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Quahog Dissection

A quahog lives between two shells, or valves. The shell is made up of calcium carbonate that the clam secretes. The thickness and toughness of this shell give the animal its other common name, the hard-shell clam. What can a quahog shell tell you about the animal inside?First, the lines on the shell tell you something about the growth of the clam. A cross-section of the shell would be needed to determine the exact age of a quahog.

Looking at the inside of shell reveals more about the clam. A hinge, made up of intermeshing teeth, forms the joint between the valve. The tough, but pliable hinge ligament holds the two valves together. At the top of the hinge, the umbo, also known as the beak, is the oldest section of the shell. All shell growth radiates out from the umbo.

The adductor muscle scars show where these important muscles were once attached. The pallial line is where the mantle attaches to the shell. Now look at some of the internal organs of the quahog. The clam uses adductor muscles to close the shell. It does this to avoid predators, and also if water conditions are not good. There are two kinds of muscle in the adductor muscles. Pink "fast fibers" are used if the clam needs to close the shell quickly. White "catch muscle" fibers can hold the shell closed for long periods of time. Compare this to red and white muscle in some fish—red muscle is used for short bursts of speed, and white muscle is used for cruising for longer periods at slower speed. (The reason that the fast fibers are pink is that they contain myoglobin, a protein that contains iron and has a red pigment. The myoglobin stores oxygen from the blood, and releases it when the muscle needs more—for example, when it must contract quickly to close the shell.)

The large, muscular foot can reach outside the shell so that the clam can burrow. The foot retractor muscles, as well as the muscles in the foot itself, control the foot. Water comes into the quahog through the incurrent siphon and leaves through the excurrent siphon. Together, these siphons make up what we call the "neck" of the clam. The clam sits buried in sediment, and sticks the siphons up into the water above so that it can suck in and spit out water. The water that the clam sucks in through the incurrent siphon contains oxygen and food (plankton). The water that the clam spits out through the excurrent siphon contains the animal's wastes.

The mantle is the part of the animal that forms the shell. This happens on the ventral side of the animal (at the bottom in the picture above). The mantle secretes calcium carbonate, the compound that we see as the hard substance that makes up seashells. The mantle also secretes a network of molecules. This network of organic (carbon-based) molecules (or proteinaceous matrix), forms a kind of framework. The calcium carbonate then fills in the gaps in the framework, and together these substances form the clam's shell.

The quahog's gills serve several important functions: obtaining oxygen and getting food. The gills have tiny, hair-like structures on them called cilia. By waving the cilia, the clam can create a current that moves water through its body.

Feeding and digestion

The gills also move food through the body. When water comes in through the incurrent siphon, particles of silt and food are trapped on the layer of mucous on the outer surface of the gills. From there, the cilia move the particles along food grooves toward the labial palps, where they are sorted. Food particles move on toward the mouth. Other particles—such as silt or excess phytoplankton—are dropped onto the surface of the mantle, where the clam eventually gets rid of them in mucous-coated balls.

Food particles move from the mouth and esophagus to a multi-chambered stomach with numerous passageways and dead-end sacs. An important digestive organ is the crystalline style, a thin, clear organ that is often mistaken for a worm, but actually contains digestive enzymes. The style is located in the style sac, which is close to the stomach. During digestion, the style is rotated by cilia on the walls of the style sac. This releases digestive enzymes and natural "detergents" (emulsifiers) that help the clam to digestfats.The style may also grind food as it rotates. Although quahogs do not have the enzymes to digest the silica "skeletons" of diatoms, diatoms are a key food source. It is possible that the diatoms' skeletons are crushed in the digestive process.

Respiration and circulation

The clam also uses the gills to breathe. The cilia move water across the quahog's gills. As the water passes over the gills, oxygen passes from the water into the clam's blood, or hemolymph. The hemolymph is moved to the gills by the heart. After it picks up the oxygen at the gills, it moves on to the outer tissues of the clam, where the oxygen is delivered. The quahog has an "open" circulatory system, so that once the hemolymph gets to the outer tissues, it leaves the blood vessels and flows into open sinuses, or cavities, where it directly bathes the tissues. In contrast, in our circulatory systems, blood always stays in some kind of blood vessel, such as capillaries. On its way back to the heart, the hemolymph passes through the kidney. Then it collects in the open space of the pericardium. Next, the hemolymph returns to the heart, and the cycle starts all over again. (For more about gills, check out the fish gills on the pogy page.) The gonad is the reproductive organ that produces eggs or sperm.