Golden mussels are devastating South American rivers. The Amazon may be next | Science

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A version of this story appeared in Science, Vol 374, Issue 6566.

On a stuffy summer day, Maria Cristina Dreher Mansur peers into the brownish water of Lake Guaíba in the southern Brazilian city of Porto Alegre. While locals watch the sunset from the shore, the 76-year-old retired biologist wades up to her neck, carefully dragging her bare feet on the muddy floor. She feels with her toes for hard shells and retrieves them with a dip net.

Mansur, an expert in bivalve mollusks, has been coming to this spot with students for more than 30 years to monitor local species. In the 1990s, the shore was covered with sedge and the lake bottom was strewn with the rounded shells of native mollusks. Now, the shore is barren and the aquatic ecosystem unrecognizable. “I could fill a bucket with native species in a few minutes,” Mansur remembers. “Now, it can take me a whole morning to find half a dozen.”

Behind the transformation are tiny mollusks with arrow-shaped, caramel-colored shells, native to the Yangtze River in China: golden mussels (Limnoperna fortunei). When Mansur first noticed them on the shore in 1998, she recognized them at first sight based on warnings from colleagues who were already tracking them in Argentina. The species had arrived in the Americas a few years earlier in the ballast water of ships coming from Asia.

Months after Mansur first spotted them, the animals had taken over Lake Guaíba, plugging the pipes that supplied the city with water. Masses of dark, thumb-size adults mixed with golden pinkie nail–size newborns encrusted bedrock, boats, piers, and bridges, forming dense reeflike structures with more than 200,000 individuals per square meter. Without any local predator to control them, they choked and rotted the roots of plants along the shore, and even grew on top of other animals such as native mollusk species and crabs, suffocating them. “In 2 years, the golden mussel transformed the lake’s sandy beaches and vegetated margins into piles of dark and stinky shells,” Mansur recalls.

Similar scenes have played out in many rivers, lakes, fisheries, and hydroelectric dams as the mussels have advanced north through Brazil at a speed of about 240 kilometers per year. Now listed as one of the three invasive species of most concern in the country, they are far from the first aquatic invader to gain a foothold in Brazil. But their speedy reproductive cycle and unique ability to cling to surfaces have made their economic and ecological impacts more severe—similar to those caused by the zebra mussels infesting the Great Lakes in the United States, but much more widespread.

Exponential invasion

After arriving in Argentina, golden mussels quickly multiplied on rocks at the La Plata River. The population there is now stable at 85,000 per square meter.

Graph showing mussel density per year

An even bigger catastrophe looms: the invasion of the Amazon and its tributaries, part of the largest drainage basin in South America, which spans eight countries and is one of the richest hot spots for biodiversity on the planet. Golden mussels have been documented in the Pantanal wetlands just 150 kilometers from the Téles Pires River, which flows into the Amazon basin and connects to the Tapajós River, a tributary of the Amazon.

“It only takes one boat encrusted with the mussel to cross the wetlands for the invader to make a new home in an Amazon river,” says biologist Marcia Divina at the state-owned Brazilian Agricultural Research Corporation. If the invader spreads in the Amazon, it could wipe out native species that scientists have not even studied yet, she adds. “We can’t even calculate the size of the impact.”

Despite a late and anemic government response, researchers funded largely by affected hydroelectric companies have developed new tools to track the mussels’ relentless advance. And some are looking to an aggressive form of genetic engineering to eradicate it. That untested strategy is still likely years from being ready to deploy, but scientists see few other ways to slow an invader so easily spread by human transit and trade.

In 1991, Argentinian biologist Gustavo Darrigran of the National University of La Plata was collecting snails for a class when he spotted the first golden mussels in South America, on rocks at the La Plata River. He knew the animals didn’t belong there; the continent has no native freshwater mussel species. He counted five mussels per square meter. Four years later, there were 150,000.

