Half a house is still a home

During October on a field trip to the Carnarvon Range northeast of Augathella, Richard, Rose and Ryan came across an interesting find – a semi-slug. We’ve all heard about snails and slugs but have you ever heard of a semi-slug? No? What is it? Glad you asked!

A semi-slug looks like it is halfway between a slug and a snail. Instead of having a shell that it can retract its body into, the shell is much smaller sitting on the back of the semi-slug with most of the animal exposed. Sounds like it should just get rid of the shell altogether, instead of carting around that added weight? Actually, there are a couple of good reasons for why a shell is still present.

Firstly, the shell offers some protection from predators such as birds. It is also good for camouflage – when the semi-slug partially wraps the mantle* around the shell it looks like a stick or sometimes even bird droppings! And let’s not forget, having a smaller shell means being able to fit into much tighter places like between rocks and crevices, allowing the semi-slug to hide from predators and to avoid drying out during the heat of the day. Another interesting way of evading predators is to exude a mucus-like substance or to break off a piece of its tail to distract the predator.

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Semi-slugs typically require moist habitats, such as rainforest and wetter types of eucalypt forest, so their range is limited to eastern Australia. Within the Brigalow Belt region, Craig and Queensland Museum staff have found them in the Carnarvon Range, Mt Zamia at Springsure, Mt Hutton near Injune, and throughout the Expedition Range. These places have small areas with suitable microclimates such as vine thickets (a form of dry rainforest), boulder piles or rocky gorges.

This particular semi-slug is a Dimidarion sp. from the Helicarionidae family. This is a southern hemisphere family, with species found in Southeast Asia, South Africa, New Guinea, the Solomon Islands and Australia. There are approximately 42 named species of semi-slug in eastern Australia (including Lord Howe Island) and likley to be many more undescribed and undiscovered species. Within this genus, there is even a famous semi-slug called the GT Striped Semi-slug. Dr John Stanisic of the Queensland Museum said when he first saw it and the 2 stripes on the animal, he immediately thought of the famous Australian race car driver, Peter Brock, and named it accordingly – the Dimadarion peterbrocki. The interesting thing here is that semi-slugs actually move about quite quickly, living life in the fast lane and the GT Striped Semi-slug is the fastest of the lot!

As for the semi-slug found by BOOBOOK staff, it may be Dimidarion minerva, however, more study is required on our local species, so for the BOOBOOK records it will remain Dimidarion sp. Who knows, it might be a whole new species!

*The mantle (the Latin word is pallium which means mantle, robe or cloak) is an anatomical structure located on the dorsal body wall of snails and semi-slugs. In some molluscs the mantle is involved in shell creation.


Stanisic, J., Shea, M., Potter, D., & Griffiths, O. (2010). Australian Land Snails Volume 1: A Field Guide to Eastern Australian Species. Mauritius: Bioculture Press.

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