Freshwater Mussels (Kākahi)
Discover the vital role of New Zealand's native freshwater mussels in maintaining healthy aquatic ecosystems
About Kākahi
Freshwater mussels, known as kākahi in Aotearoa New Zealand, belong mainly to the species Echyridella menziesii. They typically grow up to 100 millimetres in length and have elongated, oval-shaped shells. Their shell colour ranges from brown to dark green, helping them blend into riverbeds and lake margins.
Kākahi are found in rivers, streams, lakes, and slow-flowing freshwater environments throughout New Zealand, particularly where water is clean, well-oxygenated, and the substrate is stable sand or gravel.
Life Cycle
The life cycle of freshwater mussels is complex and closely linked to native fish.
Larval Release
Females release microscopic larvae called glochidia into the water.
Fish Host Stage
These glochidia must attach to the gills or fins of appropriate fish hosts, where they develop safely for several weeks.
Settlement & Growth
Once mature, they detach from the fish and settle into the sediment as juvenile mussels. From there, they grow slowly, taking several years to reach full size and sexual maturity.
Habitat & Feeding
Freshwater mussels live partially buried in the sediment with only part of their shell exposed. They are filter feeders, drawing in water through siphons and removing plankton, bacteria, and organic particles. This feeding method plays a major role in maintaining water clarity and quality.
Kākahi prefer stable freshwater systems with low pollution, minimal sediment disturbance, and strong riparian vegetation.
Morphology
Shell Structure
Morphologically, freshwater mussels have thinner and more delicate shells than marine mussels. Their smooth outer shell protects the soft body inside, while strong internal muscles allow them to close tightly when disturbed.
Age & Longevity
Growth rings on the shell can indicate age, with some individuals living for several decades under suitable conditions.
Ecological Importance
Ecologically, freshwater mussels are extremely important.
Water Quality
They improve water quality through filtration, contribute to nutrient cycling, and help stabilise sediments.
Habitat Provider
Their shells provide habitat for algae and small aquatic organisms, and they form part of the food web for birds, eels, and fish.
Ecosystem Health Indicator
Because they are highly sensitive to environmental change, their presence is a strong indicator of freshwater ecosystem health.
Threats to Survival
Freshwater mussels face serious threats throughout New Zealand.
- Habitat degradation from river engineering, damming, and wetland drainage reduces suitable living areas.
- Sedimentation from forestry, farming, and earthworks can smother mussel beds.
- Pollution from agricultural runoff, nutrients, and chemicals degrades water quality and affects mussel survival and reproduction.
- The decline of native fish also impacts mussels by reducing the availability of host species needed for their larval stage.
Additional Pressures
Invasive Species
Introduced species further increase pressure on kākahi populations. Predatory fish and invasive crayfish can disturb sediments and prey on juvenile mussels.
Climate Change
Climate change adds extra stress by raising water temperatures and increasing the frequency of droughts and extreme floods.
Conclusion
In conclusion, kākahi (Echyridella menziesii) are a vital component of New Zealand's freshwater ecosystems. Their role in filtering water, stabilising sediments, and supporting food webs makes them ecologically invaluable.
Protecting water quality, restoring rivers and wetlands, and maintaining healthy native fish populations are essential to ensure the long-term survival of New Zealand's freshwater mussels.