Bivalves are a group of freshwater and marine mollusks with bilaterally symmetric and laterally compressed bodies encased in a characteristic two-part shell. They belong to the class Bivalvia, a term coined by Linnaeus (1758) from two Latin words, bis (two) and valvae (leaves of a door). These mollusks also lack heads and even the characteristic molluskan organs, such as radula and odontophore.
Around 9,200 species exist, including clams, oysters, cockles, mussels, and scallops. Most of these mollusks reside in sediments, while others lie on the sea floor, attaching themselves to rocks and other substrates. The majority are filter feeders and possess ctenidia, a gill modified for underwater feeding.
The giant clam (Tridacna gigas) measures more than 1,200 mm (47 inches) in length and weighs about 200 kg (441 lbs), while Condylonucula maya, the smallest living mollusk, barely grows to a length of 500 μm (0.020 inches).
The largest extinct bivalves of the genus Platyceramus measured around 3,000 mm (118 inches) in length.
These mollusks have a bilaterally symmetrical and laterally flattened body covered by a hard, calcareous shell. The head is vestigial, and the foot is characteristically blade-shaped.
The shell consists of two valves made from various forms of calcium carbonate, such as calcite or a combination of calcite and aragonite. The outermost layer of the shell, the periostracum (secreted by the outer mantle), is composed of a set of complex proteins called conchiolins. In bivalves of the order Pteriida, the aragonite often forms an extra inner layer known as the nacreous.
While the foot and the byssus (filamentous bundle for attachment) are located at the anterior end of the shell, the posterior end often contains siphons (tube-like structures allowing water entry). The dorsal part contains an umbo (the highest part of the shell valve) and a beak (basal projection of the shell), joined by the hinge point.
The outer surface of these shells is often intricately sculpted into concentric rings (helpful in age determination), radial ribs, or lattice-like patterns. In some groups, such as shipworms, the shell valves are considerably reduced and are restricted to only the anterior portion of the body.
A thin membrane called the mantle covers the bodies of all mollusks and is responsible for secreting the shell valves. The mantle has a raised crest that secretes the ligaments of the hinge, the filamentous byssus threads, and the teeth. The posterior end of the mantle is often elongated into siphons that retract into a cavity called the pallial sinus.
The anterior and posterior adductor muscles control the opening and closing of the shell, while a set of retractor muscles forms a pallial line connecting the mantle to the shell’s edge. Oysters and scallops, which are mostly sedentary, lack the anterior adductor muscle, while the posterior muscle is located centrally.
The hinge ligament, an extension of the periostracum, connects the two valves and acts antagonistically to the adductor muscles, while a pair of pedal protractors and retractor muscles control the bivalve foot.
They have an open circulatory system comprising a three-chambered heart that pumps oxygenated blood (hemolymph) to the different organs. While the auricles receive blood from the gills, the ventricle distributes blood to the entire body through the aorta. Most bivalves possess two aortae, the primary and the secondary, with the latter usually pumping blood to only the hind parts of the body.
Almost all bivalves lack blood pigment, but some, like the genus Poromya, possess hemoglobin in their amoebocytes.
Bivalves possess paired gills of hollow, tubular, thin-walled filaments to facilitate gas exchange. Since these mollusks are primarily inactive, their respiratory demands are low, and thus, some freshwater species slightly gape their shells to allow the air to enter.
Their digestive tract includes an esophagus, stomach, and intestine. In primitive filter-feeding bivalves, a ciliated sac associated with the stomach releases a rod of solidified mucus called the ‘crystalline style.’ The sac’s cilia rotate the style and wrap the food in mucus, thoroughly churning it. This propulsion pushes the food particles to the rear end of the stomach, distributing smaller particles into the stomach while the heavier ones move to the intestine.
In carnivorous bivalves, the crystalline styles are considerably reduced, and their stomachs are thick and muscular with extensive cuticular lining. Waste materials are then consolidated in the rectum and expelled as pellets through an anal pore.
Body wastes are excreted through a pair of nephridia, homologous to the vertebrate kidney. Each nephridium comprises a long, looped glandular tube draining into the pericardium and a urinary bladder to store urine. Attached to the pericardium (or sometimes to auricles) are pericardial glands that serve as an extra excretory organ.
After the bladders become full, the wastes are expelled through an opening in the upper part of the mantle cavity called the nephridiopore.
Bivalves have a simple nervous system comprising a nerve network and a series of paired ganglia instead of a brain. Generally, two cerebropleural ganglia are found on either side of the esophagus. While the cerebral ganglia control the sensory organs, the pleural ganglia innervate the mantle cavity. The pedal ganglia at the base of the foot controls movement, while the visceral ganglia are located under the posterior adductor muscle. Some bivalves with long siphons may also possess extra sets of siphonal ganglia. Together, the different ganglia are interconnected with extensive nerve fibers.
