Habitat and Ecosystem
Geology and Hazards
Land Use and Economy
Links and Resources
- Mike McGiveney
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
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 fishred 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 morefor example, when it must contract quickly to close
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.
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 particlessuch as silt or excess phytoplanktonare 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.
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