Wild Side: Mussels go fishing

• Pocketbook mussel

A female plain pocketbook mussel (Lampsilis cardium) embedded in the bottom of the St. Croix River with mantle flap fish-like lures extended in the current to attract fish. The light-colored marsupium, or brood pouch, is in the middle between the mantle flaps. Note the river darter (Percina shumardi) to the left of the shell. Bernard Sietman, Minnesota DNR photo

Talk about it

The animals that live on the bottoms of lakes and streams are overlooked and underappreciated. Mussels are one group of these “out of sight” but beautiful and fascinating animals.

Mussels are mollusks that have a soft body enclosed by a hard shell made up of two halves called valves. A thin tissue called the mantle surrounds the soft body and secretes calcium carbonate and proteins that make the shell. The valves are held together with a hinge consisting of hard teeth and elastic cartilage. Mussels have the ability to “clam up” their valves for protection using two strong adductor muscles.

Freshwater mussels in the family Unionidae are long-lived sedentary filter-feeding animals that occur throughout the world. They reach their greatest diversity in North American rivers. Historically, mussels were remarkably abundant, comprising up to 90 percent of the bethic (bottom-dwelling) biota on river bottoms. Native mussels are now the most imperiled group of animals in North America as a consequence of continent-wide declines due to overharvesting and impoundment and pollution of rivers. Of the approximately 300 total species, over 200 North American freshwater mussel species are listed as extinct, imperiled or of special concern.

Mussels feed on fine particulate organic matter in the water column. They play an important role in river ecosystems by filtering water and transferring nutrients and energy from the water to the sediment, stimulating production by other animals and plants. Because they can’t move far, they are sensitive to pollution and are good indicators of habitat and water quality.

Dense communities of mussels form beds of stable substrate in rivers by armoring the bottom with their shells. Many other macroinvertebrate species, like caddisfly and mayfly larvae, live on and in mussel beds, attracting fish and making them highly productive areas, like freshwater “coral reefs.”

There are many species of mussels in the nearby Mississippi and St. Croix rivers. They provide food for fish, muskrats and raccoons. Native Americans ate mussels, leaving shell piles (middens) along the banks of rivers. Cooked freshwater mussels taste like rubber bands. They are not recommended for a wild food gourmet meal.

In 1889, the German button maker Johann Böpple invented a machine to make buttons from mussel shells. Mussels were harvested intensively from the upper Mississippi River to obtain shells for making buttons.

More recently mussels were harvested to make nuclei for cultured pearls. Mussel harvesting depleted mussel populations, and with the invasion of zebra mussels, states discontinued commercial mussel harvesting from the upper Mississippi River.

Native freshwater mussels have a fascinating life history. Male mussels release sperm into the water and females siphon them in through their gills, fertilizing eggs. The fertilized eggs develop into larvae called glochidia that look like little “PacMan” clams. The females release the glochidia into the mouths of fish. The glochidia clamp onto the fish’s gills. They take nourishment from the fish for a time and then drop off onto the river bottom to begin their filter-feeding life and grow into adult mussels.

Mussels have surprising evolved lures, baits, nets, and behaviors to attract host fish for their parasitic larvae. The female plain pocketbook (Lampsilis cardium) grows a mantle flap extension that looks like a small fish with an eye spot, ruffled “fins,” and a tail. The lure is displayed when the glochidia are ready to be released. Fish hosts like bluegill, smallmouth bass, sauger and walleye bite the lure and the female pocketbook releases a mouthful of glochidia that attach to the fish’s gills.

Mussels can move but they can’t go fast or far. They extend a muscular “foot” into the substrate and pull themselves along. You can often see mussel “tracks” in shallow water.

Native freshwater mussels are fascinating animals to study. Each species has a characteristic shape and color of shell. I like the common names like pimpleback, heelsplitter, deer toe, giant floater, hickorynut, washboard and mucket. The most common species in the upper Mississippi and St. Croix rivers is the threeridge (Amblema plicata), a heavy-shelled mussel with ridges and folds on the posterior part of the shell.

With water quality improvements in recent decades, increasing abundance of host fish species, and cessation of commercial harvesting, populations of native mussels are recovering in the upper Mississippi River. Recent quantitative surveys of three navigation pools on the river by the U.S. Geological Survey documented that mussels were actively recruiting juveniles into their populations.

The USGS estimated that in a 300-mile long reach of the upper Mississippi River, mussels filter 53 million cubic meters of water each day. That amounts to as much as 12 percent of the river flow during low flow conditions.

We should do what we can to protect river habitats and water quality to ensure that the native mussels in our region continue their recovery, fishing for hosts to give their young a start in life.

Please send any comments and suggestions for this column to me at rfjsports@rivertowns.net.

Tags: sports, outdoors, environment