These are bivalve mollusks that burrow in sand or drill into soft rock. Many are edible and are sold for food. Some were introduced to Europe from America when they were thrown from the kitchens of ocean liners.
About twelve families make up this major grouping of burrowing bivalves. Bivalves are mollusks whose bodies are encased within two shells, or valves, joined by a fleshy ligament.The shells are hinged together so that they can open and close.The hinge itself is formed from teeth on each valve that fit together, like the hinge on a box or a door. When the muscles used for closing the shell pull the valves together, the ligament is compressed (made smaller). When the muscles relax, the springy nature of the ligament forces the valves apart so they open.The shape and the way the hinge works is important in determining the family in which the animal is placed.
All these bivalves can burrow in sands and muds. They live partially or totally below the surface. In the northeastern United States, clams are divided into two groups. Soft-shelled clams, some of which are called steamers, grow in muddy, flat coastal areas.Hardshelled clams live in sandy bays and along sandy beaches.Hard-shelled clams are also known by the American Indian name quahogs. Clams and cockles are important as food in North America, Europe, and Asia. Clams are used for making foods such as soups, pies, and fritters. In northern Europe the common cockle and the razor shell are highly regarded as food items. Collecting them has become an important occupation in some regions. In the Mediterranean region, these shells are the basis of many delicious seafood dishes. Despite the abundance of sandy and muddy seabeds, many animals are unable to live on them because there is nothing to which they can attach themselves.However, burrowing in sand and mud opens up huge opportunities.There is space, food is abundant, and predators are few.Different bivalve families have exploited these conditions.They have developed structures that allow them to burrow while keeping in contact with water. This means they can absorb dissolved oxygen and food particles. The outer skin of the bivalve, called the mantle, encloses its body. It also produces the shells and ligament.At certain points, where the mantle edge forms two openings, these develop as tubes or siphons.One draws in sea water (the inhalant siphon) and the other pumps it out (the exhalent siphon). Inside the mantle cavity are the gills and the foot.The foot is an efficient digging device. Some bivalves can dig very fast.
Long and Short Siphons
Hatchet shells do not have extendable siphons. Instead, the animals use their foot to make a temporary inhalant siphon in the sand to get a supply of oxygen and food. Cockles have short siphons.They live partly buried, and their short foot holds them in position.The tellins, which are moderate burrowers, have two well-formed, separate siphons.The inhalent siphon curves out over the surface of the sand to vacuum up dead matter lying on the surface. The mollusk uses this dead matter as food.The clams and the razor shells are deep burrowers.They have long paired siphons that allow the animals to live deep below the surface of the sand.Their well-developed feet allow them to burrow fast.
Gills for Breathing and Feeding
The gills inside the mantle cavities of these bivalves are covered with tiny hairlike structures called cilia that pump water in and out.The stream of water carries dissolved oxygen and food particles in from the sea. Suitable particles are filtered out and passed to the mouth as food. Because this system is so efficient, enormous numbers of bivalves can be found living in marine sands and muds.