Conservation Corner: Freshwater Mussels, the Polite Parasites

By Dan Zarlenga, Missouri Department of Conservation

During the heat of August, people often turn to a cool Missouri stream for relief. The calmly flowing waters provide welcome refreshment from the late summer sun. While there you might notice a very interesting aquatic critter that’s so shy that it never comes out of its shell. That would be one the Show-Me-State’s freshwater mussels.

Thanks to our incredible diversity of aquatic habitats, Missouri has an impressive variety of nearly 70 different species of mussels. Clear, flowing waters of the Gasconade, Meramec, and Osage Rivers support the best diversity of mussels in the state.

Each species has distinct features and shapes to its shell. Mussels are known as bivalves because they live within two protective hinged shells, which are also called valves. The mussel inside gradually secretes materials over time that harden to form the shells.

Mussels essentially make their own houses! Much like the rings produced inside a tree trunk, the mussel produces growth lines on the outside of the shell which reveal the shapes the mussel had earlier in its life.

While the outer shell of a mussel is hard, the creature inside is soft and boneless. These soft-bodied invertebrates are mollusks, and relatives of clams, oysters, snails, slugs, squids, and octopi. As bivalves, mussels are filter feeders. When it comes mealtime, mussels play quite a shell game. They have one siphon that draws water into the shell, and food particles are extracted from the water as they are trapped by mucus in the animal’s gills. Debris and waste products are then expelled from a separate siphon. In this way, mussels help clean the water of streams and rivers, too.

As unusual as their feeding patterns might be, mussels have an even more fascinating reproductive trick up their shells. The male mussels release sperm, which are filtered from the water by females. The female holds the fertilized eggs in her gills, as they develop into tiny larvae called glochidia.

The glochidia are parasitic, and must attach to their host, which is usually a fish. Mother mussels have a variety of clever tricks to help them attract a host for their larvae. Some species, for example, tempt bass or walleye with lures that resemble small fish or crayfish. When these predatory fish strike at the lure they inhale the larval mussels.

Fortunately for the host fish, the glochidia are courteous parasites and cause them no harm. The larval mussels don’t wear out their welcome and rely on their host for only a few days or weeks before dropping off. They continue to develop into adults on the stream floor.

Mussels aren’t too adventurous when it comes to travelling as adults, however. In fact, most mussels stay in a single spot their entire lives. Because they don’t move around much, mussels require highly stable living conditions, and good water quality is especially important to them.

Nearly two-thirds of Missouri’s mussel species are of conservation concern due to a number of water quality challenges. As they are polite parasites, humans should return their courtesy. Everything we can do to help protect the integrity of our waterways is vital to their survival.