Smoky Mountain Snails

By April Byrge

Great Smoky Mountains National Park is a biodiversity hotspot for a wide variety of organisms. There is a huge diversity of salamanders, trees, flowering plants, mushrooms, millipedes, and ferns. There are 146 land snail species in the park, 27 of which are found in the area surrounding the Smoky Mountains, and 9 of which are endemic. An endemic species is one that is found only in one place in the entire world—so those 9 snail species are found only within Great Smoky Mountains National Park! 

What are snails?

So what are snails, really? They are invertebrates, so they lack a backbone. They are also in the phylum Mollusca, along with other mollusks like the slug, clam, oyster, mussel, squid, and octopus. Mollusks are characterized by their soft bodies, the presence of an internal or external shell, a mantle (a fold in the body that may line the shell), a muscular “foot” (or tentacles in some cases), a scraping mouthpart called a radula, and a fluid-filled body cavity called a coelom. Snails are in the class of Gastropoda, which makes up 80% of all mollusks. Gastropod is Latin for “stomach-foot,” which describes the muscular “foot” that snails and use to move. They can absorb nutrients through their foot, and their radula is located on their foot as well (see Figure 1).         

 Figure 1. Basic land snail anatomy (Dourson, 2013).

What makes a snail a snail?

There are many unique characteristics of land snails. Most snails have shells made of calcium carbonate, the same mineral that creates seashells and egg shells. Land snails evolved from sea snails about 600 million years ago. Land snails have lungs like us, but unlike us, they have a respiratory pore that they use to breathe (see Figure 2). Their bodies are slimy, which keeps them from dehydrating and helps them move. This mucus doesn’t taste good to some animals, so it discourages predators. Snails have two sets of tentacles. The upper tentacles are the eye stalks and the lower tentacles are chemoreceptors which help them smell chemical scent trails. This allows the snail to find food and mates. Most land snails are hermaphroditic, having both male and female sex organs. When two snails of the same species meet, they exchange sperm. This sperm can be stored for months or years (depending on the species) until the conditions are right for the snail to lay eggs. Egg laying typically takes place under logs or in moist leaf litter. 

Figure 2. Internal anatomy of a land snail (Dourson, 2013).

Did you say love darts?

      Some land snails use love darts made of calcium during their courtship (see Figure 3). These darts are shot from the body of the snail into its partner, poking into the skin. Since these snails are hermaphroditic, the snail that receives a dart is the one that is impregnated and lays eggs. The darts are covered with mucus which contains a hormone. This hormone causes the sperm storage organ to open so fertilization may be more successful. These darts are known to discourage darted snails from mating again, increasing the odds that the shooter will father the other’s offspring. Darts come in a variety of shapes and sizes. Some snails use multiple darts, while others repeatedly stab their partner with one dart.       

Figure 3. Dry land snail love dart compared to dart covered in mucus (Dourson, 2013).

What can snails can tell us?

In addition to being very strange and interesting animals, snails are also bioindicators. A bioindicator is a sensitive organism that tells us something about the environment. Because snails need calcium to build their shells, in internal processes, and in reproduction, they are excellent monitors of calcium in the environment. Snails collect calcium from rocks and soils, decaying leaf litter, and shells and bones of dead animals. In some places in the park, obtaining enough calcium may be difficult because of acid deposition. Great Smoky Mountains National Park receives the equivalent of 120 train cars of acid rain being dumped on its highest elevations every year. This acid rain is a product of air pollution that comes mainly from large cities outside the park, such as Knoxville, TN and Atlanta, GA. Acid rain leaches calcium from the soil, so it’s not as easy for snails to get. Our ongoing snail monitoring project is recording change over time in snail numbers and diversity in the park to see if acid deposition is decreasing snail abundance and species.   

So why are snails important?

But why do we care if snails disappear? The bodies of snails and other mollusks in shallow oceans created limestone. Limestone forms caves and we use to make cement. Snails are the source of many medicines—everything from pain medication for cancer and AIDS patients, to antibiotics, to eczema treatment. Snails are also a very important part of the food chain—they are the link between calcium in the soil and calcium in other parts of the ecosystem. Snails eat a wide variety of plants, decaying vegetation and leaves, woody debris, various types of fungi, decaying animal matter, scat, and other snails. Many animals rely on snails as a food source, such as small mammals, salamanders, frogs and toads, turtles, beetles and their larvae, millipedes, and flies. Birds need to ingest a lot of calcium to lay eggs, so they have to eat many snails (see Figure 4). If birds don’t eat enough snails, their egg shells are thin and likely to break. Declines in bird populations have been linked to the decline in land snails in some areas that receive large amounts of acid rain. Researchers found that the more snails birds ate, the higher their chances of reproducing successfully.   

           Snails are an amazing group of animals with a lot to teach us. By studying them and monitoring their populations, we can also learn about connections in the environment. Join us here at Great Smoky Mountains National Park to get involved in our snail study!

Resources

Dourson, D. 2013. Land Snails of the Great Smoky Mountains National Park and Southern Appalachians. Goatslug Publications: Bakersville, NC.

Carnegie Museum of Natural History. 2015. Land snails and slugs of the Mid-Atlantic and Northeastern United States. Retrieved from  <http://www.carnegiemnh.org/science/mollusks/predators.html>