INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 3 - H.Y. Shcherbak - 1997

PHYLUM MOLLUSCA

CLASS GASTROPODA

SUBCLASS PROSOBRANCHIA

Prosobranchs constitute the largest and most diverse group of gastropods. They include marine and freshwater forms, as well as some species that have transitioned to terrestrial life in very humid habitats. Most species possess a well-developed, spirally coiled shell; in some, the shell is cap- or saucer-shaped. The mantle cavity is directed forward and houses true gills, or ctenidia. In certain groups, the ctenidia, Kidneys, and atria are paired, whereas in the majority, the Organs on the right side undergo reduction. A characteristic feature of all Prosobranchia is chiasmury (the crossing of the pleurovisceral connectives). The FOOT is typically well developed and, in most species, bears an operculum that tightly seals the shell aperture.

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The Classification of Prosobranchia is highly complex. The traditional division of this class into the orders Diotocardia (or Archaeogastropoda) and Monotocardia, along with other systematic variants, is artificial. Modern analyses of these groups have demonstrated that they must be subdivided into A large number (approximately 30) of orders. The Development of a new taxonomic system is still ongoing; therefore, one of the traditional schemes is presented below (see diagram).

Order Archaeogastropoda (or Diotocardia). These are the most primitive gastropod Molluscs, possessing two or, more commonly, a single ctenidium, two kidneys with a pair of excretory pores, and two atria. Instead of pedal ganglia, the foot contains pedal nerve cords interconnected by commissures.

In Representatives of the families Pleurotomariidae, Haliotidae, and Fissurellidae, the ctenidia are paired. The shells of these molluscs feature one or more apertures through which Water exits the mantle cavity. The mantle organ complex is symmetrical only in members of the family Pleurotomariidae (Fig. 74, a). Species of the genus Pleurotomaria inhabit the Indian Ocean, with particularly rich populations found off the coast of Japan. Their turbiniform shell is relatively large (about 20 cm in length), bearing a long slit along the final whorl. Only a few species of this genus have survived to the present day, whereas hundreds of fossil species are known.

Fig. 74. Order Archaeogastropoda:

a — Pleurotomaria; b — Fissurella; c — Haliotis; d — Patella

Representatives of the family Haliotidae possess an asymmetrical mantle complex, in which the right ctenidium is smaller than the left. Haliotis, which inhabits the littoral and sublittoral zones of tropical seas, features a flattened, ear-shaped shell perforated by a row of small apertures (Fig. 74, c). By means of a powerful foot with a broad sole, these molluscs suction themselves to rock surfaces so firmly that they withstand The impact of the strongest waves. The Base of the foot is surrounded by a fold of Skin known as the epipodium, the margin of which bears numerous tentacle-like appendages that function as organs of Touch. The shells of abalones possess a well-developed, iridescent nacreous layer, making them valuable for inlay work, jewelry, and button-making. Extremely rare and highly prized pearls of unusual greenish-blue hues are occasionally found within their shells. Haliotis gigantea, with a shell measuring 10–20 cm, is found off the coast of Japan, while H. tuberculata, of somewhat smaller size, occurs along the Mediterranean coast of Europe.

Paired ctenidia and two atria are characteristic of numerous species within the family Fissurellidae. Their shell lacks a nacreous layer and takes the form of a low, ribbed cone with an apical aperture (Fig. 74, b). Members of this family are distributed in the seas of both hemispheres. In the Black Sea, their presence is restricted to the Bosporus region, where a single species, Diodora graeca, occurs.

Externally similar to the preceding family are the true limpets, which belong to three closely related families: Patellidae, Tecturidae, and Lepetidae. They possess a cap-shaped shell without apertures (Fig. 74, d). An important anatomical diagnostic feature of limpets is the presence of a single atrium rather than two, a condition associated with the modification of their respiratory organs. In members of the Patellidae, both ctenidia are reduced, and secondary (adaptive) gills develop on the ventral surface of the mantle instead. Water washes over the gills and exits the mantle cavity through the aperture. The kidneys are paired, though the right one is larger than the left.

Patellids are widespread across various marine environments, including brackish inland seas. Patella tarentina, common in the Black Sea, is a typical inhabitant of the marine surf zone and has also successfully colonized the Sea of Azov. These molluscs inhabit coastal rocks, adhering tightly to them with their muscular foot; occasionally, they crawl above the water line. Patellids feed on Algae, scraping them from rock surfaces using a radula equipped with exceptionally durable Teeth. It has been established that these teeth incorporate iron in their composition. Because the teeth wear down rapidly, the radular sheath, where new teeth are formed, is extraordinarily long and spirally coiled. Patellids are edible.

