 |  |  |
| | |
The clams, mussels, oysters, and scallops of the class bivalvia constitute 30,000 different species. These animals are called "bivalves" because they possess a shell with two valves. Many make good cuisine and others produce pearls that are harvested by prospecting jewellers. An additional value of bivalves that is becoming more and more apparent, is the utilization of their filter feeding abilities to clean polluted waters.
The two hinging concave shells that protect the bivalve internal structures are called valves. The hinge - which contains more proteineous than calcareous material - and a series of "teeth" keep the two valves together. The little knob on the exterior of the shell near one of the corners of the hinge is called the umbo. This is area of the shell where calcium carbonate secretions are first deposited, and thus the oldest part of the shell. As the bivalve matures, more layers are laid down by the mantle enlarging and thickening the shell.
If you try to open up the bivalve you will find that this is a fairly difficult task. Two large adductor muscles located on either side of the hinge maintain the protective shield for the soft tissues within. This is an adaptation that has developed in response to predation from sea stars.
The mantle is the portion that secretes the shell. If a small particle finds its way in between the shell and the mantle, nacre is secreted around it and a pearl is eventually formed. Pinctada margaritifera and Pinctada mertensi are two species of bivalve that produce high quality pearls.
Diagram of the bivalve interior.
Bivalves are rather sessile animals that are not mobile and feed by filtering water. Thus, there is no need for a muscular head-foot or radula for locomotion and grinding respectively. Enlarged gills form lamellae or folds with cilia that increase the surface area for gas exchange between the water and the blood stream. The cilia work to create currents over the gills moving water through the incurrent opening into the mantle cavity. Water is also sometimes moved through the siphon to the gills before moving on for exchange, and finally exiting through suprabranchial chamber and excurrent opening.
GAS EXCHANGE.
FILTER FEEDING.
Cilia covering the gills and labial palps filter food from the water and direct the particles toward the mouth. Particles that are too large are moved posteriorly and flushed from the animal along with water forced out by the closing of the valves.
DIGESTION.
Food particles entering the mouh are carried to the esophagus and down into the stomach. Cilia and a mucoid food string move the food against an abrasive structure called the gastric shield. As the food is being ground enzymes are released from the crystalline style which projects from the style sac. The acidic environment of the stomach breaks down the particles, but those that are large and undigestible, are taken to the intestine, and excreted through the anus, and out through the excurrent opening. Partially digested food is goes to the digestive gland for more intense digestion.
Blood flows through the heart which is wrapped around the intestine, to tissue sinuses in the foot. From there the blood travels to the nephridia and on through the gills where it is oxygenated, returning back to the heart. Gas exchange between blood and water also takes place in the mantle. The nervous system consists of 3 pairs of ganglia that innervate the foot, esophagus, and posterior adductor muscle. Sensory cells may be in the form of tentacles, photoreceptors, and statocysts.
Bivalves can be dioecious, monoecious, or protandric. External fertilization allows the release of reproductive product from the gonad into the open water where immatures can develop and disperse. It is the process of external fertilization that makes it difficult to control the spread of pest species such as the zebra mussel. Immature bivalves develop into larval trochophores, and become adults when the veliger larva settles on a substrate. The advantage of having morphogenesis from one form to another is the reduction of competition between juvenile and adult members for food and space.
In contrast, some species brood their young in their gills before releasing them to pursue trochophore and veliger stages. Others enter the glochidium stage, which parasitizes fish by hooking onto their gills or body surfaces. After a period of growth during which nutrient was obtained from fish tissues, the larva releases itself from its host and begins filter feeding.
In the case of some classes, spawning is triggered by certain environmental chemical cues. For example, the mussel Dreissena polyphora's spawning is believed to be triggered by phytoplankton blooms, which are the main food source for this filter feeding species. The hormone serotonin also plays a role in the regulation of spawning.
VELIGER LARVA
The veliger is an immature stage in the development of organisms in the Mollosca. This form is typically taken on by classes Gastropoda and Bivalvia. This form is characterized by bilobed swimming structure called the velum which has a fine row of long cilia. These cilia are used for locomotion and for bringing food particles suspended in the water towards the mouth.
This is also the stage in development that researchers have chosen to pinpoint as a means of control to prevent further spread of pest species. These minute molluscs can go undetected in waters ways and are permitted to settle and grow to adult size. Thus, this stage is a dispersal mechanism used to increase the chance of successful fertilization and proliferation of the species.
Known for their great diversity and shear number of species, Bivalvia has managed to be one of the most evolutionarily successful classes. Bivalves are able to inhabit a number of different environments adapting themselves accordingly. Some live within sand or mud sediments, or attach themselves to hard substrates or other aquatic organisms. Boring bivalves live beneath the surface of limestone, clay, coral, wood and other substrates. Those that create their own burrow by abrasion of the substrate surface, and they may produce secretions that will further dissolve the substrate. (e.g. limestone) As the bivalve grows, it soon becomes too large to escape through the opening it created and remains there indefinitely.
Return to Top
Return to Homepage