The reproductive trickery of NZ’s freshwater mussels

Michele Melchior’s time spent wading through Waikato streams with her head in a “road cone” has led to a discovery about the reproductive tricks of one of New Zealand's least known creatures.

The University of Waikato PhD student is studying kākahi, New Zealand’s freshwater mussels. The “road cone” is an underwater viewer with a glass lens, which Melchior uses to help find the mussels.

Historically an important food source for Māori and featuring in at least one New Zealand cookbook recipe, freshwater mussels are now harder to find, and off the menu.

Although many people have never heard of them, Melchior says they play an important role in the aquatic ecosystem.

“We have these really slow-growing, old-aged, filter-feeding bivalves in our streams. Because they are filter-feeding they can purify waterways.”

Like mini underwater Roomba the mussels eat algae in the water and also take out pollutants and toxins. A single mussel can filter about one litre of water an hour. The toxins though can accumulate in the tissue of the mussel, making them unsafe to eat.

Even if a clean stream can be found with toxin-free mussels, it’s advised you don’t eat them. Globally, freshwater mussels are one of the most threatened freshwater organisms, with 70 percent of species, including New Zealand’s, considered at risk, or threatened.

Melchior said both species she works with are in peril including the “super rare” Echyridella aucklandica, which is listed as threatened and nationally vulnerable and Echyridella menziesii which is listed as at risk and declining.

Echyridella aucklandica (top two mussels) and Echyridella menziesii (bottom two mussels). Photo: Supplied

Despite poor water quality in many Waikato waterways Melchior has been finding mature mussels during her hours in the field. Home for them is in sheltered areas of streams, rivers, and lakes.

“They are usually really camouflaged and are partially buried. It’s most likely if you see them, you see these two little holes which are their syphons.”

While they do seem to be able to filter out many types of pollution, one thing they struggle with is silt.

“They don't tolerate accumulated siltation or loose sediment from runoff or banks because the sediment clogs their gills and they end up dying off.”

It’s likely more than just sedimentation is driving the decline in numbers. The long-living bivalve can last for 50 years, and while plenty of mature specimens can be found there’s a concerning lack of young mussels.

“That makes it much more special because this might mean that it needs a certain host fish."

Melchior’s research, funded by the Ministry of Business, Innovation and Employment as part of NIWA's Cultural Keystone Species programme, has focused on their unique method of reproduction and how two different mussel species can co-exist, sharing the same resources. The discovery she's made may hold clues to the lack of juvenile Echyridella aucklandica mussels.

Unlike their saltwater counterparts, freshwater mussels need other species to act as an underwater baby carrier.

Saltwater mussels expel eggs and sperm and these get fertilised externally. They then float around until they settle themselves into a permanent home.

Freshwater mussels need to ensure their young are not washed out to sea. Eggs are fertilised inside female mussels where they are kept until they develop into larvae about 0.3mm long and are released by the mother.

The larvae are parasitic. Survival relies on finding a freshwater fish, latching on to them around the gills, head or fin and hitching a ride upstream until they are developed enough to drop from the host fish, into a home. Once there, they burrow around 10cm down and live hidden for around five years before they slowly make their way upwards.

For some species, the larvae is just attached to mucous strings which attach to passing fish by chance. This is the case of the more common New Zealand Echyridella menziesii which produce thousands of larvae.

Other freshwater mussel species try to trick fish into investigating their larvae.

Some North American mussels produce larvae “packages” which resemble fly larvae, as a way to attract host fish. Melchior has discovered the Echyridella aucklandica, the rarer of the two mussel species she studies, also use this type of trickery to entice host fish. This has not been seen outside of North American species before.

“They release these packages of their little baby larvae which look like leeches,” said Melchior.

“That makes it much more special because this might mean that it needs a certain host fish, it's not a host generalist possibly. It might mean it needs one host fish and if that host fish isn't available anymore, it could mean the Echyridella aucklandica could be in a pretty awful situation.”

Known host fish for the more common species of freshwater mussel include kōaro, giant bully and the common bully. Melchior has been searching for what fish carry the larvae of the Echyridella aucklandica.

“We do electrofishing, where you look at native fresh species and try to find a little larvae on the fish. We haven't found them yet, so we don't know. That's the next step.”

Michele Melchior inspects the mussels to see if they are male or female and if larvae are present. Photo: Supplied

A recent report has shown 74 percent of New Zealand’s native freshwater fish are threatened or at risk, meaning there’s a chance host fish for the Echyridella aucklandica are in a situation as precarious as the mussels. Threats to native fish include the illegal blocking of waterways and pollution.

In other countries work is being done to increase freshwater mussel numbers. In Delaware in the United States, $11.5 million has been spent to set up a freshwater mussel hatchery. It’s hoped by introducing mussels back into waterways in greater numbers water quality will improve. The hatchery hopes to produce a million baby mussels a year.

In New Zealand NIWA, has also been trying to breed the mussels. It has produced thousands of juveniles in vitro in petri dishes, but most died when they reached two months old. NIWA suspects two months represents a developmental milestone where the gut and gills are possibly formed. It hopes to find ways around this in the future so the mussels can be used to restock waterways.

Melchior, who will spend much of her summer with her underwater viewer, said a key part of conserving the bivalves is understanding their basic reproductive needs.

Once the needs are known, conservation measures can be put in place.

“Freshwater mussels are a taonga species, so they're pretty important culturally but they also filter our waters. They can take out impurities, like bacteria and toxins from our waterways.”

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