ZOOLOGY: A STUDY GUIDE - E. O. Nevedomska - 2013

LECTURE 11. PHYLUM MOLLUSCA (Molluscs)

3. Class Bivalvia

The world fauna includes about 20,000 species of bivalves. In the freshwater bodies of Ukraine, there are about 150 species, while the Black and Azov Seas are home to over 100 species. The largest marine bivalve is the giant clam (Tridacna), which can weigh up to 300 kilograms. It inhabits coral reefs in the Indian and Pacific Oceans. The Valves of Tridacna reach 1.4 m in length. The Muscles that close the valves possess immense strength; opening the valves of a live mollusc requires a crowbar. Tridacnas are dangerous to divers and especially to pearl divers. A person who accidentally puts a hand or FOOT between the valves can lose their life. Bivalves inhabit the hydrosphere from the littoral zone down to oceanic trenches (10.8 km). Their biomass is particularly large in shallow coastal waters. They are entirely absent on land. Their sizes range from 2–3 mm to 1.5 m.

In flowing Water bodies with sandy shoals, the freshwater pearl mussel (Unio pictorum) can be found (ranging from 9 to 14.5 cm in length and up to 4 cm in height). It features thick-walled valves with a well-developed nacreous (mother-of-pearl) layer, which close by means of a hinge mechanism.

In standing water bodies, the swan mussel or duck mussel (Anodonta cygnea) is widespread (reaching up to 20 cm in length). Its shell is broader and more rounded than that of the pearl mussel, thick-walled, and lacks a hinge. These animals move very slowly: 1–1.5 m per hour. In autumn, they burrow into the bottom sediment to overwinter. The lifespan of these Molluscs averages 15 years.

Among freshwater bivalves, the quagga/zebra mussel (Dreissena) holds a special place (Fig. 33). This is a small mollusc (3.5–4 cm long) with a rather

characteristic shell shape. Adult Dreissena have a poorly developed foot, but well-developed byssal glands. Using byssal threads, the mollusc attaches itself to various underwater objects, much like marine bivalves (in swan mussels and pearl mussels, byssus threads are secreted only during the larval stage to attach the glochidium larva to a fish's body). By encrusting the underwater parts of hydraulic structures, Dreissena populations cause significant damage to hydroelectric power plants and water supply systems.

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Fig. 33. Dreissena (after Dogel, 1981).

The Development of offspring in Dreissena also differs: microscopic, free-swimming larvae hatch from the eggs (Fig. 34), bearing a strong resemblance to a trochophore—the larva of marine Annelids (marine bivalve larvae look very similar).

Fig. 34. Trochophore larva of Dreissena (after Dogel, 1981) (highly magnified).

Mussels and oysters, which are edible, are widespread in the Azov and Black Seas. Mussels are long-livers among bivalves, with certain species living up to 100 years. The mussel shell (up to 20 cm long) is wedge-oval, smooth, and dark purple, brown, or yellowish-green in color. The inner surface of the shell is nacreous. These animals lead a sessile lifestyle, attaching to the substrate via byssal threads. They occur at depths ranging from 9 to 78 m. The edible blue mussel (Mytilus edulis) can withstand significant fluctuations in water salinity and brackish conditions down to 0.3‰.

Bivalve molluscs exhibit bilateral Symmetry. Their shell consists of two valves connected by an elastic ligament, and in some species, by a hinge (Teeth and sockets on the valves). The valves close through the contraction of adductor muscles and open via the contraction of abductor muscles (elastic ligament). The anterior edge of the shell is blunt, while the posterior edge is pointed. A muscular foot, which serves as The Organ of locomotion, protrudes from the lower side between the opened valves (Fig. 35). Many molluscs use their foot to burrow into mud or sand. In many marine representatives of this class, the foot contains a byssal gland that secretes a substance called byssus (from Greek *byssus* — fine flax), which the mollusc uses to anchor itself to underwater objects. In Dreissena, the foot is poorly developed, and it relies on byssal threads for substrate attachment. Marine bivalves, such as sea mussels, attach in the same manner.

Fig. 35. Anatomy of the swan mussel (after Dogel, 1981):

1 — muscular foot; 2 — anterior adductor Muscle; 3 — posterior adductor muscle;

4 — excurrent siphon; 5 — incurrent siphon; 6 — gills; 7 — mantle fold.