They possess mechanoreceptors or chemoreceptors either on the posterior margins of the mantle or on short tentacles extending out of their bodies. For example, members of the order Anomalodesmata possess vibration-sensitive receptors on their tentacles that detect the slightest movements in the surroundings. These mollusks also have a patch of sensory cells called osphradium (below the posterior adductor muscle) that measures the turbidity of water.
While eyes are absent in many bivalves, members of the families Arcoidea, Limopsoidea, Mytiloidea, Anomioidea, Ostreoidea, and Limoidea possess simple eyes (ocelli) comprising a photosensory pit and a lens. Some groups, such as scallops (family Pectinidae), have more complex eyes with a concave mirror followed by a two-layer retina.
As of 2010, around 9,200 species of bivalves are categorized under four subclasses, 46 superfamilies, 1260 genera, and 106 families. The largest marine families are the Veneridae (over 680 species) and the Tellinidae and Lucinidae (with over 500 species each).
Bivalves are abundant in the tropics and temperate regions. Some species, such as the Antarctic scallop (Adamussium colbecki), inhabit the Antarctic, whereas around 140 species, including Mya truncata, inhabit Arctic waters.
Most bivalves filter-feed on phytoplankton, bacteria, and algae on the sediment’s surface. However, some, like the granular poromya, are carnivorous, feeding on small crustaceans and even dead carrion.
Those in the genus Entovalva, being endosymbionts, passively consume the food of the sea cucumbers they inhabit.
Bivalves usually adopt a sedentary or sessile lifestyle, sometimes spending their entire life in the area where they initially settled as juveniles.
Bivalves employ their gills or ctenidia to filter phytoplanktons suspended in water (filter-feeding). However, primitive forms, like protobranchs, attach to the substrate with a pair of tentacles (each having a flap or palp) and scrape detritus. Since the tentacles are covered in mucous, the food is easily trapped and passed on to the palps. After sorting, the small, consumable particles are taken in while others are discarded.
In advanced bivalves, the water is drawn into the shell through either the ventral surface of the body or the retractable inhalant siphons.
Bivalves are expert burrowers, extending their muscular foot anteriorly through the parted shell valves for digging. On reaching the substrate, they dilate the tip of the foot to take a grip while the pedal retractor muscles pull the shell downward. The valves close simultaneously, forcing the water out of the mantle cavity, which loosens the surrounding sediments and facilitates movement. They repeat this series of actions to dig deeper into the sediment.
Freshwater clams of the family Sphaeriidae, like the European fingernail clam (Sphaerium corneum), use their long and flexible foot to climb on water weeds found on the edges of water bodies.
Although they exist in separate sexes (males or females), some species, like giant clams, are also hermaphroditic, with sperm-producing tubules and egg-producing follicles intermingled in the gonads.
Typically, the eggs and sperm are released into the water column (spawning). This process may occur continually or be influenced by day length, water temperature, or the presence of sperm in the water. While many bivalves are mass spawners, releasing their gametes in batches, some are dribble spawners that continually release them for an extended period (often weeks).
In most marine bivalves, the eggs are fertilized externally, following which they hatch into trochophore larvae. Eventually, these larvae feed on diatoms and other phytoplanktons in water, develop into veliger larvae, and settle on the seabed. They then metamorphose in adults.
However, some species, such as those in the genus Lasaea, undergo internal fertilization. In this process, females draw sperm inside their bodies from water using their siphons. The young develop in the mantle cavity and are released into the water column as veligers.
In these bivalves, fertilization is completely internal, with the sperm drawn into the female’s gills. Following fertilization, the eggs hatch into glochidia larvae that reside and grow in the female’s shell. When these larvae are released, they attach themselves parasitically to the gills or fins of a large host fish. After a few weeks, they drop on a substrate and metamorphose into adults.
Pocketbook mussels (members of the family Unionidae) follow a unique reproductive strategy. The females protrude from the shell and develop fish-like markings and false eyes to imitate a small fish. As it moves around, real fish are attracted to the female and start approaching from a distance. When the fish come too near, the female releases multiple glochidia larvae on their bodies. These larvae attach to the gills, encyst themselves, and continue digesting the fish tissue until they are ready to drop into the stream bed as juveniles.
Although brachiopods possess bivalved shells like bivalves, the two groups have different ancestral origins. Their superficial similarity results solely from occupying similar ecological niches (convergent evolution).
Moreover, there are other observable differences in the shells of the two groups. For example, in brachiopods, the two valves found on the dorsal and ventral surfaces of the body are composed of calcium phosphate, whereas in bivalves, they are purely calcareous (calcium carbonate) and found on the right and left sides. Brachiopods also have a spiral cartilaginous shell structure called the lophophore for feeding, which bivalves lack.
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