The secret lives of freshwater mussels

By Waiology 05/12/2014 3


By Kevin J Collier and Sue Clearwater

2014IconYou 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.

Part of a bed of freshwater mussels quietly doing their thing 24/7.  The inhalant siphon is fringed with tentacle-like sensory cilia and will occasionally “cough out” clumps of pseudo-faeces comprised of particles that have been rejected for ingestion. (Credit: Bruno David, Waikato Regional Council.)
Part of a bed of freshwater mussels quietly doing their thing 24/7. The inhalant siphon is fringed with tentacle-like sensory cilia and will occasionally “cough out” clumps of pseudo-faeces comprised of particles that have been rejected for ingestion. (Credit: Bruno David, Waikato Regional Council.)

The cryptic[1] 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 EchyridellaE. 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.

Glochidia (360 um length) attached to the fin of a common bully. A layer of cells has grown over the glochidia within 24 hours of attachment encysting it while it transforms into a juvenile. (Credit: Brian Smith, NIWA)
Glochidia (360 um length) attached to the fin of a common bully. A layer of cells has grown over the glochidia within 24 hours of attachment encysting it while it transforms into a juvenile. (Credit: Brian Smith, NIWA)

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.

Newly transformed juvenile mussels.  The triangular glochidial shell is augmented with shell growth around the margins, as well as a lip of protruding mantle. A large ciliated foot extends from the shell (top right) seeking food. (Credit: Karen Thompson, NIWA)
Newly transformed juvenile mussels. The triangular glochidial shell is augmented with shell growth around the margins, as well as a lip of protruding mantle. A large ciliated foot extends from the shell (top right) seeking food. (Credit: Karen Thompson, NIWA)

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.

Surveying along the edges of a river – a favourite hiding place for freshwater mussels.
Surveying along the edges of a river – a favourite hiding place for freshwater mussels.

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.

[1] 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.


3 Responses to “The secret lives of freshwater mussels”

  • 30 years ago when I was active in water ecology studies fresh water mussels were called hyridella. Obviously I have not kept up with the world.
    When did the nomenclature change?

    • Hi Mike

      I think the genus Echyridella was nominated in the McMichael and Hiscock paper of 1958. I’m not entirely sure why Hyridella stuck around for so long – Marshall et al’s recent (2014) paper on New Zealand recent Hyriidae provides the latest taxonomic review of Hyriidae based on molecular data (Molluscan Research 34:3).

  • I have been familiar with the Tiraumea River (flows into the Manawatu between Pahiatua and Woodville) and many of it’s tributary streams for the last 50 years. There is a fresh water mussel species throughout this system,but I wouldn’t know which in particular. I would like to know more about this creature. Most of the water-shed of this river is free from dairy waste, but the lower 12-15 km before the confluence with the Manawatu, so I assume there are some healthy colonies, given the number of shells I’ve seen in some areas. I think native cormorants prey on them and maybe long finned eels on smaller shellfish.

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