Numerous species of the family Trochidae possess only a single (left) ctenidium, yet retain two atria and paired kidneys. The shell is spirally coiled into a conical, spire-like shape, sometimes ornamented with projections. Shells of species belonging to the genus Trochus, which inhabit shallow waters in the Indian and Pacific Oceans, yield high-quality nacre used for manufacturing buttons and ornaments. Several small species of the genus Gibbula, lacking commercial significance, inhabit the Black Sea.

Among prosobranchs with two atria but a single ctenidium, certain forms have transitioned to life in freshwater habitats. These belong to the family Neritidae. While the majority of neritid species are marine, some, such as Theodoxus pallasi, inhabit brackish seas (the Azov, Aral, and Caspian seas), whereas T. fluviatilis populates rivers (such as the Dnieper and Southern Bug) and lakes. These small molluscs typically reside near the water's surface on coastal stones and submerged plant stems. They are frequently found even above the water line, enduring high heat. The shell aperture is tightly sealed by an operculum.

Order Mesogastropoda. Representatives of this order possess organs solely on the left side: a single (left) ctenidium, one atrium, and one Kidney. In certain forms—particularly those adapted to freshwater or terrestrial environments—the ctenidium is either entirely absent or replaced by secondary gills. Pedal ganglia are well developed within the foot, which also bears an operculum.

This order encompasses both marine and freshwater forms. Certain species have adapted to life in exceptionally humid terrestrial environments, predominantly inhabiting tropical and subtropical forests.

Typical inhabitants of the marine intertidal zone include members of the family Littorinidae. One species, Littorina littorea (Fig. 75, a), ranks among the most abundant molluscs of the Northern Hemisphere. Periwinkles inhabit algae, pilings, and rocks; during low tide, they become exposed above the water line. Under these conditions, they retract into their shell and seal the aperture with the operculum. In Eastern European countries, L. littorea is consumed as food, its widespread consumption driven by its Abundance and low cost.

Fig. 75. Order Mesogastropoda:

a — Littorina littorea; b — Cryptonatica; c — Cypraea moneta

Representatives of the family Naticidae are widespread in the northern European seas and the seas of the Far East, with the genus Cryptonatica being particularly common. They feature a thick, smooth, globular shell characterized by a callus on the umbilicus (Fig. 75, b). These molluscs are predators that feed primarily on bivalves, excavating them from the sediment. The HEAD of naticids is prolonged into a proboscis. A network of water channels penetrates the tissue of the foot, opening externally via numerous tiny pores. Once water is drawn through these pores, they close; subsequent muscular contraction forces water into the channel cavities, causing the foot to swell and envelop the prey. The saliva of cryptonatics contains an acid that dissolves the calcium carbonate of the prey's shell. Through this softened area, the predator bores a circular hole using its radula, inserts its proboscis, and consumes the soft Tissues of the prey.

In tropical seas at shallow depths, numerous species of small yet exceptionally beautiful Mollusks of the cowrie family (Cypraeidae) can be found; the best known among them are species of the genus Cypraea (Fig. 75, c). The inner part of their shells is paper-thin, whereas the outer part, forming the final whorl, is very thick. When the periostracum is removed from the shell, a magnificent porcelain layer is revealed beneath it, displaying various shades adorned with colorful spots. Cowrie shells have long been used as ornaments and even as currency (Cypraea moneta).

Among the large marine mollusks, representatives of the family Strombidae deserve mention. Species of the genus Strombus possess a massive shell; nevertheless, these snails are quite agile due to the specialized Structure of their foot. Propelling themselves on the anterior and posterior edges of the foot—the latter bearing a pointed horny operculum—conchs are capable of leaping. Furthermore, they use this pointed operculum as a weapon to inflict wounds on their prey, which earned them the name "devil's claw."

Species of the family Tonnidae are predators that hunt Echinoderms. The Salivary Glands of snails of the genus Tonna secrete a fluid containing 3–4% sulfuric acid, enabling them to dissolve even the armor of sea urchins. The large (up to 60 mm) predatory mollusk Charonia tritonis attacks the crown-of-thorns starfish (Acanthaster planci), which feeds on coral polyps. In the early 1970s, due to a decline in the population of triton shells, the ecosystem of the Great Barrier Reef was brought to the brink of collapse.

