Stokstad, E. 2012. Nearly Buried, Mussels Get a Helping Hand. Science Vol. 338, Issue 6109, pp. 876-878
Freshwater mussels are in trouble. They are the most endangered group of organisms in the United States, with most of their river and stream habitats devastated by dams, pollution, and invasive species such as the zebra mussel.
Thirty-five species have been declared extinct, others are likely gone, and more than 70 species are teetering on the brink. “It’s the biggest conservation crisis in the U.S. that no one talks about,” says Paul Johnson, who directs the Alabama Aquatic Biodiversity Center in Marion.
Lab studies show that mussels are sensitive to a number of common but poorly regulated water contaminants, such as the surfactants in the common herbicide glyphosate. In a detailed field study, it was found that the absence of juveniles was highly correlated with ammonia—likely from fertilizer or manure—in the sediment where the mussels burrow. Spikes in ammonia concentrations may be responsible for widespread declines of freshwater mussel populations, especially in agricultural areas. Some ecologists suspect that the current level permitted in surface water by the U.S. Environmental Protection Agency is dangerous for mussels.
The accelerating disappearance of mussels “really is a strong statement about what we’ve done to rivers,” Bringolf says. In the Mississippi River Basin alone, perhaps less than 10% of the original habitat of endangered mussels remains unaltered by dams.
North America is home to a record diversity of freshwater mussels with dazzling reproductive strategies and key ecological roles. But can they withstand the hard knocks of a modern world?
North American has the world’s greatest number of mussel species — 297– more than two-thirds of which are concentrated in the southeastern United States. Some rivers have more species of mussels than are found in all of Europe.
North America owes its astounding freshwater biodiversity in large part to unique geology, which has provided a stable environment that enabled mussels to thrive and diversify for 60 million years. Historically, expansive shoals of mussels served as habitat for other aquatic organisms. By filtering water, mussels move nutrients through the food web, supporting nearby terrestrial ecosystems as well.
People have also long benefited from mussels. Massive middens hint at the untold numbers harvested by Native Americans for food.
What has caused serious harm is widespread fragmentation and loss of habitat. Mining and deforestation, which polluted streams and clogged them with sediment, were already problems by the late 19th century. The worst trouble started in the early 1900s, when engineers built locks and dams in large numbers. These efforts culminated in the gargantuan dams constructed across the southeastern United States by the Tennessee Valley Authority (TVA) in the 1930s and ’40s. Most mussel species can’t live in the slow, muddy water and silty bottoms of the reservoirs formed by these dams. Nor can half the fish species that mussels need as hosts for their larvae.
The Snuffbox mussel
Every spring, a freshwater mussel called the snuffbox emerges from gravel stream bottoms for a violent bout of reproductive deception. The females have spent months buried in the sediment, brooding thousands of larvae that require a certain host to mature. Now the mussels lie on the streambed, their shells open wide. Playing dead, they wait for just the right fish to approach.
That fish, the logperch, spends its days hunting for insect larvae and fish eggs, rummaging under small stones and empty shells. When a logperch pokes its snout inside a snuffbox (Epioblasma triquetra), the mussel snaps shut. The fish is trapped between the serrated edges. For other fishes, this mistake would be fatal, but the logperch has a reinforced skull. As the fish struggles, the mussel pumps out its larvae, which clamp their tiny shells onto the filaments of the logperch’s gills. Then the mussel lets go. After several weeks of hitchhiking, the juvenile mussels drop from the gills and settle into their new habitat.
This aggressive tactic is just one of the remarkable behaviors that freshwater mussels use to reproduce and spread upstream. Other species attract their fish hosts with lures that resemble fish eggs, crayfish, or even swimming minnows. “It’s some of the most amazing mimicry in the world,” says restoration biologist Jess Jones of the U.S. Fish and Wildlife Service (FWS) in Blacksburg, Virginia.
The snuffbox was put on the U.S. endangered species list this past February; biologists estimate its population has declined by 90% over the past century. This month, FWS added another eight mussel species to its list.