Most mollusks have a brownish "outer skin" called a periostracum while they are alive, which hides the beautiful colors and patterns of the shell. After the mollusk dies, the periostracum is lost and the shells surface is revealed. A dark shell that is found on the beach, quite often black, is an indication of either a fossilized specimen or one that has been buried in sediment for a long period of time.

The phylum Mollusca is divided into seven classes, but only the two shell producing classes will be described here: Bivalvia and Gastropoda, which contain the univalves. Other classes include Aplacophora, Monoplacophora, Polyplacophora, Scaphophoda, and Cephalopoda. Please refer to the Conchologists of America mollusk page for more information on these other mollusk classes. (http://coa.acnatsci.org/conchnet/c-101a2.html)

Bivalve means two shells or valves. These two shells are attached at a hinge where one or more strong muscles inside the shell keep it tightly closed, and a rubbery ligament near the hinge holds the halves together and open. Scars can sometimes be seen on the inside of some shells where the muscles were once attached. A bivalve breathes by circulating water within its shell, which brings in oxygen. As the water leaves the shell, it carries with it carbon dioxide and other wastes. Some bivalves have siphons, which water enters and leaves through.

Bivalves usually feed on food obtained through the circulating water, which contains tiny particles of plants and animals. The gills collect this matter and move it toward the mouth. Some bivalves are suspension feeders and others are deposit feeders. Some bivalves move by using a strong foot that pulls them along the bottom or helps to burrow in the sand or mud. Others spin a strong thread, called a byssus, which they use to anchor to rocks and other objects. Others move by clapping their two valves together and ejecting water from the back of their shells.

Most bivalves have separate sexes, but some are hermaphrodites. Depending on the species, hermaphrodites may have both male and female organs at the same time, or they may switch sexes at different stages of their lives. Those with both organs usually release eggs and sperm at different times so they do not fertilize themselves, allowing for more genetic variation in the young. Most bivalves release eggs and sperm directly into the water, where fertilization takes place. The fertilized eggs hatch into young called larvae that swim with other microscopic animals in the plankton. The animal matures within several weeks and settles on the bottom to develop into an adult. The amount of time the young spend swimming differs by species, but the longer they swim, the better the chances the currents will carry them to new environments. Many young arrive at a location where they cannot survive because the habitat is unsuitable, but others find suitable habitats and are able to colonize new populations, which is important for the continuation of the species.

Gastropods, which include univalves, are mollusks usually covered by a single shell, although some gastropods, such as slugs, have no shell at all. The shelled gastropod body emerges from an opening, called an aperture, to eat and move. The outer edge of the aperture is called a lip. Each coil of a gastropods shell is called a whorl, with the last and usually the largest whorl containing the body, thus called the body whorl. All whorls above the body whorl make up the spire. Gastropod means stomach-foot because the foot is large and is the most prominent featre in most cases. The rest of the body basically contains the stomach, which rests on the foot. Univalves, unlike bivalves, have a head with tentacles, which have sense organs that can detect shadows and movement. Many gastropods have a trap door, or operculum, attached to their foot, which is actually a thin piece of shell. This operculum seals the aperture closed when the animal retreats into its shell, providing protection from predators and from drying out. As with bivalves, most gastropods breathe by taking in oxygen from the water through siphons.

Gastropods do not rely on water circulation for food as bivalves do because they have many structures that are used specifically for feeding. Most gastropods are carnivores but some are herbivores, omnivores, and others are scavengers or detritivores. The head of a gastropod has a tubelike extension called a proboscis, which includes a mouth and a tongue-like strip of teeth, called a radula. Herbivores use the radula to scrape algae off rocks, while carnivores use it to tear flesh or to drill holes in shells to get to the meat inside.

Most gastropods move along the bottom by rippling their foot.

Fertilization of gastropod eggs occurs both inside and outside the body, depending on the species. Eggs are laid in either gelatinlike globs or in one of many types of egg capsules, which are sometimes mistaken for seaweed when washed up on shore. In some species, the eggs hatch into swimming young while others hatch into a crawling form that resemble the adults. Some species even bear live young!

There are over 100,000 living species of mollusks worldwide with more than 1,000 species of shelled mollusks living in the Carolinas. Please read the conservation page to understand why shelled mollusks are important to us and what we can do to help them survive.