Bivalves Classification PHYLUM: Mollusca CLASS: Bivalvia

Bivalve Morphology 1 Look at page 32 of Black. Copy diagram 18a. The soft body is held between two shells (valves) which are usually made of calcite or aragonite. The valves act as protection from carnivores and wave energy. Bivalves are bilaterally symmetrical, the plane of symmetry occurring along the line where the two valves meet. Therefore the valves are more or less mirror images.

Bivalve morphology 2 VALVES: The shell is secreted by the animal like in brachiopods from the mantle. The valves are called left and right. What are the two valves called in brachiopods?   UMBONES: The point on the valve from which the bivalve grew.

CONCENTRIC GROWTH RINGS: Bivalve morphology 3 CONCENTRIC GROWTH RINGS: There are concentric growth rings increasing in size away from the umbones. These are added as the bivalve grows much in the same way as the growth rings on a tree. If the growth season is good then the rings will be more widely spaced.  RADIATING RIBS: The shell ornament and it may also have spines. These give the shell extra strength and rigidity.

EQUIVALVE/INEQUIVALVE: Bivalve morphology 4  EQUIVALVE/INEQUIVALVE: In most bivalve species the valves look very similar (equivalve) unlike Brachiopods although sometimes the valves do look different (inequivalve). DORSAL/VENTRAL: The dorsal side is the hinge area the opposite is the ventral. Difficult to remember. ANTERIOR/POSTERIOR: The anterior end is the mouth end. The posterior is the anus end. Difficult to work out on a fossil.

Bivalve morphology 5 SOFT PARTS: Most soft parts are in the dorsal half, which contains the main organs and muscles. MANTLE:  This is the fleshy part that extended out towards the edge of the valves. It enclosed most of the soft parts and was also responsible for the growth of the valves as the mantle wrapped over the edge of the valve. FOOT: This is a muscular organ, which can extend out of the shell and is used by the bivalve to move through the sediment. See video clip from OU.

Bivalve morphology 6 GILLS: Found in the mantle cavity. It is a respiratory organ, which also helps in food gathering. Cilia help move water through the gills. Water comes in one SIPHON and out another. DENTITION: On the inside of the hinge area the shells are thick. In this area there are teeth and sockets which fit together with those on the other valve.  Gives extra strength and rigidity to the bivalve.

Bivalve morphology 7 LIGAMENT: This occurs in the hinge region and can hold the valves together and acts as a spring forcing the valves apart when the muscles relax. ADDUCTOR MUSCLES: Mainly the shells are closed by adductor muscles, which contract to close the shell. When the muscle relaxes the valves open (because of the ligament). MUSCLE SCARS: These are the points where the muscles were attached.

Bivalve morphology 7 PALLIAL LINE: On the inside of the valves a faint groove can often be seen running from anterior to posterior roughly parallel to the shell edge. This is the point where the mantle ended. PALLIAL SINUS: In some species of Bivalve the pallial line shows a large indentation. In burrowing species which have large siphons which extended out into the burrow the sinus left space for it to be contacted back into the bivalves.

Bivalve morphology 8 GAPE: Many burrowing bivalves did not close completely and left a small gap through which the siphons or foot or both could protrude permanently out of the valve. CRENULATED MARGIN: In some bivalve species the edges of the two valves are smooth, in others a crenulation may be present represented by small ridges and grooves. These allow a tighter more rigid fit. BYSSUS: This is a horny extrusion extending from the body cavity, which attaches the animal to a firm surface.

Interesting Facts About Bivalves Some oysters may shed over one million eggs in a season. Only about one of these lives to adulthood. Some oysters alternate their gender. Male one year, female the next! Some scallops (Pectens) have dozens of eyes. They help the scallop to see predators, so it will know whether to swim away or clamup! The ocean quahog can live to be 220 years old. Boring clams (Family  Xylophagidae "wood eater" in Latin) can sink a ship!  They are often called "ship worms", even though they aren't worms at all!!

Interesting Facts About Bivalves 2 Most molluscs are capable of making pearls when foreign substances enter their shell. They coat the foreign substance with shelly material. Some clams can grow pearls as big as golf balls.   Freshwater clams are often used as nuclei, around which pearls can grow small, round pieces of the clam's shell are placed inside a Pearl Oyster and they are coated with iridescent nacre and form a pearl. The largest known bivalve harvested was a Tridacna gigas which weighed 330kg  and was 1.4m (nearly four feet) in length! Some molluscs, such as the oysters, change sex. They start off life as a male and they usually end up life as females.