by John Walsby
On seashores, bivalve
shellfish are varied and plentiful but in freshwater habitats they are less common. The most notable are the freshwater mussels
that can be found in rivers and lakes buried just below the surface
of soft muddy bottoms. They are more common than is generally
appreciated, but you have to feel for them with your finger or
toes to find them in the muddy beds of lakes and streams.
Hyridella menziesi is the most common of the three species in New Zealand. Over the fragile wedge shaped shell it has a thick, dark brown glossy covering which is often worn away near the hinge. All are quite large shellfish, commonly as big as a good sized pipi (75mm long) and occasionally as big as a small toheroa (100mm long).
They are seldom collected for food nowadays but in pre-European times, Maoris sought the mussels, or kakahi, as a special food for young children and sick people. Before eating any today you would need to be certain that the waterway was not contaminated with cowshed wastes, effluent from septic tank seepages or municipal sewerage discharge.
For all sedentary or slow moving freshwater animals, the dispersal of young is a hazardous event. Like most bivalve shellfish, these mussels produce minute, free swimming larvae. When such larvae are released into the water that flows only one way, they must have a means of moving upstream or they will be washed out to sea.
Mussel larvae have a fascinating method of moving against the current. Each larva, or glochidium, has a long sensory filament through which it can detect the presence of small fish, such as bullies and koaro on which it hitches a ride. Along the inner edges of the larval shell are a series of hooks and these "teeth" allow it to hang on tightly to the fish. They may be carried along for several weeks before dropping off upstream.
After finding a suitable soft, muddy bottom they change into miniature versions of the adults and become sedentary filter feeders.