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Biosystematics of New Zealand terrestrial molluscs

Our biosystematics research on the New Zealand land snail fauna combines morphological, molecular, ecology and geography to provide revisions, descriptions of newly discovered species, analyses of past and present distributions, and analyses of conservation priorities. We aim to supplement our publications in scientific journals with development of identification keys and fact sheets to assist biodiversity and biosecurity managers and members of the publiic.

Why study the biosystematics of land snails?

New Zealand fauna globally significant – the conservation imperative

By global standards, New Zealand has an extraordinarily rich land snail fauna (with c. 1400 species), which, being highly distinctive in composition, represents a major contribution to global land snail diversity. All but five of the species are endemic to the New Zealand region (Barker 2005).

Internationally, land snails are recognised as highly vulnerable to anthropogenic disturbances and, sadly, are the taxonomic group with the highest recorded rate of extinction over the past millennium. New Zealand land snail species feature in the IUCN Red List and make up a significant component of the listed threatened animals in the New Zealand Threat Classification System (Hitchmough et al. 2007). Indeed, many New Zealand land snails are recognised as of high conservation concern and under active conservation management. Many other, mostly undescribed, are listed as ‘range restricted’ or ‘data deficient’ and proper systematic characterisation of these is urgently needed to guide conservation planning.

Relevance to biosecurity in New Zealand

Molluscs are a highly successful invertebrate group. In terrestrial environments the global diversity is in the order of 35 000 species. Among these there are a number of species that are being widely dispersed via human commerce, travel, and classical biological control, and include many recognised pest species. Thirty-one exotic species are already established in New Zealand and additional exotic species are not infrequently intercepted at our ports or managed as incursions.

There is increasing demand for improved biosecurity management. Molluscs are just one taxonomic group that are demanding the attentions of Biosecurity New Zealand, but are important in terms of (1) the magnitude of the global diversity, (2) the difficulties in identifications and recognition of biostatus, (3) several species are declared as unwanted organisms and thus specifically targeted by border management, and (4) the suite of invasive species, potentially of concern to New Zealand, continues to grow with changing patterns of trade and travel.

Invasive mollusc species are of significance to New Zealand with impacts on the indigenous flora (herbivory), indigenous fauna (predators; disease and parasite vectors) and agriculture/horticulture (herbivory; vectors of plant pathogens and animal parasite; contaminants of commodities).

Relevance to ecosystem management in New Zealand

Most of New Zealand terrestrial species are detritivores – in the order of 20 000 to 25 000 species of microbes and invertebrates that principally occur in the soil litter and A-horizons. These species are critical to decomposition processes that drive nutrient cycling and ecosystem function. Yet, currently, decisions about conservation management or resource use provide for little consideration of this major component of biodiversity. Land snails are numerically an important component of the New Zealand soil fauna, with high sympatric richness (20–80 species), abundance (up to 10 000 m2), and contributions to detritus turnover in indigenous ecosystems.

Biogeographic and evolutionary models

Terrestrialism in molluscs is ancient. The earliest fossils of terrestrial forms date from the mid-Paleozoic, and the adaptative radiation into the higher taxa (superfamily/family) that characterise modern faunas had occurred by the Lower Cretaceous (Solem & Yochelson 1979; Barker 2001). Their ancient origin and slow evolution at higher taxonomic levels, combined with low vagility, high rates of cladogenesis, and high rates of allopatric speciation (Solem 1984, 1990), make terrestrial molluscs superb biogeographic and evolutionary models.

Plesiomorphically, terrestrial molluscs possess a coiled shell within which the visceral organs are contained and into which the cephalic structures can be fully withdrawn for protection. While shell loss has occurred independently in a number of lineages, giving rise to the slug body form, in most regions the modern terrestrial mollusc faunas are dominated by animals that retain the external shell (snails). The shell is particularly useful in investigations of evolution, palaeoecology, archaeology and ecology, in being (1) species-specific yet malleable in both form and size by environment (e.g. Goodfriend 1986; Emberton 1994, 1995a; Chiba 1996) within measurable developmental constraints (e.g. Gould 1992); (2) with the individuals’ entire ontogeny conserved and displayed in the shell of the mature animal; and (3) at the faunal level, the range of shell morphologies is highly diagnostic of evolutionary ecology (Cain 1977, 1978a, b, 1981).

The New Zealand region represents an evolutionary hotspot for Mollusca, and understanding the New Zealand fauna will lead to major contributions to global understanding of evolutionary and biogeographic processes.

