By Guest Author 30/10/2020

Recently, the Science Media Centre ran the third round of its 2020 SAVVY Video Competition for science researchers. With entries ranging from kea tracking to Beethoven’s piano pieces, we judges were incredibly impressed by the creativity and quality of submissions. This week, we’re featuring the work of the winner, conservation biologist Bridgette Farnworth.

Our streams and lakebeds host unsung heroes, who work 24-7 to filter contaminants and toxins out of New Zealand’s waterways. They are underappreciated employees…with no tax cuts or pay rises. There is no company vehicle or reserved carpark for these sessile organisms, and they are never invited to the annual Christmas function.

Instead, their work places are buried within the sediment where, submerged within their aquatic worlds, they silently purify water for all other freshwater species. Collectively these ‘workers’ are known as kākahi (freshwater mussels); a culturally significant species for Māori that are now rapidly declining.

Note: This video is an extended version of the competition-winning entry.

Without intervention, all three of New Zealand’s unique species of freshwater mussel are currently threatened with extinction: a loss that could have significant impacts on our water quality. The largest mussel species, Echyridella aucklandica (Threatened: Nationally Vulnerable), is found throughout the northern North Island, and in a few southern lakes, while the smaller Echyridella menziesii (At Risk: Declining) is found throughout New Zealand. The third species Echyridella onekaka (At Risk: Naturally Uncommon) is the least understood species, with a very small range within north-west Nelson.

Kākahi have a complex lifestyle whereby the females release larvae (called glochidia; pronounced “glock-kid-ee-a”) that attach themselves to the gills of certain species of native fish. Like tiny vampires, the glochidia cling on and hitchhike their way to reach the soft, sandy sediments of rivers and lakes. Here, the glochidia drop off as juvenile mussels and begin their life in river and lakebeds. Although adult kākahi can bury themselves and wiggle around in the sediment, they are mostly immobile, so this amazing relationship between fish and mussels allows kākahi to distribute offspring far and wide. Buried in the sediment with their siphons sticking out, adult kākahi filter feed by sucking in water and extracting nutrients for the remainder of their lives – 50 years or more.

Adult kākahi are inconspicuous – they never feature their struggles on celebrity Instagram accounts and they certainly don’t tweet about their public services – so many people are unaware of the plight that they face. However, I’m hoping to raise awareness for these forgotten freshwater treasures by publicising the threats that kākahi face. Habitat destruction, the loss of the special fish species that are essential for the kākahi life cycle and water pollution all place freshwater mussels at risk. In addition, one emerging problem for kākahi is that predators are eating them – potentially, at a faster rate than kākahi can reproduce. 

As a conservation biologist, I’m particularly concerned that something is dining-out on too many kākahi. Piles of broken mussel shells around waterways first alerted scientists to this important ecological problem and the predated shells opened a plethora of questions about whether invasive mammalian predators might be crossing from their traditional hunting grounds on land, to conduct foul play in the water. The main suspects are Norway rats (Rattus norvegicus) because they are very strong swimmers and often live close to stream and lake edges. Norway rats are the largest of the four rodent species that were introduced to New Zealand and they have contributed to the decline of many animals including insects, snails, frogs, lizards, tuatara, birds and bats, which they eat (alongside the flowers, fruits and seeds of plants). 

With their excellent swimming capabilities and their proficiency at predating other native animals, it’s possible that Norway rats are also eating freshwater mussels and leaving the empty shells on stream banks and lake edges as the only evidence. I was particularly captivated by this conservation puzzle and, with my background in rodent behaviour and foraging ecology, I’ve become the perfect detective to discover who exactly is eating our hard-working kākahi. For this complex project, I embarked on a real-life game of ‘Cluedo’ where I used motion-activated cameras to film various stream sites around the Waikato Region and collected samples of predated kākahi to analyse back at the lab to find out ‘who-dun-it’.


In a recent workshop, hosted by Baz Caitcheon (Bazzacam) and Daniel Walker (NZ Science Media Centre), I had the opportunity to create a video overview of my research. The SAVVY Video Workshop provided the perfect occasion to summarise why kākahi are important and the threat that predation poses. To inspire New Zealanders to do their bit for the conservation of our unsung heroes, I’ve shared the fascinating mystery of mussel predation online as a part of the Royal Society Te Apārangi Early Career Researcher Video Contest. You can view it online and vote for it for the People’s Choice award.  

Further reading:

Kākahi – NIWA

The freshwater mussel housing crisis: eviction by invasive weeds? – Sciblogs

The secret lives of freshwater mussels – Waiology

Scientific muscle meets freshwater mussels – NIWA

Conservation status of New Zealand freshwater invertebrates, 2018 – DoC

Watch the extended cut of Bridgette’s winning video entry here.