A compact, spiral shell rarely exceeding five millimetres in length. The shell is a mottle of grey, brown or black, providing perfect camouflage against river stones. Its most formidable tool is the operculum, a small, leathery trapdoor that the snail pulls shut to seal itself inside. This seal is so effective that the snail can survive for days out of water or pass entirely through the digestive tract of a predatory fish or bird, emerging alive and ready to crawl away. Most New Zealand water snail populations are parthenogenetic. Females produce genetically identical daughters without needing a male. A single snail can colonise an entire new stream system with staggering speed. This reproductive strategy allows them to exploit disturbed habitats where other species struggle to establish. They are scrapers by trade. A specialised, ribbon-like tongue called a radula, covered in thousands of microscopic teeth, rasps algae and organic biofilm off rock surfaces. This constant cleaning prevents algae from smothering the riverbed, ensuring the water remains oxygenated for other species. A single square metre of healthy stream bed can support hundreds of thousands of water snails, each one grazing a tiny patch of algae. By consuming massive amounts of primary waste and algae, they convert raw energy into high-protein snacks for native fish like galaxiids and bullies. They are the janitors of the stream, working around the clock to process nutrients that wash down from the forest canopy. Because they are so tough, they act as a biological baseline. If even the water snails begin to disappear from a stream, it is a sign that the waterway is in deep trouble. New Zealand species have been accidentally introduced to Europe, North America and Australia, where they have become invasive. In their new homes, they outcompete native snails and alter the ecology of lakes and streams. What is a native success story here is a pest elsewhere.