Clam Anatomy: Interactive 3D Model of a Bivalve

A clam is a bivalve mollusc: a soft, headless body enclosed by two hinged shell valves. The body has four main parts. The mantle lines the shell and secretes it. The gills hang in the mantle cavity and both breathe and filter food. The muscular foot digs. The visceral mass holds the gut, digestive gland, gonad, heart and kidneys. Two adductor muscles close the shell, and an elastic hinge ligament opens it.

Bivalves have no head, no jaws and no radula (the rasping tongue of snails). Water enters through the incurrent siphon, passes through the gills and leaves through the excurrent siphon, carrying food in and wastes out. The hard clam is a typical member of the venus clam family (Veneridae) and is used in most school and university labs, so the anatomy here applies to bivalves in general, with differences noted below.

The hinge is dorsal, the opening edge is ventral, and the beaks point to the anterior end. In Mercenaria the umbo (beak) sits in front of the middle of the hinge and curves forward, over a small heart-shaped depression called the lunule. Behind the beaks, the dark brown hinge ligament lies in a long, flattened area called the escutcheon. The siphons come out at the posterior end.

Left vs right valve: hold the shell with the hinge up and the anterior end (lunule) pointing to your left. The valve facing you is the left valve; the one facing away is the right valve. Hard clams are equivalve, so you must use orientation, not shape, to tell them apart.

Inside each valve you can see the anterior and posterior adductor muscle scars, joined by the pallial line where the mantle edge attaches. Near the posterior scar the line bends into a short V, the pallial sinus, which makes room for the siphons. The hinge plate carries three cardinal teeth under the beak and a lateral tooth behind them. A dark violet border near the posterior margin is the trademark of Mercenaria: Indigenous peoples of the Atlantic coast cut it into purple wampum beads.

The shell has three layers: a thin protein periostracum on the outside (mostly worn away in quahogs), a chalky outer calcareous layer, and a smooth inner layer. Unlike freshwater mussels and pearl oysters, the hard clam has no nacre (mother-of-pearl); its inner layer is porcelain-like and white.

From the outside you can see the two valves, the umbo, hinge ligament, lunule, escutcheon and growth lines, and in a live clam the siphons and foot. The short siphons are fused together. The lower, wider incurrent siphon has a fringe of short sensory tentacles; the upper excurrent siphon carries water out. The white, blade- or plough-shaped foot extends from the anterior-ventral gape. Inside the shell edge you can see the frilled mantle edge.

With the left valve and left mantle lobe removed, you see the gills, labial palps, foot, visceral mass, both adductor muscles and the heart. The large, ridged, brownish gills cover much of the side of the body. Each gill has an outer and an inner plate (demibranch). At their front ends are the flap-like labial palps, which surround the mouth just behind the anterior adductor. Below the gills lies the foot, and above it the visceral mass, filled with creamy gonad and the green-brown digestive gland. Just under the hinge is the thin pericardium around the heart, with the brown kidneys below it.

These are all 61 structures in the 3D model, grouped by system. Click any of them in the viewer above to find it.

A clam is a filter feeder: its gills strain phytoplankton and other fine particles from the water it pumps through the mantle cavity. Cilia on the gill filaments create the water current. Particles caught on the gill surface are wrapped in mucus and carried by cilia down to the food grooves along the lower gill edges, then forward to the labial palps. The palps sort the particles: food goes into the mouth, and rejected material is bound in mucus and expelled as pseudofaeces.

Inside the stomach, the clear crystalline style rotates against the hard gastric shield, winding in the mucus food string and releasing digestive enzymes. Fine particles pass into the digestive gland to be digested inside its cells; the rest go to the intestine and leave as faecal pellets through the anus, into the outgoing water.

Clams breathe with their gills, using the same water current that brings their food. Oxygen diffuses from the water into the blood flowing through the gill filaments, and the thin mantle also takes up oxygen. Oxygenated blood returns from the gills to the two auricles of the heart. When the shell is shut, for example at low tide, a clam can switch to anaerobic metabolism for a while.

A clam burrows by pushing its foot into the sediment, swelling the tip with blood as an anchor, and pulling the shell down after it. The foot has no skeleton: it is a hydrostatic organ stiffened by blood pressure. Contraction of the adductors squeezes blood into the foot, then the anterior and posterior pedal retractor muscles pull the shell towards the anchored foot. Repeating the cycle rocks the clam into the sand. Hard clams live just below the surface, from a few centimetres to about 15 cm deep, with their short siphons reaching the water.

A clam's heart has three chambers, one ventricle and two auricles, and the rectum passes straight through the ventricle. The heart sits dorsally, just under the hinge, in a fluid-filled sac called the pericardium. The ventricle pumps blood forward through the anterior aorta and backward through the posterior aorta. Bivalves have an open circulatory system: blood leaves the arteries and flows through open spaces in the tissues before returning through the kidneys and gills to the auricles.

Below the pericardium lie the paired kidneys (nephridia, or organ of Bojanus), which take fluid from the pericardial cavity and release urine into the exhalant chamber. Reddish pericardial glands (Keber's organ) on the pericardial wall also help filter the blood.

Clams have no brain; they have three pairs of ganglia linked by nerve cords. The cerebropleural ganglia lie beside the mouth, the pedal ganglia at the base of the foot, and the visceral ganglia on the front of the posterior adductor muscle. Senses are simple: balance organs (statocysts) in the foot, an osphradium in the exhalant chamber, and touch- and light-sensitive cells on the siphon tentacles and mantle edge.

Most adult hard clams are either male or female, and they release eggs and sperm into the water. The gonad fills much of the visceral mass around the intestine and opens through small genital pores into the exhalant chamber, so gametes leave through the excurrent siphon. Males and females look the same: you need a microscope to see eggs or sperm. Young quahogs usually mature first as males, and many later become females. Fertilised eggs develop into a swimming veliger larva that settles after one to a few weeks.

A clam grows by adding new shell at the margin, so each growth line marks a former edge of the shell. The mantle edge lays down new shell during the growing season; in winter or during spawning, growth slows and a dark band forms. Counting these annual bands, usually in a cut section through the shell, gives the clam's age. Hard clams grow fastest in their first few years, so later bands are packed tightly near the edge. One quahog from Massachusetts was aged at about 106 years this way.

All bivalves share the same basic plan (two valves, mantle, gills, foot and adductors), but they differ in muscles, foot and lifestyle.

A scallop has a single, very large adductor muscle, rows of eyes on its mantle edge, and swims instead of burrowing. It claps its valves together to jet water out near the hinge, so it moves in a series of jerks. The adductor has a large quick part for swimming and a smaller catch part for staying closed; the white “scallop” sold as food is this muscle. Scallops have dozens of small eyes, up to about 200 in some species, that focus light with a curved mirror of guanine crystals rather than a lens. They have no siphons and only a small foot.