Freshwater mussels.
Freshwater mussels are generally found out of the main flow of rivers and streams, under overhangs or around fallen branches.
New Zealand has three species of freshwater mussels. One of these amazing bivalves is found in Nelson, one in the northern North Island and the other all through the country.
They were once numerous (there are stories of collecting canoes-full) but are now much rarer due to pollution, new fish species being introduced, habitat destruction and the eutrophication of waterways.
Unlike their marine counterparts, the freshwater mussel cannot anchor itself with threads. Instead it buries itself into the sediment, leaving just the filters, inhalant (frilled) and exhalent (plain), exposed on their shell edge.
As they are so well hidden they are often difficult to see, and it is not until a flood deposits dead shells on the river bank that their presence is known.
Freshwater mussels are generally found out of the main flow of rivers and streams, under overhangs or around fallen branches. They do not like sandy or stony-bottomed waterways, as they need sediment to bed into. In lakes they are found in the zone where shallow water changes to deep.
The mussels filter water to obtain nutrients, algae and bacteria, which help purify the water. Filtration rates of one litre per hour have been recorded, and if numbers are high (up to 800 per square metre) water quality can improve quickly. They also oxygenate the sediment by moving around.
Living in water travelling in only one direction can make dispersal of offspring tricky. The larvae overcome this by using fish for transport. In summer males release huge amounts of sperm into the water. The females then inhale this, and the fertilised larvae (glochidia) are ejected.
They then have a few days to find a fish, usually a koaro, using their long sensory filament. Small hooks then latch the larvae on to the fins and gills, and the fish transports the young mussels upstream for a few weeks.
The larvae feed by catching small particles, algae and bacteria, with their ciliated foot. Once they have ditched their ride the juveniles (less than 0.5mm) bury themselves into the sediment. Over the next 50 years they can grow up to 10cm long.
Recent research has shown a unique relationship, where the mussel is parasitised by a worm-like midge larva that causes the mussel shell to change shape and reduces its thickness reduce.
Maori used these mussels raw, cooked and dried. They were fed to orphaned infants and the sick. The shell was used to cut hair and umbilical cords and to scrape vegetables.
They are very sensitive to ammonia and copper, and, as these are often found in polluted waters, reproductive rates have dropped.
The remaining population of geriatric mussels then slowly dies off. The loss of fish species is also be a major contributor to mussel decline.