Freshwater mussels
The River's Liver but not Dinner
Unlike saltwater mussels, they aren't delicious and don't reproduce from spat on a rope. In fact, freshwater mussels (kākahi or kāeo in Māori) are physiologically cold-blooded parasites. Their threatened status in New Zealand is of concern because mussels help filter contaminants from our waterways and store them for slow-release: like a sucker buffer. However, sparse knowledge of their intricate life-cycle and juvenile habitat hinder targeted conservation action.
Adult mussel (Echyridella menziesii)
The life-cycle of freshwater mussels is a curiosity: we know adults of Echyridella menziesii release thousands of larvae (glochidia) that resemble pacman-vampires to drift in the water column, as well as on mucus 'fishing lines' that rise up from their siphons. Upon contact, glochidia snap shut on unsuspecting fish-hosts, ideally on the fins, gills, or operculum. Attachment is the first hurdle for mussel reproduction, but are all fish suitable? How about non-native species, or in the absence of natives, are freshwater mussels up fish creek without apt tackle?
On suitable native fish-hosts such as the common bully (toitoi), glochidia encyst, assimilate nutrients, and detach as juveniles mostly within a fortnight. Warmer laboratory conditions expedite this process and can produce over 100 juveniles per bully and transform up to 80% of those attached. Although not representative of real-world conditions, successful transformation on fish-hosts in trials demonstrates the second hurdle, which is the consequence of a mussel and native fish 'arms-race' carried out over an evolutionary time-scale.
On the other hand, non-native species are new players to the host-fish game, and as you may expect, are inexperienced. Our research found non-native brown bullhead catfish, rudd, and goldfish (morihana) transformed less than 1% of initially attached glochidia: undoubtably an ineffectual invasion of mussel privacy.
On the other hand, non-native species are new players to the host-fish game, and as you may expect, are inexperienced. Our research found non-native brown bullhead catfish, rudd, and goldfish (morihana) transformed less than 1% of initially attached glochidia: undoubtably an ineffectual invasion of mussel privacy.
Two-day old juvenile mussel (Echyridella menziesii)
The third hurdle spans a coming-of-age story from juvenile to adult. Behavioural habitat preferences of fish-hosts determine the location where juveniles are deposited. Affluent neighbourhoods offer prosperity in food resources that juveniles can pedal feed using their muscular foot, and life-sustaining sediments supportive of the healthy homes act. Unfortunately, if the host-fish dispersal lottery is unfavourable, juveniles may be stuck with lamentable landlords in the form of dense non-native aquatic weeds (macrophytes). Here, juveniles can be exposed to hypoxic and anoxic conditions that occur diurnally overnight during summer, as well as high sediment ammonia concentrations and smothering sediments.
Tom wrangling a macrophyte (Egeria densa)
Our research has shown these adverse oxygen conditions produced by macrophytes are context-dependent based on the management of hydrogeneration reservoir flow regimes. However, establishing causation between the lower numbers of juvenile mussels and adverse macrophyte conditions in some situations requires further investigation. In other words, there may be a freshwater mussel housing crisis, but we don't know if mussels should go with the flow or appeal to the tenancy tribunal with current knowledge.
Above is a summary of the mussel life-cycle, with parts particularly threatened by non-native species displayed. The unmentioned non-native predators are predicted to consume mussels opportunistically. Although, predation rates are likely to be higher when mussels are exposed during droughts or after floods if washed onto the bank. Check out Bridgette Farnworth's work for the most up-to-date advancements on this threat.
In conclusion, freshwater mussels are sucker buffers important to conserve in-part due to their positive effect on water quality and ecosystem health. My PhD thesis and scientific publications have gone some way to understand the impact non-native species have on mussels. However, there is still plenty of fertile ground for future research, particularly in their fundamental ecology such as: where do threatened mussel populations exist, are other non-native fish suitable as mussel hosts, what are the mechanistic underpinnings of juvenile survival, and how does Echyridella menziesii reproduction compare to other New Zealand mussel species (see Michele Melchior's work on Echyridella aucklandica).
In conclusion, freshwater mussels are sucker buffers important to conserve in-part due to their positive effect on water quality and ecosystem health. My PhD thesis and scientific publications have gone some way to understand the impact non-native species have on mussels. However, there is still plenty of fertile ground for future research, particularly in their fundamental ecology such as: where do threatened mussel populations exist, are other non-native fish suitable as mussel hosts, what are the mechanistic underpinnings of juvenile survival, and how does Echyridella menziesii reproduction compare to other New Zealand mussel species (see Michele Melchior's work on Echyridella aucklandica).
