This species belongs to the Austropyrgus sparsus group, whose members are characterised by the following shell features: small to medium sized shells, conical, with convex to slightly convex whorls. Furthermore, in females, the coiled oviduct is of an inverted U-shape or with two or more bends, loops or twists.
Austropyrgus pusillus differs from other members of the group in the following combination of characters: shell small, with straight to weakly-convex spire outline and convex whorls; ctenidium with 9–12 filaments; prostate gland about 2/3 in pallial roof; ovary simple.
This species has been found in sympatry with four other species of Austropyrgus (A. exiguus, A. glenelgensis, A. tumidus and A. latus) throughout its currently recognized range. It differs from A. exiguus in its very small, squat shell, and weakly thickened outer lip (heavily thickened in A. exiguus), from A. glenelgensis in its much smaller shell with a more truncated spire, from A. tumidus in its much smaller, narrower shell, and from A. latus in its much smaller shell with convex whorls. The only other taxon with a similar shell is A. salvus from Tasmania, which is larger in size.
Austropyrgus pusillus Clark, Miller & Ponder 2003
Original name: Austropyrgus pusillus Clark, Miller & Ponder 2003. Clark, S. A., Miller, A. C. and Ponder, W. F. (2003). Revision of the snail genus Austropyrgus (Gastropoda: Hydrobiidae): A morphostatic radiation of freshwater gastropods in southeastern Australia. Records of the Australian Museum Supplement 28: 1–109.
Type locality: Small tributary of Glenelg River, at Dartmoor, Victoria.
In streams on water weeds, hard substrate (rocks etc.) and crawling on litter and sediment. Can be locally abundant. Assumed to feed by scraping bacteria and microalgae. Lay solitary capsules containing a single egg. Direct development.
This species is known from a number of small springs and streams which flow into the lower part of the Glenelg River, western Victoria.
Clark, S. A., Miller, A. C. & Ponder, W. F. (2003). Revision of the snail genus Austropyrgus (Gastropoda: Hydrobiidae): a morphostatic radiation of freshwater gastropods in southeastern Australia. Records of the Australian Museum 28: 1–109.