Bivalves are two-shelled invertebrates belonging to phylum Mollusca and
include clams, oysters, mussels and scallops. They are characterized
by their very developed gills or "ctenidium" which usually function in
biotic repiration and filter feeding. Most bivalves are found burrowing
into sand, mud, wood and even rock, although some attach to the surface
of hard substrates. Some species are free swimming.
All bivalves have rigid shells to provide support for their soft bodies.
The shell is divided into three parts: the two calcareous
valves and the ligament. The ligament is a stiff, organic hinge that
holds the two valves together and holds the valves of the shell open when
the adductor muscles are relaxed.
The valves are composed of three general layers. The outer most
layer, the periostracum, is a thin, paper-like, organic skin on most bivalves.
It acts as a base for the other layers to build on and protects the shell
from abrasion and dissolution in sea water. The middle layer is called
the perismatic layer, and is composed of calcite, a crystaline form of
calcium carbonate, which provide a high degree of strength for the shell.
The nacre or "mother-or-pearl," on the inner side of the valve, is made
of aragonite, a form of calcium carbonate. This smooth, shiny surface
protects the animal itself from abrasion
against the shell and is the same material that pearls are made from.
Nacre is formed around a foreign object, like a grain of sand, that gets
lodged between the shell and the mantle so that it won't irritate the mantle.
Both the periostracum and the perismatic layer are secreted by the mantle
edge where glands remove calcium carbonate from the animal's blood
which has a high concentration of this molecule. The nacre is produced
by the entire mantle surface from calcium carbonate in the animal's
At the hinge point,within the shell, many bivalves have rounded projections
called "teeth." There are two types of teeth in those bivalves that
have them: lateral teeth and cardinal teeth. Cardinal teeth are tall
and are located close to the umbo and chondrophore. Lateral teeth
are long, low and roughly parallel to the umbo and below the cardinal teeth.
The teeth help align the valves of the shell so that they close properly.
Collectively, the cardinal and hinge teeth are called "hinge teeth."
Shell External Sculpting
1. Concentric Sculpture (Saxidomus giganteus)
2. Cancelate Sculpture (Protothaca staminea
-Showing evidence of predation by
3. Radial Sculpture (Clinocardium nuttallii)
Although not all bivalves have hinge teeth, they all possess a hinge
ligament. The ligament, whether internal, external or partially internal,
opens the shell by acting as a hinge spring. The animal thereby expends
less energy to keep itself open while feeding and strengthens the connection
between the valves.
If the ligament is completely or partially internal, it will either
attach to a projection called a chondrophore or a depression called a
resilifer. Both of these features are located on the hinge shelf
of the shell
interior near the umbones.
Along the interior of the valves are various scars left behind where
adductor muscles or the mantle attached directly to the valve. The
most noticeable are the adductor scars, where the adductor muscles attached.
Depending on the species, there will be either one or two of these scars
and in some species with two scars, one may be so small that is hidden
from casual viewing.
The other obvious scar is the pallial line, or mantle line,
where the mantle attached to the valve interior. In most of the sediment
burrowing species, the pallial line switches back on itself and loops dorsally
and posteriorly to the posterior adductor scar forming a letter "C."
This scar represents the pallial sinus and signifies the place where the
siphons retract into the shell.
1. Posterior Adductor Muscle
2. Anterior Adductor Muscle
3. Pallial Sinus
4. Pallial Scar
5. Lateral Teeth
6. Cardinal Teeth
Bivalves feed on microscopic organisms and organic matter brought into
the mantle cavity through the incurrent (dorsal) siphon. Food particles
are strained by the ctenidia, and indigestible particles are expelled from
the animal by way of mucus pellets called pseudofeces. Digestible
particles are moved down the ctenidia to the labial palps which direct
food to the mouth. From the mouth, food passes through the esophagus
to the stomach where the enzyme rich, crystalline style helps in digestion.
Wastes are passed down through the anus and expelled through the excurrent
(ventral) siphon. The excretory system is similar to the digestive
system. Water, wastes and salts are passed into the nephridium where
they are stored while the water and salts are reabsorbed into the body.
Wastes are then released into the suprabranchial chamber and moved outside
the body through the excurrent siphon.
1. Anterior Adductor Muscle
4. Posterior Adductor Muscle
5. Mantle Edge
6. Pericardial Sac
7. Labial Palps and Mouth
1-7. Visceral Mass
A SIZE AND SHAPE COMPARISON OF THE BIVALVES OF THE EVERGREEN STATE
Page author: Brian L. Kegel email@example.com
Last modified: 980906