Bivalves are animals that have shells with two halves, or valves. The word bivalve means “two valves.” Bivalves are a class, or group, of mollusks. Mollusks are a type of animal with a soft body. There are about 8,000 species, or kinds, of bivalve. They include scallops, cockles, clams, oysters, and mussels. Most bivalves live in the ocean, but about 15 percent of bivalve species live in freshwater. Physical FeaturesBivalves range greatly in size, shape, and color. Tiny freshwater seed shells may be only 0.06 inch (1.5 millimeters) long. Giant clams in the southern Pacific Ocean may be more than 4 feet (1.2 meters) long and weigh more than 500 pounds (225 kilograms). A bivalve's shell protects its soft body. Muscles connect the two valves of the shell on one side. These muscles allow the shell to close rapidly and tightly. When a bivalve relaxes the muscles, the shell opens. A bivalve's body includes a nervous system, a digestive system, and a heart. Bivalves have gills instead of lungs. When a bivalve opens its shell, water washes over the gills. The gills then strain out oxygen and food particles. Many bivalves have a muscle called a foot. This foot grows out from the body and sticks out between the valves. A bivalve uses its foot for moving around and for digging. BehaviorDifferent bivalve species live in different ways. Oysters and marine (ocean) mussels spend their lives attached to solid, underwater surfaces. Cockles live near or on the ocean floor. Some bivalves, such as scallops, swim by clapping their valves together. This pushes water out of the shell and moves them forward. Clams and freshwater mussels have a strong digging foot. They escape enemies by using this foot to burrow into sand. They collect water by sending a tube up through the sand. Life CycleMost bivalves are either male or female. Some change their sex or combine male and female parts. To reproduce, most female bivalves release eggs into the water. There the eggs meet with sperm from male bivalves. Young bivalves swim freely. They do not grow shells until later. Some types of bivalves live very long lives. For example, Icelandic cyprines may live for more than 200 years. UsesBivalves are an important source of food for fish, birds, and people. People also make jewelry and crafts out of bivalves' shells. The shiny material of some shells, called mother-of-pearl, can be made into buttons or other decorations. Some bivalves build shiny round beads around particles of dirt that enter their shells. People make jewelry out of these beads, which are called pearls. |