Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Metazoa
Mollusks (Mollusca)
Class Bivalvia (Lamellibranchia)

The Class Lamellibranchia comprises approximately 20,000 species of slow-moving or completely sessile Mollusks. The vast majority of them inhabit marine environments, though some have adapted to life in freshwater bodies, such as freshwater pearl mussels, swan mussels, fingernail clams, and pea clams. In bivalve mollusks, the body is enclosed in a shell consisting of two typically symmetrical Valves (hence the name bivalves), which are joined on the dorsal side by an Cytology/practical/47.html">Elastic ligament of organic origin. Beneath the shell lies the mantle, consisting of two cutaneous folds that hang down from the DORSAL SIDE OF the mollusk and envelop its elongated, bilaterally symmetrical body, which is usually laterally compressed (Fig. 39). The body of bivalves consists of a trunk and a FOOT; the HEAD is reduced.

Fig. 39 Internal Anatomy of the swan mussel:

1 — mantle; 2 — foot; 3 — branchial siphon; 4 — cloacal siphon; 5 — adductor Muscles; 6 — labial palps; 7 — Mouth opening; 8 — Stomach; 9 — digestive gland (Liver); 10 — intestine; 11 — anus; 12 — Heart; 13 — pericardial cavity; 14 — Kidney; 15 — Keber's organ; 16 — cerebral ganglion; 17 — pedal ganglion; 18 — visceral ganglion; 19 — gills; 20 — mantle cavity.

The valves of the shell are typically convex. The most prominent parts lie near the dorsal margin of the shell and are referred to as the umbones (beaks) of the valves.

The umbones represent the oldest, initial PARTS OF THE valves, to which new annual growth layers are added along the margin, running parallel to the free edge of the shell and clearly visible as rings. By counting these rings, the age of the mollusk can be determined. The inner nacreous (mother-of-pearl) layer is well developed in many species, such as freshwater pearl mussels, marine pearl oysters, and freshwater pearl oysters. In some bivalves, pearls may form between the shell and the mantle. They arise when sand grains entering beneath the shell irritate the outer surface of the mantle, which begins to abundantly secrete a nacreous layer around the foreign object. Pearls are harvested from marine pearl oysters of the genera Pteria and Pinctada. Traditional antique Russian pearls were extracted from the European freshwater pearl mussel, which still inhabits northern rivers and lakes today.

Due to the elasticity of the ligament, the valves open. They close upon the contraction of the adductor muscles, of which there are typically two—anterior and posterior. The adductor muscles are powerful bundles passing through the body with their ends attached to the shell valves. In some lamellibranches, such as the freshwater pearl mussel, a hinge takes part in joining the shell valves. It consists of tooth-like projections and corresponding depressions on the dorsal margin of the valves. The hinge prevents the valves from shifting relative to each other. The mantle, hanging down along the sides of the trunk as two folds, encloses a spacious mantle cavity that communicates with the external environment via two siphons.

The siphons are formed by the incomplete fusion of the mantle fold margins at the posterior end of the body. The lower siphon is termed the branchial, or inhalant, siphon, through which Water enters the mantle cavity; the upper one is the cloacal, or exhalant, siphon, through which water exits. Located within the mantle cavity of bivalves are the foot, gills, Organs of chemical sense, labial palps, and openings (mouth, excretory, anal, and the outlet ducts of the Gonads). The foot is a ventral muscular region of the body, often wedge-shaped, capable of protruding between the shell valves. Using the foot, the mollusks burrow into the substrate or move very slowly. For instance, the movement speed of a swan mussel along the bottom does not exceed 30 cm/hour.

The respiratory organs of bivalves are represented by two ctenidia, or dermal gills, each of which hangs from the top of the mantle cavity on either side of the foot. In true lamellibranches (such as the freshwater pearl mussel and swan mussel), the gill consists of two lamellae. Water passing through the mantle cavity supplies oxygen to the gills and removes gaseous Metabolic waste products.

The mouth opening in lamellibranch mollusks is situated between the anterior adductor Muscle and the foot. Labial palps are typically located on either side of the mouth. They are covered with cilia that direct food particles toward the mouth opening. After passing through the mouth, the food bolus enters the Esophagus and proceeds through The Stomach, midgut, and hindgut. In many bivalves, the hindgut passes directly through the ventricle of The Heart and opens via the anus into the mantle cavity just above the posterior adductor muscle. Bivalves are classified as biofilters capable of Processing tens of liters of water daily, extracting fine food particles and thereby purifying aquatic environments of suspended organic matter.

The heart of lamellibranches is located on the dorsal side within a thin-walled pericardial sac. Two aortas—anterior and posterior—originate from the heart and branch into a series of Arteries that open into the spaces between organs.

The excretory organs are represented by two Kidneys. Each begins with an opening in the pericardial cavity and terminates with an excretory pore leading into the mantle cavity. The Nervous system is of a typical dispersed-ganglionic type. Sense Organs are represented by tactile Cells, which are abundant along the mantle margin and labial palps. Organs of chemical sense and equilibrium (statocysts) are also present. In some bivalves, simple photoreceptors (eyes) sensitive to light are located on various parts of the body (mantle, siphons, gill filaments, etc.).

Fig. 40 Glochidium larva of the swan mussel:

1 — larval shell; 2 — adductor muscle; 3 — larval gland; 4 — bundles of sensory setae; 5 — denticles with micro-Teeth.

The majority of bivalves are gonochoric (dioecious), though hermaphroditic forms also occur. Paired gonads, both male and female, are located within the parenchyma of the foot. Their ducts open into the mantle cavity. Fertilization of the eggs takes place in the water. Development typically involves several larval stages. In freshwater bivalves (such as freshwater pearl mussels and swan mussels), the eggs develop on the gills of adult mollusks, where they hatch into larvae known as glochidia (Fig. 40). These appear in autumn and spend the winter on the gills of the maternal Organism. In spring, the glochidia leave the mother's body and attach to the gills of fish, where they parasitize, deriving nourishment from the host. By the end of summer, the young mollusks drop off the fish gills, settle on the bottom, and transition to a free-living existence.

Swan mussels, freshwater pearl mussels, fingernail clams, and pea clams are widespread in the freshwater bodies of our country. Swan mussels are frequently found in shallow waters of slow-flowing rivers, lakes, and ponds. They derive their generic name (Anodonta, meaning "toothless") from the absence of a hinge. Their large, greenish-brown, elongated-oval shells are usually half-embedded in the substrate. Freshwater pearl mussels inhabit the sandy bottoms of rivers and lakes. They differ from swan mussels in having a more elongated and thick-walled shell, the valves of which possess hinge teeth on the dorsal side.

The shells of freshwater pearl mussels are utilized for manufacturing buttons and other items, and when pulverized, they are also used as a feed Supplement for livestock.

Fig. 41 Some commercially important edible mollusks: A — oysters; B — mussel.

Fingernail clams and pea clams also inhabit the sandy bottoms of rivers and lakes; their shell valves can be found in almost any heap of sand. Fingernail clams possess a brownish-yellow, nearly round shell approximately 1 cm in diameter, whereas the shells of pea clams are half that size.

Many species of marine bivalves are consumed as food (oysters, mussels, cockles, scallops) (Fig. 41). Their flesh possesses high nutritional value and contains significant amounts of Vitamins. In our country, commercial mollusk fisheries are well established in the Black Sea. In recent decades, the harvesting of bivalves has also been increasing in the seas of the Far East.