Moore details
In New Zealand, three species of freshwater mussel (kākahi or kāeo) inhabit our waterways; found either buried in soft sediment on lake bottoms or hiding out of the main flow along stream margins. This taonga species was traditionally a food source for Māori, and the shells were used as tools to help produce fibre from harakeke (flax). New Zealand Freshwater mussels all belong to the genus Echyridella, with E. menziesii found nationwide, E. aucklandica encountered in northern New Zealand, and E. onekaka restricted to north-west Nelson. They can live 40-50 years and provide important services to support ecosystem health such as water filtration (~1 litre per hour), carbon and nutrient transformation, sediment oxygenation, and bed habitat stabilisation. Furthermore, freshwater mussels are indicators of aquatic pollution as their early life stages of glochidia and juveniles are highly sensitive to contaminants from agricultural or urban land-use (e.g., ammonia and copper). Unfortunately, these native freshwater mussels are currently in decline or consist of remnant geriatric populations that are unable to reproduce. Due to their irreplaceable ecological role, as well as being a taonga species, New Zealand’s Biological Heritage National Science Challenge under the Reducing Ricks and Threats Across Landscapes theme investigated if the freshwater mussel reproductive cycle is affected by non-native species. This helped develop management information to understand the decline of freshwater mussels, and provide knowledge to help restore the mechanisms underpinning their complex reproductive cycle.
Globally, freshwater mussels have a diverse number of reproductive strategies, however in New Zealand a single approach involving a fish ectoparasitic stage (glochidia) is used for larval growth and dispersal. Historically, the native fish koaro (Galaxias brevipennis - part of the whitebait catch as juveniles) was the host to freshwater mussels for completing their reproductive cycle. This occurred in Lake Taupo, where Māori harvested Koaro by the hundredweight using big seine nets. Today, shoals of koaro are rare in Lake Taupo and other lakes, with the primary cause of decline linked to the introduction of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss). These predators consumed native fish in Lake Taupo until common smelt (Retropinna retropinna) had to be introduced as a replacement food source to sustain the trout fishery. Since koaro has been replaced with these non-native species, which also includes the introduced brown bullhead catfish (Ameiurus nebulosus), rudd (Scardinius erythrophthalmus), and redfin perch (Perca fluviatilis), the question of whether freshwater mussels can still complete their reproductive cycle without koaro is important; some parasitic freshwater mussel species have host-specific relationships and require fish species that frequent bottom lake sediments. With the predicted expansion of non-native fish distribution, protecting areas where freshwater mussels are able to reproduce is critical for this “at risk” or “declining” threat classified species.
As well as potential disruption to the ectoparasitic stage, freshwater mussel reproduction may be inhibited as juveniles by the proliferation of non-native macrophytes occupying bottom sediments. Impacts on freshwater mussels could include an increase in organic matter, decrease in sediment particle size, and reduction of oxygen concentration resulting in slow organic matter breakdown or potentially anoxia. Since freshwater mussels are a symptom and mechanism to improve poor water quality, determining if the reproduction of geriatric populations can be restored will provide important information about their ability to filter contaminants in degraded waterways. This could potentially be used as a management tool to promote good water quality, and may also prevent proliferation of non-native macrophytes by encouraging competition for sediment space and nutrients.
Globally, freshwater mussels have a diverse number of reproductive strategies, however in New Zealand a single approach involving a fish ectoparasitic stage (glochidia) is used for larval growth and dispersal. Historically, the native fish koaro (Galaxias brevipennis - part of the whitebait catch as juveniles) was the host to freshwater mussels for completing their reproductive cycle. This occurred in Lake Taupo, where Māori harvested Koaro by the hundredweight using big seine nets. Today, shoals of koaro are rare in Lake Taupo and other lakes, with the primary cause of decline linked to the introduction of brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss). These predators consumed native fish in Lake Taupo until common smelt (Retropinna retropinna) had to be introduced as a replacement food source to sustain the trout fishery. Since koaro has been replaced with these non-native species, which also includes the introduced brown bullhead catfish (Ameiurus nebulosus), rudd (Scardinius erythrophthalmus), and redfin perch (Perca fluviatilis), the question of whether freshwater mussels can still complete their reproductive cycle without koaro is important; some parasitic freshwater mussel species have host-specific relationships and require fish species that frequent bottom lake sediments. With the predicted expansion of non-native fish distribution, protecting areas where freshwater mussels are able to reproduce is critical for this “at risk” or “declining” threat classified species.
As well as potential disruption to the ectoparasitic stage, freshwater mussel reproduction may be inhibited as juveniles by the proliferation of non-native macrophytes occupying bottom sediments. Impacts on freshwater mussels could include an increase in organic matter, decrease in sediment particle size, and reduction of oxygen concentration resulting in slow organic matter breakdown or potentially anoxia. Since freshwater mussels are a symptom and mechanism to improve poor water quality, determining if the reproduction of geriatric populations can be restored will provide important information about their ability to filter contaminants in degraded waterways. This could potentially be used as a management tool to promote good water quality, and may also prevent proliferation of non-native macrophytes by encouraging competition for sediment space and nutrients.