INVERTEBRATE ZOOLOGY - H. I. Shcherbak - 2008

KINGDOM METAZOA

SUBKINGDOM EUMETAZOA

DIVISION TRIPLOBLASTICA, OR BILATERIA

SUBDIVISION SPIRALIA

PHYLUM MOLLUSCA

CLASS BIVALVIA

SUPERORDER AUTOBRANCHIA

This superorder comprises the majority of bivalves. They inhabit all seas and oceans, as well as freshwater bodies, occurring at various depths—from the intertidal zone to the deepest oceanic trenches.

Autobranchians are characterized by shells with a hinge whose Teeth radiate from the umbo, though secondarily they may shift position or disappear entirely. The gills are enlarged, filamentous, or lamellar, modified into a filter-feeding apparatus with ciliated Water-current epithelium. The FOOT is wedge-shaped, lacking a flat sole; juveniles always produce a byssus, and the foot is occasionally reduced.

Unlike protobranchs, which collect food particles from the substrate surface using appendages of the labial palps, the vast majority of autobranchians feed by filtering water and straining food particles from it.

This group of bivalves exhibits extreme diversity in size, shell shape, and lifestyle; it includes crawling, swimming, jumping, and sessile forms.

The superorder Autobranchia encompasses over 120 families, grouped into seven to nine orders (according to various authors).

The order Unionida includes large freshwater Mollusks with a well-developed nacreous shell layer and a hatchet-shaped foot. The mantle margins are unmerged, and the siphons are very short. Hinge teeth are typically divided into two groups: tubercle-like central teeth located near the umbo, and lamellar lateral teeth extending along the posterior-dorsal margin of the shell. Hinge teeth are sometimes entirely absent,

for instance, in swan mussels (Anodonta). Unionids brood their eggs within the gill cavities of the females. The larva, known as a glochidium (Fig. 218), features a bivalve shell with a sharp tooth on the ventral margin of each valve and a larval byssal gland. The mollusk expels glochidia through the excurrent siphon into the water when a fish swims past. The glochidium attaches to the fish's gills or fins using its byssus and marginal shell teeth, embeds in the Tissues, and leads a temporary parasitic lifestyle. Once developed into tiny mollusks, the juveniles leave the fish and drop to the bottom. Fish facilitate the dispersal of unionids, enabling them to colonize the upper reaches of rivers.

Among unionids, the freshwater pearl mussel family (Margaritiferidae) is widely known for its ability to produce pearls. They are distributed across Eurasia and North America. In northern Europe, the freshwater pearl mussel (Margaritifera margaritifera) is well known, with a shell length of up to 12 cm (Fig. 220). Due to the pollution of European rivers and overfishing, it has nearly vanished. In the rivers of Ukraine, the most prominent Representatives of the family Unionidae belong to the genera Anodonta (swan mussels), Unio (river mussels), and Crassiana. The most widespread species is the swan mussel (A. cygnea), reaching a length of 20 cm (Fig. 213).

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Fig. 220. European freshwater pearl mussel (Margaritifera margaritifera) with a pearl (from Zatsepin et al.):

1 - posterior end of the shell; 2 - external ligament; 3 - anterior end of the shell;

4 - pallial line; 5 — pearl

Marine autobranchians commonly include sedentary or byssally attached mollusks of the order Mytilida. Oysters (family Ostreidae) possess an uneven, ribbed shell with unequal Valves: the left valve is more convex and deeper, serving to attach the mollusk to the substrate while mirroring its contours and irregularities. Adults lack a foot, although juveniles possess one (Fig. 221). Shell length typically ranges from 8-12 cm, though the Pacific giant oyster (Crassostrea gigas) can reach up to 38 cm. Around 50 species of oysters are known; almost all are warm-water forms inhabiting rocky bottoms at depths from 1 to 70 m, occurring either solitarily or in dense clusters that form coastal beds and reefs where population density is so high that individuals fuse together. Oysters are considered a prized delicacy. One of the main commercial species, Ostrea edulis, inhabits European coastal waters, including the Mediterranean and Black Seas. Mussels (family Mytilidae) feature a smooth shell with the umbo strongly shifted toward the narrowed anterior margin, alongside a well-developed byssal gland. Shell length can reach 10 cm. They inhabit the intertidal zone down to depths of 80 m, forming dense beds. The best-known and most widespread species is the blue mussel (Mytilus edulis), which ranges along the Atlantic coasts of Europe and North America, the Barents, White, and Baltic Seas, as well as Far Eastern seas (Fig. 217). A closely related species, the Black Sea mussel (M. galloprovincialis), lives in the Black Sea.

Fig. 221. Mytilids (from Zatsepin et al.): a - Black Sea oyster (Ostrea taurica); b - pearl oyster (Pinctada margaritifera):

1 - ligament; 2 - lower (left) valve cemented to the substrate; 3 - upper (right) valve encrusted with acorn barnacles

A distinct group is formed by marine pearl oysters (suborder Pteriina); they possess a foot with a well-developed byssal gland and a robust nacreous shell layer. These mollusks are capable of producing quite large pearls. The finest and most valuable pearls are produced by species of the genera Pinctada and Pteria (Fig. 221). These are precisely the ones referred to as true pearl oysters. The largest pearl oyster, Pinctada margaritifera, reaches a shell diameter of 30 cm and a mass of 10 kg, although such large specimens are rare.

