A gastropod is a single valve (shell), soft-bodied animal belonging to the mollusk phylum. Gastropods, which are also known as univalves or gasteropods, are the largest class belonging to the mollusk family. Estimates of how many species of gastropod are alive today ranges from 65,000 to 90,000. The name gastropod is derived from the Greek words gaster, meaning stomach, and poda, meaning feet. Snails and slugs, both freshwater and saltwater, and other animals that make coiled seashells such as conches, abalones, cowries and limpets belong to the gastropod class.
The gastropod’s most identifiable physical characteristic is its “torsion” process. During its growth and development, the internal organs rotate 180°, so that the anus migrates to just above the gastropod’s head. It has either two or four tentacles, which are used for sensory purposes. While a few gastropods may have more sophisticated eyes, most have simple eyespots at the tip of the tentacles that can discern only light and dark.
Another feature that distinguishes a gastropod is its ventral or muscular foot—this foot has a gland that secretes a mucous-like fluid that facilitates easier movement. Although most slugs don’t have the characteristic shell, most gastropods have spiral shells opening on the right side, made out of a horn-like or calcareous material.
Most of us will only come in contact with the typical garden snail or slug, but nearly two-thirds of gastropods live in water, both fresh and salt. Typically, the gastropod is herbivorous, feeding on decaying plant matter, but some are actually carnivorous. Most have a radula, a ribbon-like tongue “system” which cuts and chews the animal’s food before traveling through the esophagus. Burrowing gastropods siphon water to glean food and oxygen through siphoning tubes.
Most water dwelling gastropods have gills, but some water and most land species actually have a lung to breathe oxygen. The garden variety snail or slug is typically brown for purposes of camouflage, but some sea slugs are brightly colored, ruffled animals meant to fade into a colorful reef, or warn predators of a nasty sting or poisonous flesh.
With so many different species of gastropod, there is great variance in size. Snails can range from .02 inches (<1 mm) to 30 inches (77 cm). The largest land snail is the Giant African Snail, measuring nearly 16 inches long (39 cm) and weighing two pounds (900 g).