Bivalves have a hard shell made of minerals, typically layers of calcite and aragonite. The shell has two halves (valves) connected by a flexible ligament hinge. Powerful muscles contract to close the valves into a tight fit. Teeth along the edges of each valve interlock to keep them from sliding apart if the bivalve is attacked by a predator. The pattern of teeth, because it is often species-specific, is one feature used to identify bivalves. The shell grows over time, using calcium taken up from seawater or ingested food. Bivalves have no heads at all, and a flattened foot. The foot is shaped for wedging into sand, explaining why bivalves have also been called pelecypods (hatchet-feet). To keep from getting moved by water currents, bivalves tend to attach themselves to hard surfaces or burrow into sediments. They burrow by probing down with the foot (lengthening) and then retracting it (shortening) to tug the shell downward.

Most bivalves filter food with their gills, then transport it to their mouths. While still used to get oxygen, their gills are modified to also act as food strainers. Typically, a special tube called a siphon serves as a straw to suck water through the bivalve's gills, and another siphon lets the water out after filtering has occurred. Tiny grooves in the gills are covered with hair-like cilia that trap particles passing by, such as microscopic plants (phytoplankton). Particles not accepted as food are sent to where they can be purged from the shell. Particles accepted as food are wiped off the gills by flaps (palps) and sent on to the bivalve's mouth. From there they travel to its complex stomach, where more cilia sort it out by size. Some bivalves are scavengers, scraping food from the sea bottom, or sucking in crustaceans or other prey with their siphons.

Mollusks have been exploited by people around the world for thousands of years. Both shelled (snails, clams, scallops, conchs) and unshelled mollusks (squid and octopus) have been popular food items since prehistoric times. Several early societies used shells such as cowries for money. Today, mollusk shells are often collected and sold. Even mollusk waste products have value. A pearl is just shell layers that the mollusk uses to cover debris that gets under its shell. All shelled mollusks make them, but it is the pearl oysters that sometimes make the symmetrical, shiny ones, essentially decorative pieces of dirt. Over the years, mollusks have been used for many other purposes: dyes, decorative inlays, medicines, blades, fishing lures, tweezers, and horns. Overharvesting has endangered many mollusks species, and cultivation of mollusks had emerged as one solution.