By Kevin J Collier and Sue Clearwater
You may not see them, but they are probably out there somewhere…hiding under overhangs and around fallen branches out of the main flow along stream banks, or buried in soft sediments on lake bottoms with only their siphons showing. And where you find one freshwater mussel there are likely to be more, sometimes many more.
The cryptic habits of these mussels belie their ecological significance. They are ecosystem engineers – when present in large numbers they filter impressive volumes of water, transform carbon and nutrients, oxygenate sediment as they move around, and create beds of stable habitat that can benefit other freshwater life. A single mussel can filter around a 1 litre of water per hour, and so dense mussel populations can process the whole volume of a shallow lake in a matter of days, removing fine organic particles and sediment suspended in the water and increasing water clarity. Densities can be extremely high in some lakes, reaching over several hundred per square meter in parts of Lake Taupo and up to 800 per square metre in Lake Rotokawau.
New Zealand has three species of freshwater mussel all belonging to the genus Echyridella – E. onekaka is restricted to north-west Nelson, E. aucklandica is most commonly encountered in northern New Zealand, and E. menziesii is found throughout the country. This diversity is low compared to some other parts of the world such as the ancient drainages of the eastern United States, with over 100 species found in the state of Tennessee alone. Surprisingly little is known about the biology of the less common New Zealand species E. aucklandica and E. onekaka, or about the ecology of our native mussels in streams and rivers generally.
Some freshwater mussels have evolved elaborate lures to attract fish which act as hosts for mussel larvae, known as glochidia, that are ejected and attach onto gills and fins of the host fish (see the Unio Gallery http://unionid.missouristate.edu/). As far as we know, New Zealand mussels are less showy, modestly discharging fertilised glochidia into the water column or attached to strings of mucus. This is done during summer after males have released sperm into the water column. Fertilisation is dependent on females inhaling sperm, a risky strategy probably requiring dense populations in streams and rivers where sperm are rapidly dispersed and diluted in the moving water column.
After release glochidia have 2-3 days to find a fish host to provide nutrients and dispersal to other habitats. Some overseas mussels are host-specific but in New Zealand they appear able to use a range of fish species including koaro, bullies and trout. Following a period of time hitch-hiking, the larvae fall off the fish and settle into the sediments.
Juvenile mussels (< 0.5 mm length) are extremely hard to find and possibly live in the interstitial spaces between sediment grains within the beds of rivers and lakes, but eventually they emerge to the surface to perform their important ecological roles. As larvae they have the unusual habit of feeding with their ciliated foot (pedal feeding) which is used to capture small algae, bacteria and organic particles.
An astounding feature of freshwater mussels is their longevity. It has been estimated that E. menziesii can live 40-50 years based on annual growth rings (annuli) laid down in their shells. E. aucklandica is larger and may live even longer. They can therefore perform ecological functions for a substantial period of time.
The glochidia and juveniles are very sensitive to contaminants such as ammonia and copper – both common in urban and agricultural pollution. This contaminant sensitivity might explain why mussels are probably failing to reproduce in small streams and in eutrophic lakes – leaving only populations of geriatric adults that will slowly die out. This phenomenon is known as ‘extinction debt’. Pressures such as changes towards flashy flow regimes, predation, sedimentation, deoxygenation in eutrophic lakes and loss of their preferred larval hosts are probably also contributing to a decline in freshwater mussels globally.
 Cryptic = hidden, as in ‘crypt’.
Dr Kevin Collier is an Associate Professor at the Environmental Research Institute, University of Waikato. Dr Sue Clearwater is a freshwater ecologist at NIWA.