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If you’ve spent time canoeing or kayaking on rivers in the Midwest, you’ve probably come across the shells of freshwater mussels from time to time. On the outside, mussel shells are seldom pretty, but the pearly shine of the interior surfaces often prompts people to pick them up.
Scientists distinguish among different species of mussels by focusing on the shape of certain parts of the shell. Here, for example, is how the Field Guide to Freshwater Mussels of the Midwest (Illinois Natural History Survey: click here to see free online version) describes the appearance of a species that goes by the scientific name Quadrula quadrula: “Shell quadrate to rounded, and somewhat inflated. Anterior end rounded, posterior end squared or truncated.”
Now, if that doesn’t bring to mind a very clear picture for you, try the common name for the same species: it’s “mapleleaf.” [Photo of Quadrula quadrula by Kevin Cummings, from the field guide. It reminds me of a maple leaf, anyway.]
Other Illinois mussels carry similarly evocative common names, which tell both what the creatures look like and what objects were familiar to the people who named them. Among them some of my favorites are washboard, pistolgrip, wartyback, heelsplitter, deertoe, spectaclecase, and pocketbook.
According to Kevin Cummings, a mussel expert at the Illinois Natural History Survey on the U of I campus, North America is home to a greater diversity of freshwater mussels than any other continent, with nearly three hundred species and subspecies. Some eighty of these are or were once found in Illinois. Many mussels have become locally extinct in former habitats, and only about forty species are regularly found in the state now.
Freshwater mussels live a low-key life for the most part. They pass their days hunkered down in the sand or gravel, usually in flowing water. They feed on microscopic plant and animal life, as well as other tiny bits of organic matter, which they filter from water they take in through one siphon and eject from another. Mussels are fed upon by a variety of fish and birds, as well as muskrats, otters, and minks. Minks leave the cleaned shells of mussels they’ve eaten in a pile near the water’s edge called a midden, which can be a great place to find and identify shells.
The early development of mussels is a bit more complex and dramatic. Mussel eggs are fertilized within the female, with sperm that has been released into the water by nearby males. Inside the female, the fertilized eggs develop into larvae, which scientists call “glochidia.” To grow further, these glochidia must be expelled and attach themselves to the gills or fins of a fish for some weeks, where they will take on their adult form, in miniature, before dropping off to live at the bottom of the stream again. By sending forth their young attached to fish, mussels are able to disperse much farther than they would under their own power.
Over time, freshwater mussels have served a variety of human purposes. Native Americans ate their flesh and used their shells for utensils, tools, and jewelry. During the nineteenth and early twentieth centuries—before the advent of plastics—mussel shells were used on an industrial scale to make buttons. Since the 1950s, mussel shells have been exploited commercially for use in the production of cultured pearls in Japan.
It is unfortunate for mussels that they are not more cute and cuddly, because as a group they are among our most endangered animals, suffering from overexploitation, the pollution and physical degradation of waterways, and the introduction of exotic species to their habitats. Perhaps our best hope for preserving them comes from the growing awareness that the health of our rivers and streams is really a component of our own health.