Golden mussels attach to surfaces by releasing superadhesive, almost invisible filaments known as byssal threads. The threads allow them to take over areas unexplored by native mollusks—none of which has this sticking power. Their reproductive powers are also formidable. If water is warmer than 15°C (as most Brazilian rivers are year-round), they spew thousands of eggs and sperm. These unite to form microscopic, free-floating larvae that attach to a new surface and grow. At 1 year old, the sesame seed–size mollusk starts to reproduce.

The population Darrigran observed in the La Plata eventually stabilized at 85,000 per square meter in 2019, the last count before the pandemic. But by then, everything around it looked different. Golden mussels are “ecosystem engineers,” Darrigran explains, “altering the interactions between the local fauna and flora forever.” As mussels build up on a riverbed, they impede water flow, which allows organic matter to accumulate, nourishing a new set of insect larvae, snails, and small gastropods. On average, the mussels attract 10 to 15 new species when they take over, while crowding out previous occupants. “You always find the same species in invaded areas,” Darrigran says, “and the local diversity is lost.”

The mussels eat by filtering plankton, monopolizing nutrients that would otherwise feed other river-dwelling species or flow into the ocean. A single adult filters, on average, half a liter of water per hour—about 10 times more than the zebra mussels infesting the U.S. Great Lakes. The water gradually becomes clearer and lets in more light, which encourages blooms of toxic cyanobacteria. A 2008 assessment of Southern Brazil’s Alto Paraná River found the mussels had completely wiped out colonies of freshwater sponges—species endangered in Brazil that act as nurseries for larvae of several types of aquatic insects.

Similar damage now threatens the Amazon. More than 2500 fish species unique to the basin depend on the nutrient-rich Amazon water and its resident plants and algae. The Amazon accounts for 15% of the world’s river discharge into the oceans; depleting nutrients that support sea life in the Atlantic Ocean “will affect this balance on a still-unknown scale,” Darrigran says.

Masses of golden mussels clog a cooling pipe
Masses of golden mussels clog a cooling pipe at the Governor José Richa Hydroelectric Plant.GOVERNOR JOSÉ RICHA HYDROELECTRIC POWER PLANT

Humans are aiding the mussels’ advance. In 2009, veterinarian Carlos Belz, who was doing his Ph.D. on golden mussels at the Federal University of Paraná, Curitiba, set out to understand how these tiny creatures that can’t swim have spread north—against river currents—so quickly. He and a colleague stood at a highway patrol post near the banks of the already-infested Paraná and, with help from the police, stopped 32 trucks ferrying sand to artificial beaches along its tributary, the Iguaçu. Sifting through the sand, they found golden mussel shells and two living mussels. The species can survive for up to 10 days out of water, Belz says.

In other rivers, Belz found mussels and larvae in water retained in fishing boats and in the stomachs of fish, which eat the mussels and defecate them without digesting their hard shells. “To control the spread of the golden mussel, we would need to attack on so many fronts … it’s almost impossible,” Belz says. “And that is why no one is really doing anything about it.”

Mansur had hoped the Brazilian government would spring into action when she first reported the golden mussels in Lake Guaíba in 1998. She urged measures such as requiring fishers and riverboat operators to bleach boat hulls, bilges, and wells after passing through an affected area. She also recommended a ban on throwing the mussels back into the water after cleaning them off dams and other structures. But when Mansur called local and federal environmental agencies, she got no response.

Only in 2003 did the Ministry of the Environment create a task force to propose control measures. Implementation of such proposals “should have started 10 years ago,” Mansur says. In contrast, North America appears to have kept golden mussels out so far, likely thanks in part to interagency plans for early detection of invasive species put in place after the U.S. zebra mussel invasion, and requirements that ships treat ballast water and empty it far off the coast before docking. Such ballast water disposal has been mandatory in Brazil since 2005, but oversight has been lacking, Belz says.

The golden mussel invasion in Brazil did raise alarms, however, for the hydroelectric power industry, whose dams are being overrun. By now the mussel has invaded about 40% of Brazil’s hydropower plants—with costly consequences. The Governor José Richa Hydroelectric Plant on the Iguaçu, which generates energy for 4 million people, spends about 1 million reais (about $200,000) per year pumping toxic chemicals that curb mussel proliferation into the water tubes that surround and cool its turbines. If the tubes clog, water stops running and the turbines can overheat and stop. Sharp mussel shells coat the stairs and handrails in the dam’s chambers, making it hard for crews to do routine maintenance.

