I would venture to guess that everyone reading this blog has some familiarity with Zebra Mussels (Dreissena polymorpha) and their close cousins, Quagga Mussels (D. rostriformis) but many will be much less familiar with our native freshwater mussels, the Unionids. Freshwater mussels - "clams" as so many refer to them as - have one of the most interesting life histories. Our native mussels are rather simple filter-feeding creatures that live mainly in rivers, living a seemingly uninteresting life until you dig a little deeper. After all, how exciting can an eyeless, mostly immobile filter feeding animal really be? In this case, much more interesting than the cover might lead one to believe.
North America is the center for diversity of freshwater mussels. Unionidae is the largest and most diverse group (family) of bivalve Mollusca. There are nearly 900 species of freshwater mussels and just a bit over a third of them are native to the United States - most of those are found in the southeastern United States, the global center for diversity of freshwater mussels. And our native mussels are dwindling at rather alarming rates. In fact, mussels are easily the most imperiled of the freshwater fauna. Seventy percent of freshwater mussels are rare - meaning that they are at least listed as globally vulnerable, federally threatened or endangered, or presumed to be extinct. Diversity is lower in the Midwest (about 78 species) but they are nearly equally as imperiled with about half of those species listed as federally threatened or endangered.
The story of why freshwater mussels are so imperiled parallels that of many other species but the life history of mussels exacerbates their declines. The causes of rarity have been given the acronym HIPPO which refers to habitat destruction and fragmentation, invasive species, pollution, human over-population (yeah, they cheated a bit there...), and over-harvest. All of these are part of the story of freshwater mussel imperilment.
If you are mostly a trout angler, you probably have not seen a lot of our native bivalve mussels. They tend to occur in warmwater streams. In Wisconsin, the Mississippi, St. Croix, Wisconsin, and Mukwonago Rivers have the greatest mussel diversity. That should not be a surprise as rivers grow larger, they increase in species richness for most taxa (see River Continuum Concept). The Mukwonago is an interesting outlier in this list in that is is a smaller river and a thread of biodiversity in a sea of human development. There is a reason that The Nature Conservancy has a large stake in the watershed.
A quick bit about the life history of mussels. In a previous post, I wrote about r- and K-selected species and mussels are one of those interesting exceptions to many of the rules. Mussels are long-lived, in many species a 20 to 50 year old mussel is not an anomaly. While they reproduce at a relatively old age - a K-selected trait - they produce a ton of eggs, an r-selected trait. Probably the most unique part of mussel reproduction is that female mussels have lures, part of their mantle tissue that is modified to draw the attention of fishes, the next host in their life cycle. Rather than try to explain it in any great detail, the video below show these lures and how the glochidia (larval mussels) are released into their host fish's mouth and then attach to their gills. As glochidia, they are parasitic and this is a critically important stage as this is how mussel disperse from their birth place. Adults mussels can move themselves to deeper water as water levels recede but they do it very slowly and they are unable to make long movements. As larva, fishes may move them tens or even hundreds of miles before the glochidia release from the fish to develop as juvenile mussels.
How cool is that?! What I find most amazing is that mussels, without any sort of eyesight, have evolved a range of different lures and other ways to attract their specific fish hosts. Evolution has shaped these attraction devices to fool their fish hosts through natural selection and differences in fitness among individuals within the population.
Imperilment of Unionid Mussels
As mentioned earlier, mussels are affected by all of the HIPPO variables. Their habitats have been altered and fragmented by dams. Invasive species, most notably Zebra Mussels, have competed with and will grow so heavily upon their shells that they can not feed. As filter feeders and species living in stable cobble, gravel, and sand substrates, pollution - particularly sedimentation - is another reason for their declines. Of course all of these impacts are tied to human populations. And lastly, while today there is very little harvest of mussels, due to their long-lives, historic over-harvest continues to impact them today.
Habitat Alteration and Fragmentation
Being denizens of large rivers, mussels now live in greatly altered habitats. Our large rivers - the Mississippi, Missouri, Ohio, Tennessee, Mobile, and others are greatly altered to facilitate barge traffic and commerce. The dams that facilitate barge traffic have greatly altered the rivers they impound, altering the habitat of the mussels that live there. There are some number of mussels species that are expected to go extinct as there is no evidence of reproductive success but because of their long lives, these species are not yet extinct.
Maybe even more critical than the habitat alteration caused by dams are the effects of fragmentation. Through evolution, mussels have timed their reproduction with the spawning movement of their host species. I did not mention it earlier as I knew it would be critical here - fish hosts are specific to mussel species. Many of the fishes - some of the sturgeons, paddlefish, shad and herring, Mooneye and Goldeye, and other large river fishes are also experiencing declines due to dams. Instead of having continuous populations up and down the Mississippi River, fishes and mussels are more or less relegated to living within a single navigation pool. This fragmentation takes a larger, better connected population and creates smaller, insular populations. This insularity and smaller habitat area decreases the population size and likelihood of long-term survival. As go the fishes, so go the mussels.
Historic Overharvest
Before there were plastic buttons, there were buttons made of mussel shells - mother of pearl. Up and down the Mississippi River and others, mussels were harvested by the barge load to harvest native mussels for pearls and their shells which were used to create buttons. Eventually over-harvest and plastic put those mussel harvesters out of business but the effects are still felt today.
Today, the visible evidence of the pearl and button factories is mostly gone. We have Pearl Street in La Crosse, Wisconsin and a number of businesses such as Pearl Street Brewery, The Pearl (an ice cream and candy shop), and Pearl Street Books but there is little else - other than the lack of mussels - that provide much evidence of the decades of mussel harvest. For the long-lived mussels, this immense over-harvest is not past history but part of their present predicament, another hurdle to overcome. Habitat alteration and fragmentation, pollution, and other factors are more difficult to overcome after mussel populations were decimated by over-harvest.
The Future
Quite honestly, the conservation of mussel species is an uphill battle. Our native mussels have evolved in a landscape that no longer exists. Once large, connected rivers are now a series of riverine lakes punctuated by nearly impassible dams - at least for mussels and their fish hosts. We have dedicated a significant amount of effort to try to preserve these unique native species but there are no guarantees that trends do not continue. In fact, we are almost certain to see more extinctions as species that have not successfully reproduced for years and even decades die out. But all is not lost. Many of the species - with our help - are holding on as are their fish hosts.
Links for more information about our native mussels:
Scientific American - America's Freshwater Mussels are Going Extinct - Here's why that sucks!
Wisconsin's Rare Mussels and Clamming Regulations (there is no legal harvest)
US Fish and Wildlife Service - Mussels of the Midwest and St. Croix River Mussels