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 of 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, also called valves. The mussel inside gradually secretes materials that harden to form the shells over time. 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 hell, which reveals 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 to 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 females filter from the water. 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, 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 the lure, they inhale the larval mussels.
Fortunately for the host fish, the glochidia are courteous parasites that cause 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.
However, mussels aren’t too adventurous when traveling as adults. 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 critical to them.
Due to water quality challenges, nearly two-thirds of Missouri's mussel species are of conservation concern. 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.