CLASS BIVALVIA (bis, twic + vala; leaf)
There are 30,000 species of this class. It is the second largest molluscan class. This class includes the clams, oysters, mussels, and scallops.
1.They have sheet like mantle.
2. Shell consists of two valves. Shell covers the laterally compressed animals.
There are collowine use of bivalves.
1. Many bivalves are edible.
2. Some bivalves form pearls.
3. Most bivalves are filter feeders Therefore. they are used in removing bacteria from polluted water.
SHELL AND ASSOCIATED STRUCTURES
Shell: The two convex halves of the shell are called valves. A proteinaceous hinge is present at the dorsal margin of the shell. A series of tongue-and-groove modifications called teeth are also present at dorsal margin. Hinge and teeth prevent the valves from twisting. The oldest part of the shell is called umbo. Limbo is a swollen area near the shell’s anterior margin. The shell forms as a single structure in embryo.
Hinge ligament: The shell is continuous along its dorsal margin. But the mantle secretes greater quantities of protein in the region of the hinge and secretes relatively little calcium carbonate. Thus it produces an elastic hinge ligament. The muscles relax and hinge ligament opens the valves due to elasticity.
Adductor muscles: These muscles-are present at both end of the dorsal half of the shell. They contract and close the shell. This is important for bivalves. These valves are pri lary defense against predator like sea stars. The bivalve does open their shells and remain save.
Pearl formation: The mantle attaches to the shell around the adductor muscles and near the shell margin. Sometimes, a sand grain or a parasite comes within the shell. The mantle secretes nacre around it and form pearl. The Pacific oysters, Pinctuda margaririfera forms highest-quality pearls.
EXCHANGE, FILTER FEEDING, AND DIGESTION
Bivlves are sedentary and filter-feeding.. Therefore, they have lost the head and radula. The cilia cover gills. Gills forms folded sheets called lamellae. One end of gill is attached to ti c foot and the other end attached to the mantle. The mantle cavity ventral to the gills fon is inhalant region. The cavity dorsal to the gills is the exhalant region.
(a) Cilia moves and water enters into the mantle cavity through an incurrent opening of the mantle. This opening is present at the end of a siphon. Siphon is an extension of the mantle. A bivalve can extend its siphon to the surface. Therefore, it can feed and exchange gases when it is buried in the substrate.
(b) Water moves from the mantle cavity into small pores of the gills. Then water moves into vertical channels in the gills. called water tubes. Blood is present around the water tube. Therefore gases are exchanged by diffusion.
(c) Water leaves the bivalve through suprabranchial chamber and ex- current opening in the mantle. Suprabranchial chamber is part of the mantle cavity. It is present at the dorsal side of the gills.
The gills trap food particles. Zoologists thought that cilia action was responsible for the trapping of food. But recent study indicates that cilia and food particles have little contact. The food-trapping mechanism is unclear. Cilia move the particles to the gills after trapping of food. Cilia along the ventral margin of the gills move food toward the mouth. Cilia also cover the labial palps. The labial pains are present on both side of the mouth. The cilia of laibial palps also filter food particles. Cilia carry small particles into the mouth. They move larger particles to the edges of the palps and gills. This rejected material called pseudofeces. Pseudofeces are transferred to the mantle. The ciliary of the mantle transports the pseudofeces posteriorly. Valves are forcefully closed. Therefore, water rushes out. This water washes pseudofeces from the mantle cavity.
The digestive tract of bivalves is similar to other molluscs.
(a) Oesophagus: Food enters into the esophagus. It is covered in mucous. It forms mucoid food string . Cilia line the digestive tract. They move this mucoid od string in to the stomach.
(b)Stomach: A diverticulum called the style sac open into stomach. A crystalline style mucoid mass) moves into the stomach from style sac. Enzymes for the digestion of carbohydrate and fat are present in the crystalline style. Stomach has a chitinous astric shield. Cilia of the style sac rotate the style against the gastric shield. The friction and acidic conditions in the stomach release the enzymes from crystalline style. The crystalline style rotates. Therefore, mucoid food string winds around it. It pulls the food string into the stomach from the esophagus. The action and the acidic pH in the stomach remove food particles from the food string. Further rocess separates fine particles from the indigestible materials.
