BIVALVE BIOLOGY
& PHYSIOLOGY

 
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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.

Shell Structure

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 blood.
Shell External Sculpting
1.  Concentric Sculpture (Saxidomus giganteus

2.  Cancelate Sculpture (Protothaca  staminea  
     -Showing evidence of predation by 
     Polinices lewisii

3.  Radial Sculpture (Clinocardium nuttallii)

1   
2    3   
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."

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.
Protothaca staminea
(Interior)

1.  Posterior Adductor Muscle 

2.  Anterior Adductor Muscle 

3.  Pallial Sinus 

4.  Pallial Scar 

5.  Lateral Teeth 
                         }Hinge Plate 
6.  Cardinal Teeth

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.


Internal Structure

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.
Tresus capax

1.  Anterior Adductor Muscle 

2.  Foot 

3.  Ctenidium 

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 COLLEGE
 
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Page author: Brian L. Kegel  kegelb@elwha.evergreen.edu
Last modified: 980906