Freshwater mussels have long been used by humans as decorations, tools, and for food. Ancient Europeans used freshwater mussel shells as ornaments. Ancient Mayans used the shells as paint pots, pendants, and clothing decorations. The mound-building tribes of the the American Midwest gathered freshwater mussels for food, the tempering of pottery, and for making tools.
Freshwater mussels cut by the Maya and sewn into clothing for use as sequin-like “sparklers”.
Kristen Grace, Florida Museum
North American freshwater mussels were first recognized for their commercial value in the 1800s by the American button industry. The mussel’s pearly shell was used for buttons while the meaty interior was used in livestock feed. By 1912 there were nearly 200 button factories across the country making over one billion buttons a year. The industrial use of these mussel shells, coupled with the simultaneous degradation of their freshwater habitats due to rapid industrialization, led to major declines in freshwater mussel populations.
Then, in the 1900s, the Japanese discovered a new market for freshwater mussel shells – as a source material for cultured pearls. Freshwater mussel shells were cut and finished into beads, then inserted into oysters to start the pearl-making process. Even today, thousands of tons of mussel shells are exported to China and Japan each year to support the pearl-making industry.
Beyond their marketable functions, mussels also have important environmental roles. Like other bivalves, mussels are filter feeders. For food, mussels pull microscopic phytoplankton out of the passing water. Through this filter feeding process, mussels also collect suspended particles and pollutants from the surrounding water, thereby cleaning up the water column.
Freshwater mussel, Dreissena polymorpha.
ullstein bild via Getty Images
This same filter feeding also makes mussels particularly sensitive to water quality. The loss of mussels from an ecosystem typically indicates poor water quality, such as chronic water pollution. What’s more, the collection of toxins within mussels can be measured to determine not only which toxins are present in a given water system, but also when the toxins first appeared, helping determine the source of a water contamination.
Even the mussel’s shells are important to the surrounding ecosystem. When a mussel dies, leaving a vacant shell behind, other animals use the shell for habitat and protection.
Today, freshwater mussels are rapidly approaching extinction. In the United States, 70% of freshwater mussels species are at risk of extinction, and 38 species have already been lost. In China’s Xin River Basin, where more than 43 freshwater mussel species reside, populations of many freshwater mussel species have declined by over 80% from historical levels, and many are believed to be extinct. Declines are also evident in the Philippines, Laos, Indonesia, and Malaysia, although data is limited.
While these freshwater mussel declines began with the commercial exploitation of mussels for their pearly shells, mussel populations also suffer from habitat degradation, river pollution, and climate change. However, researchers now have a new concern regarding the future of freshwater mussels: leeches.
A freshwater mussel (Lamellidens savadiensis) with leeches inside.
Ilya V. Vikhrev
Leeches were first discovered within freshwater mussels in the 19th century, but were initially thought to only be accidentally inside the mussels, or to only be using the mussels for shelter. The idea that these leeches were feeding on the mussels was dismissed.
In research recently published in Nature Scientific Reports, scientists describe their discovery of seven new leech species that live within freshwater mussels. Prior to this new discovery, only two species of freshwater mussel leeches were known.
Of the over 3,000 freshwater mussels tested, researchers found 12% housed leeches. Upon further investigation, these leeches were discovered to represent nine new species. Seven of these new leech species are considered to be ‘mussel-associated’ leeches, meaning these leeches require freshwater mussel hosts to survive. These parasitic leaches were discovered in mussels in East and Southeast Asia, India, Nepal, Africa, and North America.
It is not yet understood how costly these parasitic leeches are to their freshwater mussel hosts. However, given the imperiled status of freshwater mussels around the world, scientists hope to further understand the many threats these animals face.