By Dan Zarlenga, Missouri Department of Conservation
Photo caption: Plain pocketbook mussels are unsung heroes of the stream bed, creating a better habitat for all aquatic creatures. Photo by Jim Rathert, Missouri Department of Conservation.
A largemouth bass makes its way through a clear Missouri stream. Atop predator of the waters, the bass glides above the rocky bed, confident, hungry, and unsuspecting. A small fish wiggling just above the gravel bottom catches its eye—perhaps a tasty gizzard shad. The predator sweeps down for the capture. But the hunter becomes the prey. Suddenly bursting toward the bass comes thousands of tiny larvae that surround it, penetrating its gills. The predator is helpless to resist the microscopic attack.
This was no attack by a para site. It was a remarkable reproductive strategy.
The largemouth has been co-opted by a female plain pocketbook mussel. The fish will now become an unwitting nursery to support her larvae.
The plain pocketbook is among the most widespread mussel species in the Show-Me State. It is typically found within water bodies ranging from creeks to large rivers, especially if there is a gravel bottom and some level of moving current. It’s common throughout most of Missouri except the northwest portion.
Pocketbook mussels are freshwater bivalves— the river cousins of the clams most people know. They have two protective shells (the “valves”), which are built over time by protein and calcium secretions generated by the soft, boneless creature inside. Missouri is one of North America’s freshwater mussel strongholds, with some 70 or so native species still found here.
While it may look like brown rock to us, the pocketbook mussel is very much a living animal that can crawl, eat, and sense the presence of light. It will live for decades—for as much as 50 years—but the shells exist long after the inhabitant inside has died and decomposed. So, in a sense mussels are creators of their own immortality.
Befitting such an unusual creature is an equally strange reproductive strategy, which begins in August. The female pocketbook has a set of mantle flaps that move to mimic a fish. This is the lure it uses to attract a predatory fish. When close enough, the crafty mussel releases a cloud of larval mussels which attach themselves to their new host.
While this may seem a deceptive and parasitic practice, the larval mussels cause little harm to the fish. They stay as stowaways under the protection of the host just until the tiny mussels develop enough so they can break away and float to the stream bottom. Here they begin a new life and repeat the cycle.
The pocketbook certainly does take advantage of unwitting fish hosts, yet it offers something valuable in return. It, and other freshwater mussels, are nature’s ultimate water filters. The pocketbook constantly draws water through an internal siphon where it consumes algae, bacteria, and fine particles of decaying organic matter. As it extracts oxygen and tiny food particles, it returns cleaner water to the stream—filtering many gallons every day.
As a population, mussels are quietly and constantly improving the water quality of our streams and rivers. A healthy mussel bed can tirelessly filter thousands of gallons of water daily. They make a better habitat for the rest of their aquatic partners. Perhaps seeking some reproductive assistance from a passing bass isn’t too much to ask after all.