An invasive mussel is devastating ecosystems as it spreads through South American rivers, threatening the Amazon basin

_SCIENCE

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.

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.

Read the entire feature in Science, Vol 374, Issue 6566.