The mantle cavity communicates with the external environment via two siphons (from Greek *siphon* — tube):

1) the incurrent (or branchial) siphon, through which water enters;

2) the excurrent (or cloacally associated) siphon, through which water exits.

Water flow is maintained by the beating of epithelial cilia covering the mantle, gills, and siphons.

The body consists of two main regions: the visceral mass (trunk) and the foot. The HEAD is reduced. Due to their sluggish lifestyle, Sense Organs are poorly developed. Osphradia are located near the Base of the gills, and statocysts are found in the foot. In some species, eyes are situated along the edge of the mantle.

The Mouth is located at the anterior end of the body, just above the base of the foot. Flanking the mouth are labial palps, whose cilia direct food particles toward the oral aperture. Salivary Glands are absent. Bivalves are passive filter-feeders, consuming primarily planktonic organisms, Bacteria suspended in the water entering the mantle cavity, and detritus. Detritus (from Latin *detritus* — worn down) consists of fine organic particles (remains of dead animals, plants, and Fungi along with bacteria) that have settled to the bottom of a water body or remain suspended in the water Column.

They breathe using gills located in the mantle cavity on both sides of the foot. For Respiration (as well as feeding), bivalves pump a large volume of water through their bodies, which is drawn into the mantle cavity via the inhalant siphon and filtered through the gills. Dissolved oxygen in the water enters the Blood Vessels of the gills, while suspended particles settle on their mucous membrane. Cilia on the gills drive the mucus with trapped particles forward toward the mouth. Microscopic plants and animals, along with organic debris that serve as food for the Mollusks, pass into the Digestive System, whereas inorganic matter is encased in mucus and expelled through the exhalant siphon.

The Heart of bivalves is three-chambered (consisting of 1 ventricle and 2 atria).

Most bivalves are dioecious, although hermaphrodites are also known (such as oysters). The ducts of the Testes and Ovaries open into the mantle cavity. Egg Fertilization is internal. In males, microscopic Gametes (spermatozoa) are discharged outward through the exhalant siphon. Some of these gametes are carried by water currents into the mantle cavity of the female, where eggs have already been laid between the gill lamellae. The eggs are fertilized, and larval development begins, after which the larvae are expelled outward through the exhalant siphon.

In most bivalve species, development proceeds with metamorphosis. The larva of marine forms—the veliger—resembles the trochophore of annelid worms. The veliger swims using cilia (the ciliary velum).

The larvae of freshwater mollusks have a somewhat different Structure. Upon leaving the female's mantle cavity, these larvae are covered by a bivalved shell with serrated edges. Larvae with these structural features are called glochidia (from Greek glochis — a barb or arrowhead) (Fig. 36).

Fig. 36. Glochidium of an swan mussel (after Dogel, 1981):

1 — byssus thread; 2 — hook; 3 — shell valve; 4 — adductor muscle; 5 — sensory hairs./em>

Glochidia possess a bivalved shell with a hook on the ventral edge of each valve, a strong adductor muscle, sensory hairs, and a long byssal thread. By clapping their valves, the larvae swim until they attach via an adhesive thread to the gills or Skin of a fish. Using the microscopic teeth on their shells, they penetrate the fish's integument and parasitize there for several weeks before dropping off and settling to the bottom to transform into adults. This temporary larval parasitism is an adaptation for long-distance dispersal via hosts.

Bivalve mollusks are a vital component of aquatic communities. They serve as a primary food source for various aquatic animals (cephalopods, starfish, sea urchins, fish, marine mammals). As biological filters, they purify bodies of water.

The Significance of bivalve mollusks in human life:

Positive impact:

1. Humans consume bivalves such as oysters, mussels, and scallops as food.

2. The shells of fossil mollusk species form a special type of limestone—coquina—which is used in construction.

3. Bivalve shells are used as souvenirs and for crafting mother-of-pearl items.

4. The well-known value of pearls as jewelry.

Negative impact:

1. Certain mollusks (such as Dreissena), by forming colonies on lock gates and pipelines, can clog them and spoil drinking water upon their death.

2. By attaching to the hulls of ships, certain mollusk species (such as Dreissena and shipworms) impair their hydrodynamic performance.

3. Among bivalves, There are also species (such as the freshwater pearl mussel) that serve as intermediate hosts for parasitic Flatworms.