A preprint published in 2018 in PeerJ estimates the mussel plague costs the Brazilian electrical sector $120 million per year. And available control methods can only keep mussel populations stable enough to keep plants running—not eradicate the species or prevent its ecological damage. Because there is no regulation on how to dispose of the mussels, most plants throw them back into the river by the ton, fueling the spread.

The threat to their business makes electric utilities the biggest supporter of research on golden mussels. With funding from power plants, a startup company called ATGC, incubated at the Federal University of Paraná, has developed a simple way to detect and measure infestations using traces of genetic material the mussels leave in rivers.

ATGC geneticist Marcio Pie has identified two genetic sequences unique to the golden mussel and developed a polymerase chain reaction (PCR) test, like the ones used for COVID-19, that detects these sequences in water. In lab experiments, the test could detect a single mussel in 950,000 liters of water. The researchers also use a fluorescent labeling technique to count particles of mussel DNA, which allows them to track changes in the density of infestations. “Now we can monitor the golden mussel in real time,” Pie says.

Each month, the company receives plastic bottles of river water from 15 hydroelectric dams already infested or worried about an infestation. Pie hopes the strategy can be expanded into a national monitoring network. For now, information on the mussels’ spread comes from sparse network of research teams and hydropower plants that voluntarily report new invasions on a collaborative website. “The biggest challenge will be to coordinate actions in a country as huge as Brazil,” Pie says.

A path of destruction

Since the 1990s, golden mussels have been advancing north from Argentina at a speed of 240 kilometers per year. In 2015, a second invasion began at the Sobradinho hydropower plant in northeastern Brazil. The mussels are now within 150 kilometers of the Amazon River Basin, one of the richest biodiversity hot spots in the world.

Map showing where mussels have been detected in South America
(Map) K. Franklin/Science; (Data)

In 2020, Brazil’s Institute of the Environment and Renewable Natural Resources (IBAMA) finally launched a national plan to monitor and control the golden mussel. The plan, collectively created by authorities, academics, and industry, established goals for the next 25 years to mitigate the invader’s impact and prevent its arrival in the Amazon basin. The measures include fostering research, informing the public about the invasive species, and creating legislation that would, for example, require hydropower plants to check for the mussels in their dams.

But the pandemic and federal budget cuts have delayed the planned actions, says biologist Rogerio Santos at the Federal University of Mato Grosso, Cuiabá, who is part of the national task force for controlling the mussel’s spread. Underfunded control efforts have focused on local measures that protect individual dams and private companies, he says. Meanwhile, scientists know more about the invader than the local fauna it is destroying, which makes it hard to measure the mussels’ toll. He believes the arrival of the golden mussel in the Amazon basin is inevitable and worries that stopping it is still not a priority of the federal government.

Requests for comment from IBAMA on its response to the golden mussel went unanswered.

Studies suggest Amazonian rivers’ water temperature, salinity, and nutrient content are perfect for the golden mussel. In 2010, a mathematical model created by Fabiano Alcísio Silva at the Bioengineering Centre of Invasive Species and his team predicted a high risk of invasion in the Amazon basin by 2050. Those calculations may have been conservative. They ignore, for example, the fact that new Brazilian hydropower plants are legally required to breed fish in their reservoirs to compensate for damage to local fisheries, which often means importing exotic species. Golden mussels can hitch a ride in the guts of these fish or in the water used to transport them. According to Brazil’s Ministry of Mines and Energy, 79 new Amazonian hydroelectric dams are in the planning stages—each one a potential site of a new mussel introduction.

In July, Silva found a few golden mussels in a water sample from a power plant in an Amazonian river. But two subsequent PCR tests did not detect the animal’s DNA in samples from the same dam. He is now testing more water to determine whether that first sample signals the start of an invasion. “Not all invasions are necessarily successful,” he points out. “Usually, an invader has to try many times.”

