Why We Need to Improve the Sex Lives of Mussels
Published by the Natural Resources Defense Fund
The U.S. is home to hundreds of species of freshwater mussels, but many are endangered
U.S. Fish & Wildlife Service
The Amazon Rainforest, the Okavango Delta, and the Great Barrier Reef are all famous for their spectacles of biodiversity. But there’s a similar display of life right here in the United States. Only instead of sloths, lions, and sharks, we’ve got freshwater mussels.
Orangefoot pimplebacks, rough rabbitsfoots, winged spikes, sugarspoons, purple cat’s paws, and snuffboxes—these are all common, if fantastic, names for our country’s mussels. And there are many more like them. All in all, our great nation harbors nearly 300 species of freshwater mussel, a third of all the known such species on earth. “You can go out to a stream and in one single site, you might be able to encounter 30 different species,” says Carla Atkinson, an ecologist at the University of Alabama.
But our freshwater mussel bounty is much, much smaller than it once was. When the American Fisheries Society last did a census of all the mussels in the United States and Canada, it found that 71.7 percent, or 213 out of 297 species, were either endangered, threatened, of special concern, or possibly extinct. (By the way, that includes every one of the adorable names above.) That census was back in 1992, and the situation is thought to have gotten worse, not better.
“For many species, things have gotten pretty bleak,” says Atkinson. One significant reason: “We’ve dramatically changed river habitats,” with dams causing much of the mussels’ decline. Damming a river can change the way water flows and the quality of the water that remains, both of which can have hefty impacts on wildlife. But perhaps the biggest reason that dams are bad for mussels is that they cut the mussels off from the fish that help them reproduce.
Freshwater mussels can’t procreate like their cousins do out in the ocean. Saltwater mussels do it a lot like coral and abalone—which is to say, they simply pump a bunch of sperm and eggs into the water column and hope for the best. But if freshwater mussels did that, all of their young would get washed further and further downstream until the whole lot of them were in the ocean. This would, of course, be rather bad for a creature with “freshwater” in its name. Instead, these bivalves have evolved to have pieces of their bodies act as living lures. Some look like minnows, others look like worms, and there’s even a species that resembles a crayfish. In every case, the lure tricks predatory fish into coming down for a nibble. And really, you just have to see these things to believe them.
However, the fish’s curiosity is not repaid with lunch. Instead, the mussels spray the fish with tens of thousands of their larvae, which then attach to the fish’s fins, skin, and gills. The larvae then ride around on the fish for a few weeks or months as they develop into juvenile mussels, at which point they drop off, hopefully in a hospitable new territory. The problem is, many mussels seem to rely on just one or two species of host fish. When those hosts become scarce or disappear, an entire mussel population can be rendered infertile. For instance, when a dam was erected in Keokuk, Iowa, it prevented skipjack herring from going any farther upriver, says David Strayer, a freshwater ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York.
That was in 1913. And because both ebony-shell and elephant ear mussels rely on skipjack, Strayer says there are whole populations of these mussels that haven’t been able to reproduce in more than a hundred years.
Mussels are extremely long-lived, he says. Some individual bivalves may have even been around when Abraham Lincoln was alive.
In southeastern New York, Strayer says, he’ll go into a stream and see what appears to be a healthy population of adult mussels, with an average age of about 70. The youngest mussels in the area, however, may be 33 years old. For some reason, reproduction has ground to a halt. “You don’t need a Ph.D. in ecology to see that’s a problem,” says Strayer. “Eventually, if you wait long enough, they’ll die out.”
And here, we come to the other central reasons mussels are struggling. Strayer says in this instance it’s not so much dams that are the problem, but perhaps the effect of pollution from nitrogen, like that found in fertilizers. In a study published in Ecological Adaptations in 2012, Strayer found that freshwater mussel reproductive failure could not be tied to invasive crayfish, loss of host fish, sedimentation, or even low oxygen levels. But he did find a link between such failure and levels of un-ionized ammonia, which is a hallmark of agricultural runoff.
Because mussels are filter feeders, they are vulnerable to a whole host of environmental issues. Mussels are sensitive to lead and zinc introduced to water systems from mining. Drought or overuse of water leaves them without habitat. And too much sediment can bury them and cause them to suffocate. Climate change could also be a threat, since many species may already be living at the height of their temperature limits.
And all of this isn’t just bad for mussel diversity. It’s bad for us, too.
Enough mussels in one place can actively clean the water. Think of it like free water filtration. That’s not important just for drinking, but also for fishermen, boaters, and all the other groups that like to hang out on the water.
Losing these species would also be a detriment for innumerable other animals, from the insects and algae that live in the muck mussels sift out of the water, to the fish that eat those species, to the birds that eat those fish, and on and on.
Thank goodness for people like G. Thomas Watters, curator of mollusks at the Ohio State University. Watters and other researchers across the country are working on captive breeding programs to ensure we don’t lose any more mussel species. He says North America has around a dozen propagation facilities, and each one is racing against the clock. Because saving mussels is complicated. “You have to conserve and manage two animals—the host and the mussel—and obviously those are very different animals with different kinds of needs,” says Watters. “It’s an uphill battle.”
Watters even has a student working on an “in vitro” method of reproduction that would essentially cut the host out of the mussel’s life cycle. That way they’d be able to raise mussels in the lab without also dealing with the fish. The solution would be far from ideal, he admits, but if a mussel’s still around and its host fish is not, there may be no other options.
Desperate times call for desperate mussels.
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