or granules enclosed within the head-foot tissues, or lost completely. These animals, known as slugs, are simply snails with a reduced shell, and their identification is more dependent on the external morphology and internal anatomy of the head-foot. There are 14 species of slug in our naturalised fauna.