Land snails represent a highly distinctive element of the New Zealand invertebrate fauna. As patterns of diversity (e.g. regional endemism; hyperdiversity radiations) mirror those in other invertebrate groups, land snails represent useful models for biosystematic and ecological research.

New Zealand land snail communities are characterised by extremely high rates of alpha diversity (20–80 species per site cf. global norm of 5–10 per site). Understanding evolutionary and ecological processes shaping in these communities should make a major contribution to community assemblage ecology (Solem & Climo 1985; Barker & Mayhill 1998; Emberton 1995b; Barker 2005).

References and suggested reading

Barker GM 2001. Gastropods on land: phylogeny, diversity and adaptive morphology. In: Barker GM ed.The biology of terrestrial molluscs. Wallingford, CABI Publishing. Pp. 1–146.

Barker GM 2005. The character of the New Zealand land snail fauna and communities: some evolutionary and ecological perspectives. Records of the Western Australian Museum, Supplement 68: 53–102.

Barker GM 2006. The astonishing diversity of land snails. In: Harvey B, Harvey T eds Waitakere Ranges. Ranges of inspiration. Waitakere City, The Waitakere Ranges Preservation Society. Pp. 130–139.

Barker GM, Mayhill PC 1998. Patterns of diversity and habitat relationships in terrestrial mollusc communities of the Pukeamaru Ecological District, northeastern New Zealand. Journal of Biogeography 25: 215–238.

Cain AJ 1977. Variation in the spire index of some coiled gastropod shells, and its evolutionary significance. Philosophical Transactions of the Royal Society of London, Ser. B, Biological Sciences 277: 377–428.

Cain AJ 1978a. The deployment of operculate land snails in relation to shape and size of shell. Malacologia 17: 207–221.

Cain AJ 1978b. Variation in terrestrial gastropods in the Philippines in relation to shell shape and size. Journal of Conchology 29: 239–245.

Cain AJ 1981. Variation in shell shape and size of helicid snails in relation to other pulmonates in faunas of the Palaearctic Region. Malacologia 21: 149–176.

Cain AJ, Cowie RH 1978. Activity of different species of land-snail on surfaces of different inclinations. Journal of Conchology 29: 267–272.

Cameron RAD, Cook LM 1989. Shell size and shape in Madeiran land snails: Do niches remain unfilled? Biological Journal of the Linnean Society 36: 79–96.

Chiba S 1996. Ecological and morphological diversification within single species and character displacement in Mandarina, endemic land snails of the Bonin Islands. Journal of Evolutionary Biology 9: 277–291.

Cowie RH 1995. Variation in species diversity and shell shape in Hawaiian land snails: in situ speciation and ecological relationships. Evolution 49: 1191–1202.

Emberton KC 1994. Partitioning a morphology among its controlling factors. Biological Journal of the Linnean Society 53: 353–369.

Emberton KC 1995a. Sympatric convergence and environmental correlation between two land-snail species. Evolution 49: 469–475.

Emberton KC 1995b. Land-snail community morphologies of the highest-diversity sites of Madagascar, North America, and New Zealand, with recommended alternatives to height-diameter plots. Malacologia 36: 43–66.

Goodfriend GA 1986. Variation in land-snail shell form and size and its causes: a review. Systematic Zoology 35: 204–223.

Gould SJ 1992. Constraint and the square snail: life at the limits of a covariance set. The normal teratology of Cerion disforme. Biological Journal of the Linnean Society 47: 407–437.

Hitchmough R, Bull L, Cromarty P 2007. New Zealand Threat Classification System lists 2005. Wellington, Department of Conservation. 194 p.

Solem A 1984. A world model of land snail diversity and abundance. In: Solem A, van Bruggen AC edsWorld-wide snails, biogeographical studies on non-marine Mollusca. Leiden, Brill & Backhuys. Pp. 6–22

Solem A 1990. Limitations of equilibrium theory in relation to land snails. In: Proceedings of an 'International Symposium on Biogeographical Aspects of Insularity'. Rome, 18–22 May 1987. Atti Dei convegni Lincei 85, Rome, Accademia Nazionale dei Lincei. Pp. 97–116.

Solem A, Climo FM 1985. Structure and habitat correlations of sympatric New Zealand land snail species. Malacologia 26: 1–30.

Solem A, Yochelson EL 1979. North American Paleozoic land snails, with a summary of other Paleozoic nonmarine snails. Geological Survey Professional Paper 1072. Washington, DC, US Government Print Office.

Solem A, Climo FM, Roscoe DJ 1981. Sympatric species diversity of New Zealand land snails. New Zealand Journal of Zoology 8: 453–485.