Dot Snails

A dot snail Punctum pygmaeum crawls over a mountain bulin Ena montana (itself not a very large snail). Copyright Stefan Haller.

The dot snails of the family Punctidae are one group of animals that certainly lives up to their name. For the most part, these are absolutely tiny terrestrial snails, sometimes barely passing the millimetre mark. They are united as a family by anatomical features of their radulae and reproductive systems (Solem 1983), but they show a variety of external morphologies, from higher-spired trochoid forms to flatter discoid forms. They also vary in ornament, with some being fairly smooth but others marked with distinct ridges, in some cases with spiny extensions of the periostracum. Most punctids are found in leaf litter, in which they graze on microalgae and detritus. The diverse fauna of punctids on Lord Howe Island, however, includes at least two species, Dignamoconcha dulcissima and Allenella formalis, that are arboreal, found living on palm tree leaves (Stanisic et al. 2010). Molecular analysis supports the inclusion of punctids in a group of small pulmonate gastropods known as the 'endodontoid clade' (Wade et al. 2001).

Live individual of Paralaoma servilis, from Christensen et al. (2012). Scale bar = 1 mm.

One would expect that such tiny snails would be easily overlooked, and it would not be surprising to find them amongst the tally of species that have been transported outside their native ranges by humans. One species, Lucilla singleyana, is associated with greenhouses in Europe; though known there from the fossil record, it is believed to have become extinct at the end of one of the interglacial periods and been subsequently reintroduced from North America (Alexandrowicz 2010). Another species, Paralaoma servilis, is believed to have originally been native to New Zealand but has been transported around the world, with expanding populations in Australia, Europe, North and South America (Christensen et al. 2012). Seeing as the fauna of my native New Zealand has suffered a lot at the hands of exotic introductions, I have to confess to a certain satisfaction at the idea that we're giving our own back.

REFERENCES

Alexandrowicz, W. P. 2010. Lucilla singleyana (Pilsbry, 1890) (Gastropoda: Pulmonata: Punctidae) in recent flood debris in the Beskidy Mts (southern Poland). Folia Malacologica 18 (2): 83–92.

Christensen, C. C., N. W. Yeung & K. A. Hayes. 2012. First records of Paralaoma servilis (Shuttleworth, 1852) (Gastropoda: Pulmonata: Punctidae) in the Hawaiian Islands. Bishop Museum Occasional Papers 112: 3–7.

Solem, A. 1983. Endodontoid land snails from Pacific islands (Mollusca: Pulmonata: Sigmurethra). Part II. Families Punctidae and Charopidae, zoogeography. Field Museum of Natural History: Chicago.

Stanisic, J., M. Shea, D. Potter & O. Griffiths. 2010. Australian Land Snails vol. 1. A field guide to eastern Australian species. Bioculture Press: Mauritius.

Wade, C. M., P. B. Mordan & B. Clarke. 2001. A phylogeny of the land snails (Gastropoda: Pulmonata). Proceedings of the Royal Society of London Series B 268: 413–422.