On the door of a small laboratory at the Federal University of Rio de Janeiro’s main campus, next to a sign with a biohazard symbol, is a drawing of a saber-toothed tiger, the logo of the ’80s cartoon series ThunderCats. The show follows a group of heroic humanoid cats who defend their planet from invaders. The lab’s main investigator, biologist Mauro Rebelo, says he draws inspiration from the show and its motto, “Sight beyond sight.” “Here, we all have sight beyond sight, working with molecular biology and things you can’t see with the naked eye,” Rebelo says.

On the other side of the door, Rebelo’s team is developing a mutant mussel that could mate with wild, invasive mussels and produce infertile offspring, wiping out the population. Rebelo, whose work is supported by electric companies, is not only picking a fight with the golden mussel, but with inheritance itself. Normally, when two animals mate, their offspring inherit half of their genes from each parent, and the next generation ends up with only 25% of the genes from their grandparents. But a genome-altering strategy called gene drive breaks those rules, allowing a modified gene to spread through a population much faster than natural inheritance allows.

Tweezers holding a golden mussel
One golden mussel filters half a liter of water per hour to obtain nutrients.AP PHOTO/LEO CORREA

Rebelo’s group aims to spread genetic modifications that interfere with fertility. The technique relies on the gene-editing tool CRISPR-Cas9, which uses molecular “scissors” to make precise changes to DNA. The researchers will use these scissors to cut DNA in golden mussel sperm. They’ll insert both a sequence that disrupts fertility genes and a sequence encoding the scissors themselves. When mutant sperm fertilize eggs in the lab, the offspring will have the fertility-blocking mutations—and the genetic instructions for the scissors—in their germ cells, which give rise to gametes (sperm and eggs).

That’s where the gene drive steps in to turn these offspring into superspreaders of infertility: When they reach adulthood and produce gametes, the scissors, which are designed to only be active in germ cells, will cut out and replace the normal fertility genes inherited from the mother so that all the gametes carry the altered genes. When these mutant mussels breed with wild mussels, the offspring will still carry normal fertility genes from their wild parent, so they won’t be infertile. But all of their gametes will carry the mutations. As the population of modified mussels grows, the chance of two mutants breeding—creating fully infertile offspring—increases. And as mutants slowly replace the original population, it collapses entirely.

Rebelo’s team estimates that infertile mussels would make up 50% of the population 2 years after the release of the modified mussels, and 90% after 10 years. “One modified mussel can release millions of modified sperm,” Rebelo says. “My modified mussel factory is the mussel itself.” The researchers expect to have the first mutant mussel next year and will then test its power to crash populations in tanks.

But they are still far from releasing the animals in nature. Although several teams are developing gene drives to control disease-carrying mosquitoes and other pests, the strategy has never been tried outside of the lab. Brazilian law doesn’t regulate gene drives specifically, but it requires time-consuming studies to assess the environmental impact of genetically modified animals before approving their release.

Other researchers fear that once released, a gene drive will be hard to control. Sandra Ludwig, a geneticist studying golden mussels at the Federal University of Espírito Santo, Goiabeiras, worries the fertility-blocking modification might disappear over time—or that mussels could even develop and spread mutations that increase fertility, making the population “worse than the golden mussel we have today.”

After more than 20 years of following the golden mussel’s conquest, Mansur would like to see the genetically modified mussel working. But she sees the species advancing at a faster pace than the sparse scientific efforts to stop it. “I am not seeing much hope,” she confesses. At best, she says, the mussel invasion serves as a warning to anticipate and stop the next threat.

In a drawer in her home office, she keeps her recent nightmare: the palm-size, brown striped Chinese pond mussel (Sinanodonta woodiana). Producers in Europe and Costa Rica imported it to cultivate pearls in artificial ponds. But it has escaped into the wild and is now spreading, threatening native biodiversity and affecting water quality in more than 21 countries. Mansur has alerted the Brazilian Ministry of the Environment about the threat, and there’s still time to restrict its import to Brazil. “If this leaves a tank and enters the environment here … oh God!” she says. “We already have the golden mussel, we cannot let this animal enter, too.”

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