Bill Petty, of Mosgiel, asks :-

Can you please tell me what these are? I find these regularly on the concrete at the bottom of the garage door but no where else. They are about 3mm across they look like baby hardback snails but I don't have any of these in my garden

Hamish Spencer, a zoologist and snail expert at the University of Otago, responded.

Your snails look like young individuals of the Cellar Glass-Snail, Oxychilus cellarius. This species was accidently introduced to New Zealand from Europe (probably Britain) over 150 years ago. It lives in damp, shady habitats, often near human habitation, on rock walls and even in cellars (hence its name), but sometimes also in our native forests. The species is a world-wide traveller, having also spread to North and South America, Australia, South Africa and the Philippines.

Two other species of Oxychilus, the Garlic Glass-Snail, O. alliarius, and Draparnaud’s Glass-Snail, O. draparnaudi, have also been introduced, but they are not so widespread and not yet recorded from the Dunedin area. These three species are very difficult to tell apart by their shells, but, uniquely, living Garlic Glass-Snails smell strongly of garlic if the animal is irritated, for example by being rubbed. This odour is apparently a defence against predation, especially by hedgehogs, which avoid eating this species.

Glass-Snails are omnivorous, eating plants, small snails and slaters. In New Zealand they are a threat to some of our smaller native land snails, which mostly occur in native forests. O. cellarius has been observed preying on several native snail species; patches of bush where it is common have fewer native snails. There are about 800 species of these tiny native snails, and the level of diversity, as measured by the number of species that can be found in one area of forest, is much higher than in other parts of the world. The vast majority of our species are endemic; they occur only in New Zealand.

Like most land snails, the Cellar Glass-Snail is hermaphroditic, having both male and female functions. They lay clusters of tiny eggs of about 1.5 mm in diameter in the soil in spring. Small snails hatch directly from these eggs and grow rapidly up to an adult size of about 12 mm in diameter.