by John Walsby
The freshwater mussels are the largest molluscs found in our lakes and rivers. There
are some other bivalves but they are very small and difficult
to find in their muddy habitats. The most common molluscs are
small snails with pointed spires that live on stones, mud, weed,
and logs at the water's edge grazing the broths and slimes of
microscopic algae that grow on these surfaces.
The commonest, Potamopyrgus antipodarum, is often found away from strong water flow, often in densities of several hundred per square metre. They grow to 10mm tall but are usually smaller and vary in colour from tan to dark brown. Many of the snails have a spiral crown, or corolla, of bristles protruding from the shell.
The largest native freshwater snail Melanopsis trifasciata, grows to 30mm tall. Adults often have the spire tip broken clean off, exposing the white shell below its shiny black covering. Melanopsis is found on and under stones. It shuns bright light but may be found in the open where there is dense overhanging vegetation.
Did you know that limpets are found in freshwater as well as on the seashore ? The commonest and largest freshwater limpet Latia neritoides reaches 10mm in diameter and is found throughout the North Island. As with marine limpets, the embryo shell is coiled, but the adult spiral twists so slowly that the shiny flat shell forms a simple cone.
The smooth, streamline shells of both marine and freshwater limpets allow these grazers to colonise places where swift water movements would dislodge other snails. Latia can often be found most easily by running your hand over smooth stones which are slippery with algal slimes, in briskly flowing streams.
This limpet is unique among freshwater snails in New Zealand in glowing in the dark.
Take a couple home and keep them in the refrigerator on a white saucer with just a tablespoon of water. At night, with the lights out, enjoy their spectacular light show. Under their shells, the limpets bodies glow bright greenish yellow and, if you move them, a trail of luminescent mucus spreads into the water.