Among the order Pectinida, the best-known are the scallops (family Pectinidae), which inhabit almost all seas and oceans at various depths, including the deepest trenches. They feature a rounded shell with a straight hinge line that extends at the front and back into ear-like projections called auricles. The upper (left) valve is flatter, while the lower (right) one is more convex. The foot in scallops is poorly developed and rudimentary. Juveniles attach to the substrate by a byssus, whereas adults lose this ability. Scallops are the most active of all bivalves. They can swim and dart about by periodically clapping their valves and expelling water from beneath them (Fig. 222). The mantle edge of a scallop is fringed with numerous tentacles (tactile Organs) and eyes that help orient the animal during swimming. Many marine scallops are commercially important species. The northern seas of Europe and America are inhabited by the Iceland scallop (Chlamys islandicus), a fairly large species (shell diameter up to 8 cm); the Far Eastern seas host the Yesso scallop (Patinopecten yessoensis); and the Black Sea is home to the Black Sea scallop (Flexopecten ponticus).

Fig. 222. Pectinids (from Zatsepin et al.):

a - Yesso scallop (Patinopecten yessoensis), exterior of the right valve; b - successive swimming movements of a scallop:

1 - posterior margin of the shell; 2 - posterior auricle; 3 - umbo; 4 - anterior auricle; 5, 6 - anterior and ventral margins of the shell, respectively

The order Lucinida is extremely diverse and species-rich, comprising about 30 families whose representatives inhabit both marine and freshwater bodies. Among the marine forms, the most notable are members of the family Astartidae, which are widespread in the seas of the Northern Hemisphere and constitute a common component of benthic biocenoses. Small (up to 5 cm) yet striking species of the family Donacidae are distinguished by their brightly colored shells, earning them the moniker "marine butterflies."

This order also includes freshwater mollusks of the families Pisidiidae and Sphaeriidae. These are tiny bivalves (with the smallest species measuring up to 2 mm in length), the largest of which belong to the genus Sphaerium; for instance, the river pea clam (S. rivicola) (Fig. 223) reaches 2.5 cm. Pea clams are ovoviviparous: their eggs develop within brood chambers located on the inner hemibranchs, and fully formed juveniles emerge from the chambers. Some species of the genus Pisidium have mastered a habitat type unusual for bivalves: they live in waterlogged soils, where the negligible amount of moisture trapped between soil particles is sufficient for their Respiration and feeding.

Fig. 223. Lucinids (from Zatsepin et al.):

a — pea clam (Pisidium amnicum); b - river pea clam (Sphaerium rivicola)

The order Venerida is the largest group of bivalves, uniting about 40 families. Venerids inhabit both marine and freshwater environments. The most popular among them are giant clams (tridacnas), the largest of all bivalves. The giant clam (Tridacna gigas) reaches lengths of nearly 1.5 m and a mass of 250 kg, the bulk of which is accounted for by its heavy shell. Tridacnas are inhabitants of tropical shallows, found in the Indian and Pacific Oceans within coral reef zones. They rest motionlessly on the substrate with their dorsal side down and open valves pointing upward (Fig. 224). Their thickened mantle edges harbor countless symbiotic unicellular flagellates known as zooxanthellae.

Fig. 224. Venerids (from Zatsepin et al., modified):

a - boring giant clam (Tridacna crocea) (with the body oriented as it sits in the substrate — ventral side up); b - common edible cockle (Cerastoderma edule); c - Heart cockle (Glossus humanus); d - a cluster of freshwater zebra mussels (Dreissena polymorpha) bound together by byssal threads

Heart cockles constitute a major group of warm-water marine bivalves (Fig. 224). Lamarck's cockle (Cerastoderma lamarckii) and several other species are widespread in the Black Sea.

Razor clams are also found in the Black Sea, notably the sword razor clam (Solen Vagina), which possesses an elongated, nearly rectangular shell up to 12 cm long. They inhabit depths of up to 10 m and can burrow into the substrate very rapidly while extending a pair of siphons to the surface; they are also capable of jet-propulsion swimming by expelling a stream of water from their siphons.

Freshwater bodies feature an Abundance of sessile mollusks of the family Dreissenidae, which attach to substrates using a byssus. European waters are inhabited by the zebra mussel (Dreissena polymorpha), which has a triangular shell 4–5 cm long (Fig. 224) and penetrates into the estuaries of the Black Sea as well as the desalinated areas of the Sea of Azov. The southern part of Ukraine is also home to the bug mussel (D. bugensis), which is currently actively expanding its range. Dreissenids form massive aggregations, carpeting stones, pilings, various hydraulic structures, as well as water intakes, turbine supply pipes, protective gratings, and the like. Cleaning these structures incurs substantial financial costs.

The order Venerida also includes two families whose members have adapted to life inside burrows they bore into rocks, limestone, and wood. Members of the family Pholadidae drill into hard rocks using their shells, which feature movable valves armed with ribbed, spiky projections. The common piddock (Pholas dactylus) is widespread in the Black Sea. Woodborers (family Teredinidae) are specialized for boring into wood. The genera Bankia and Teredo possess elongated, worm-like bodies, earning them the common name "shipworms." At the anterior end of the body sits a very small shell (1/30 to 1/40 of the total body length) armed with sharp ridges, while the posterior end bears long siphons (Fig. 225). Woodborers damage timber, posing a severe threat to wooden piers and ships. Three species are known in the Black Sea: Teredo navalis, T. pedicellatus, and T. utriculus; the first two have also invaded the Sea of Azov.

Fig. 225. Shipworm Teredo (from Zatsepin et al.):

a, b - adult mollusk inside its bored burrow; c - veliger larva; d - young stage:

1 - burrows drilled by Teredo; 2 - body of the mollusk; 3 - shell; 4 - siphons; 5 - calcareous lining of the burrows;

6 - foot; 7 - pallets; 8 - velum