(c) Intestine: Indigestible materials are sent to the intestine. Partially digested food enters into a digestive gland for intracellular digestion.
Cilia carry undigested wastes from the digestive gland back to the stomach and then to the intestine. The intestine release wastes through the anus. The anus is present near the excurrent opening. The excurrent water carries feces away.
OTHER MAINTENANCE FUNCTIONS
Blood vascular system
Blood flows from the heart to tissue sinuses, nephridia and gills. It then moves hack to the heart. The mantle is an additional site for oxygenation. Therefore, a separate aorta supplies blood directly to the mantle.
Two nephridia are present below the pericardial cavity (the coelom ). Their duct system connects to the coelom at one end. It opens at nephridiopores in the anterior region-of the suprabranchial chamber.
Their nervous system consists of three pairs of interconnected ganglia. These ganglia are present near oesophagus, foot, and posterior adductor muscle.
Sense organ: The margin of the mantle is the principal sense organ. It always has sensory cells. It has sensory tentacles and photoreceptors. In some species photoreceptors are present in the form of complex eyes. Eye has a lens and a cornea. Other receptors are statocysts. Statocysts are present near the pedal ganglion. Another receptor osphradium is presentin the mantle beneath the posterior adductor muscle.
REPRODUCTION AND DEVELOPMENT
Most bivalves are dioecious. A few species are monoecious. Some of the monoecious species are protandric. Gonads are present in the visceral mass. They surrounded the looped intestine. Ducts of these gonads open directly in to the mantle cavity. Or it open by the nephridiopore in to the mantle cavity.
External fertilization takes place in most bivalves. Gametes are released through the suprabranchial chamber of the mantle cavity and exhalant opening. The sperm enter into mental cavity through inhalant water. Fertilization occurs in the mantle cavity.
Trochophore and veliger stages are formed during development. The veliger settles on the substrate and it becomes adult.
- Trochophore and veliger larvae: Most freshwater bivalves brood (internal development) their young. Some brood their young in maternal gills. They have reduced trochophore and veliger stages. Young clams are shed from the gills.
- Glochidium larvae: Others brood their young to a modified veliger stage called a glochidium. Cilochidium is parasitic on fishes. These larvae possess two tiny valves.
Some species have tooth like hooks. Larvae moves out through the exhalant aperture and sink to the substrate. The giocnidium. attaches to the gills. fins, or another body part of fish. It begins to feed on host tissue. The fish may form a cyst around the larva. The mantles of some freshwater bivalves produce a fish like lure to attract predatory fish. The fish try to feed on the lure and the bivalve ejects glochidia onto the fish. After several weeks the glochidium starts developing adult structures. A mall clam falls from its host. It takes up its filter-feeding lifestyle. The glochidium is a dis Dersal stage for sedentary animal. It is harmless to the fish.
Bivalves live in nearly all aquatic habitats. They may completely or partially bury themselves in sand or mud. They may be attached to solid substrates. Or they may bore into submerged wood, coral, or limestone.
1. Boring bivalves: The margins of mantle of burrowing bivalves are fused to form an opening in the mantle cavity. This opening is called siphons. Water enters through this siphon and washes the mantle cavity during burrowing. Therefore. the sediments do not accumulate in the mantle cavity.
Boring bivalves lie beneath the surface of limestone, clay, coral, wood, and other substrates. The larvae settle to the substrate and boring starts. The anterior margin of their valves mechanically removes the substrate. Acidic secretions dissolve limestone. The most recently bored portions of the burrow are larger in diameter than portions bored earlier. Therefore as the bivalve grows, it is often entangles in its rocky burrow.
2. Surface-dwelling bivalves: They are attached to the substrate by proteinaceous strands called byssal threads. It is secreted by a gland in the foot. It is found in common marine mussel. (Jr it nun form cementation to the substrate like oysters.