I recently went to the Jupiá and Ilha Solteira reservoirs, on the Paraná River in Mato Grosso do Sul, to visit the Jupiá hydroelectric plant and Aguaruga aquaculture.
Hydroelectric dams and aquaculture are the two main sectors with direct economic damage caused by the golden mussel.
Looking at the map, the aquaculture is located in a small cove of a small arm of the Ilha Solteira reservoir, which is more than 1000 km² in area. But there, when we looked around, the sensation was of immensity. The distances and the amount of water were enormous!
Google Maps
Paraná River, Mato Grosso do Sul (Mauro Rebelo, 2022)
This got me thinking about the challenge of golden mussel control.
Our goal is not just to control mussels inside power plants and on the hulls of ships, which would already be a huge challenge, but to control the species in the environment. And that environment is huge.
Inside the laboratory, working with invisible gene editing in tiny volumes, we can lose sight of the colossal volume of hydroelectric reservoirs in Brazil.
And the mussel is there… in every liter of water.
How can we aim to treat every liter of water in every Brazilian reservoir? It’s very daring, to say the least. Is that crazy?
Adult mussels are not omnipresent
Although mussels are potentially in every liter of water, they are not everywhere. In fact, if you walk along the banks of the reservoir on the aquaculture beach, you won’t see any mussels. They don’t stick to river rocks, tree trunks or boat hulls, but they fill every square inch of the net tanks.
This suggests to me that, although mussels can attach themselves anywhere, they prefer artificial substrates*.
Could I test this hypothesis? In fact, it was already being tested by our research project.
The Hubz team, co-executor of the golden mussel biological control project, led by Raquel Figueira, had measured mussels along three reservoirs with quadrants, sonar and imaging, developing an automatic and scalable measurement method. They rightly observed that the distribution of mussels was neither uniform nor random, but aggregated.
It is not uncommon for an organism to have an aggregate distribution as a result of some biological or environmental factor.
What Raquel’s team discovered was that the factor that seemed to aggregate the mussels was artificial substrates: dams, canals, abandoned buildings, shipwrecks and aquaculture farms.
One such place is the Pereira Barreto canal, in the state of São Paulo. It is considered the largest artificial canal in South America and the fifth largest in the world. It connects the Ilha Solteira reservoir to the Três Irmãos reservoir with the aim of regulating the volume of the two reservoirs to maximize energy production.
The canal also has a large volume of boats, as it serves as an outlet for a large part of Mato Grosso’s soy production towards the port of Santos and is a major nursery for mussels.
Google Maps
The impact of this nursery is not negligible. After 5 months of reversing the flow of water from Três Irmãos to Ilha Solteira (usually the opposite), the mussel infestation in the Três Irmãos reservoir decreased. The Pereira Barreto canal has become a major exporter of larvae.
The evidence, although anecdotal for the time being, is reinforced by mathematical models and references in the literature. Our next step is a controlled study.
You can’t imagine that aquaculture is any different. With a protected area, artificial substrate and two meals a day, they are a perfect environment for mussels to settle and disperse.
This finding was a little uncomfortable because it turned one of the mussel’s traditional victims, aquaculture farms, into perpetrators.
But the observations and data were robust: there aren’t mussels on every surface, and even less where we don’t have aquaculture.
Breaking down paradigms
We don’t find mussels on every possible surface, but we do find mussels on some more than others. This finding overturned an important paradigm of infestation: that mussels stick to any surface.
So I decided to revisit other premises
The first is that the mussel has no natural competitors. This is because, during the mussel counts, Raquel’s team took several photos of sponges covering mussels and suffocating them. We’ve already reported how sponges also attack installed banks of sun coral, another marine invasive species. I got the impression that, if you look around, you’ll find more and more examples of competitors.
Humberto’s presentation at the 23rd International Congress of Zoology on the possibility of sponges controlling golden mussel populations. Photos from 7′:28″
The second is that mussels have no predators. We have observations to challenge this premise as well. In fact, for years! Fish, such as the armadillo, are preying on mussels, especially younger mussels with less calcified shells. This doesn’t mean that they’re the ideal diet, but it doesn’t mean that they’re not a food resource for anyone.
Photos: Rodrigo Beltz
The third is that mussels change the composition of the water. Although this is possible in smaller lakes and reservoirs, I don’t believe it’s possible in reservoirs of the magnitude of Ilha Solteira. In fact, we found no evidence of this impact on water quality in any of the reservoirs we studied.
Finally, the fourth premise: mussels resist dissection for long periods of time. The observations suggest that exposure to air and sun is actually the mussel’s main control tool. Aquaculture farms expose infested tanks to the sun. After 4 days, the dead mussels begin to detach from the net tanks.
Mussels drying in a net tank. Photo: Mauro Rebelo
New premises
I’ve generated new premises for the mussel that we should test:
- Mussels only live in large clusters and must need some element that only the cluster can provide. It’s not enough to have a substrate: they need to be agglomerated. It is possible that the mussel needs chemical elements supplied by other mussels, by the cluster, or by other species of accompanying fauna. It could also be protection, but I think that’s less likely.
- Mussel populations are regulated by the availability of food. Our main evidence for this comes from the laboratory. The difficulty of keeping mussels in the laboratory is great, mainly because of the food. Mussels are insatiable and it is easier to spoil the aquarium water by overfeeding than to satiate them.
- Mussel populations are controlled by exposure to sun and air. In addition to the massive evidence of aquaculture, the phenomenon of decoada, the drop in oxygen in rivers due to the seasonal reduction in the volume of water, has for years been reported as the main barrier to the expansion of the golden mussel into the Amazon.
- Adult mussels seem to remain very resistant to chemicals in the water
- Aquaculture farms are mussel multipliers.
- The artificial canals are nurseries for mussels.
- Dams reduce the number of mussels downstream. The latter is also based on the observations of Raquel’s team, that downstream of any dam, the count is lower. As much as mussel larvae get through the grates and stick to the walls of the pipes, the kinetic energy of the turbines and spillways doesn’t seem to do them any good.
*Artificial substrates are simulators of different types of habitats used for sampling.