Freshwater mussels

New Zealand freshwater mussels are a poorly known component of our waterways, but are found throughout New Zealand in streams, rivers and lakes, from pristine areas surrounded by native vegetation to often degraded and polluted lowland lakes and streams. They are found in sediment and feed by filtering small particulate matter, algae and bacteria from the water. The early life-cyle includes a tiny parasitic larval stage that attaches itself to native fish, then drops off when they have then been transported upstream. Growth is very slow and annual growth rings on shells indicate they may live over 50 years.  
    
Dead shells found scattered along riverbanks following flood events are a good indicator of the presence of living mussels.

Freshwater mussels can be difficult to spot as they are often buried in the sediment with just the edges of the shells showing. Often the inhalant (“frilled”) and exhalant (“plain”) siphons may be visible. Occasionally an orange or other coloured encrustation on the shell may be present as a result of a build up of minerals from the lake waters.

In lakes the mussels burrow into the bottom sediments in a area that limnologists call the “shell zone” which is associated with a steep drop off from shallow to deeper water.

In lakes with dense mussel populations their filter feeding activity significantly reduces algae and sediment suspended in the water column and thus improves water quality. However, farm animal effluent, and/or chemical runoff from weed spraying and fertilisers degrades many waterways resulting in reduction or extermination of mussel populations. The decline of freshwater mussel populations in New Zealand can be linked directly to the recent deterioration of the water quality in many lakes and rivers following land conversion to more intensive farming practices. 

Unlike their marine relatives, freshwater mussels do not attach themselves to a hard substrate. Trails left behind in the mud indicate that the mussels migrate to find the best conditions.

Most New Zealand mussels have thick shells. Research at the Museum of New Zealand Te Papa Tongarewa discovered that mussels parasitised by the worm-like larvae of a small midge suffer from thin shells, and the shape of their shells is sometimes severely affected. This relationship between a freshwater bivalve and a fly is unique to New Zealand.

Echyridella onekaka, a new species was recently recognised and described by Te Papa scientists. This species was initially detected in a DNA sequencing study of freshwater mussels from around New Zealand. The species had previously been misidentified as the common kakahi, but following the DNA result, further study of museum specimens revealed distinctive shell characteristics that enable easy field identification during biodiversity surveys.

Reference: Mark Fenwick and Bruce Marshall (2006). A new species of Echyridella from New Zealand, and recognition of Echyridella lucasi (Suter, 1905) (Mollusca: Bivalvia: Hyriidae). Molluscan Research 26(2): 69–76

Further information: Mussel Project Website: The MUSSEL Project (MUSSELp, for short) is an on-going study aimed at the global revision of the classification of the Unionoida, otherwise known as freshwater mussels.