The George Washington Memorial Parkway is home to forty species of mollusks: twenty-seven species of terrestrial snails and slugs, five species of aquatic gastropods, and eight species of freshwater mussels. This list includes rare species such as the fine-ribbed striate, and the Appalachian springsnail.
"Mollusks" describes all organisms within the invertebrate phylum Molluska. They are an extremely diverse group of organisms, with living members in eight classes. Their anatomical features vary greatly, although generally they possess a shell and a fleshy mantle which excretes it. (There are numerous exceptions.) Two classes of mollusks inhabit the park: the gastropods, which consist of several families of snails and slugs, and the bivalves, which are represented by freshwater mussels.
Gastropods in the park are primarily nocturnal and are most frequently seen on spring and summer nights crawling across the forest floor or on rocks and vegetation after fresh rainfall. They can also be seen during the day hiding under loose leaf litter and rotting logs or—in the case of slugs—wedged under loose tree bark. They may occasionally be seen moving across the rocks of exposed stream beds. Aquatic gastropods may be found in bodies of water throughout the park. They may be moving across submerged rocks or even suspended upside-down from the surface of the water, feeding on algae at the surface.
Gastropods—as primary consumers of algal growth—occupy an important niche in the ecosystem. Algae are most readily found growing on hard surfaces like rocks and wood. In order to exploit this food source gastropods have specialized mouthparts consisting of several rows of chitinous teeth (a "radula"). The radula functions as a sort of tongue that gastropods use to scrape off algae. Muscular contractions then force the food backwards down the esophagus into the stomach.
While the feeding habits of gastropods ensure that they have access to a plentiful food source, it also leaves them vulnerable to a variety of predators. To compensate for this, gastropods have evolved a shell composed of calcium carbonate embedded in a protein mesh. As in all mollusks, a gastropod's shell is secreted by a structure called the mantle. The mantle continuously secretes the shell throughout the snail's life, causing the shell to grow larger and wider with age. Slugs, on the other hand, have lost their shells at some point in their evolution in a tradeoff between the added protection afforded by a shell and the ability to compress their bodies to maneuver into tight spaces, such as a crack on a rock face or under the bark of a rotting logs.
While walking along one of the park's many waterside trails, you may notice large quantities of broken, discarded shells strewn about the shoreline and accumulating along the water bottom. This attests to the large populations of freshwater bivalves that inhabit the waterways of the George Washington Memorial parkway. These mussels are most frequently found partially buried in sandy gravel in slow moving sections of the river.
The wide distribution of our bivalves is due to their unique juvenile dispersal strategies and the relationships they maintain with native fishes. After fertilization, the mussel larvae develop within the gills of the parent mussel. This causes the mussel's gills to distend. (These elongated gills are termed "gill marsupia.") After a brief period of development. the larvae (called "glochidia") are released into the water column through the mussel's exhalent siphon. Then, the glochidia attempt to attach themselves to the gills of passing fish. Once attached, they derive sustenance from the blood plasma of their host until they complete their development. Then they drop off of the fish's gills and settle on the bottom. This relationship with native fishes allows mussels to distribute both upstream and downstream.
Bivalves feed by drawing water into their gills through their inhalant siphon. This is accomplished by rhythmically beating tiny hairs, or cilia, that line the labial palps near the mussel's internal mouth. The incoming water passes over the gills where tiny organic particles become trapped in a net of mucus strung between the gills. Beating cilia gradually draw the mucus-entangled food towards the mouth. By doing this shoals of mussels are capable of filtering large amounts of water, removing suspended particles, and improving water clarity.
Mollusks as a Source of Food
Mollusks occupy an important niche in park ecosystems. They serve as an important food source for wildlife: waterfowl, raccoons, turtles, and snakes frequently eat mollusks, as do several species of fish, including American shad, channel catfish, and striped bass. Historically, mussels served as a food source to both indigenous peoples and early colonists in the region.
Conservation StatusAlthough mollusks are abundant in the park, they face many threats. Overuse of pesticides for maintaining lawns, gardens, landscaping, and agriculture threaten gastropods because they often feed on affected surfaces or live in water contaminated with pesticide laden runoff. Increasing land development and urbanization throughout northern Virginia over the past few decades has increased the amount of water runoff from impervious surfaces like roads, sidewalks, buildings, and parking lots. This increase in runoff decreases the amount of time water has to infiltrate into the ground. Large quantities of fast moving water suspend more sediments in the water column. This can overwhelm mussels' ability to sort particles and interfere with their respiratory functions. If you would like to learn more, visit the Chesapeake Bay Program website.