Among marine inhabitants, completely sessile mollusks belonging to several families can be found. Notable among them is the worm-snail family (Vermetidae). Their shells resemble the tubes of sessile polychaetes in shape. The upper part of the shell forms a typical turbospiral, while the remaining whorls appear uncoiled, giving the shell a worm-like appearance (Fig. 76). The round aperture is closed by an operculum. The crawling SOLE OF THE foot is reduced in these species. Vermetids feed primarily on detritus, drawing it in along with mucus secreted by the pedal gland. A typical representative of this family is Vermetus lumbricalis, which inhabits warm seas.

Fig. 76. Vermetus lumbricalis among coral and hydrozoan polyps

A distinct group is formed by the pelagic heteropods (Heteropoda), which comprise three families: Atlantidae, Carinariidae, and Pterotracheidae. Heteropods are typical planktonic animals with a Glass-like, transparent body through which the dark visceral mass is visible. They derived their name from the peculiar STRUCTURE OF THE foot: its middle part is laterally compressed to form a keel-like fin used for swimming, while the posterior part of the foot is elongated into a tail-like appendage. Heteropods occur predominantly in warm seas. All of them are predators, feeding on small planktonic animals, including fish larvae.

A representative of the family Atlantidae is Atlanta peronii, a small (about 1 cm) snail with a transparent shell coiled in a single plane (Fig. 77, a). The anterior part of the foot has transformed into a fin; the middle part bears a sucker, and the detached posterior part carries an operculum. Atlanta lives in the open ocean; using its sucker, it can attach to any floating object or release itself to swim freely.

Fig. 77. Heteropods (Heteropoda):

a — Atlanta peronii; b — Pterotrachea coronata; 1 — eyes; 2 — shell; 3 — operculum; 4 — foot; 5 — Pharynx; 6 — intestine; 7 — anus; 8 — ctenidium; 9 — posterior part of the foot; 10 — tail appendage; 11 — anterior part of the foot

In representatives of the family Carinariidae, the shell is small and does not cover the entire body, whereas in species of the family Pterotracheidae, it is entirely absent. Species of the genus Pterotrachea have a fusiform body (Fig. 77, b) with well-developed musculature. They swim rapidly not only by means of the fin, but also by undulating their entire body. Pterotracheids possess well-developed organs of Vision and equilibrium, which is crucial for such an active lifestyle. Their eyes are very large and complex in structure.

The order Mesogastropoda also includes a peculiar group of prosobranchs known as the Aglossa (aprtivate of a radula). They lack both jaws and a radula, and in some parasitic forms, even the shell is reduced. The most profound modifications are observed in members of the family Entoconchidae, which parasitize the body cavity of holothurians.

Representatives of certain families have adapted to life in freshwaters, and some have even successfully colonized terrestrial habitats. Among them is the family Hydrobiidae, whose members inhabit brackish and fresh waters. In the freshwaters of Ukraine, widespread species include the small (4–5 mm) Hydrobia steini as well as the somewhat larger (10–12 mm) Bithynia tentaculata and B. leachi (Fig. 78, d); the latter species is well-known as the intermediate host of the cat Liver fluke, Opisthorchis felineus. These mollusks feature a high, turreted shell with an operculum. They inhabit both running and standing water bodies. Bithyniids frequently form large aggregations on coastal stones, in silt, and on aquatic vegetation. As with other freshwater mollusks, the planktonic larval stage has been lost in these species.

Fig. 78. Freshwater Prosobranchia:

a — Bithynia leachi; b — Viviparus contectus; c — Valvata cristata

Snails of the family Viviparidae are extremely common in the freshwaters of Ukraine. The marsh snail (Viviparus contectus, Fig. 78, b) occurs in great abundance in stagnant water bodies. This relatively large snail (up to 60 mm in height) possesses a spirally coiled shell with an operculum. Viviparids are dioecious, with internal Fertilization. In males, the right tentacle Functions as a copulatory organ. The eggs, fertilized within the female's body, develop inside the oviduct, and fully formed miniature snails emerge from the mother's body. Their shells bear fringed conchiolin outgrowths that are absent in adults, giving the juvenile shell a "hairy" appearance. Eventually, these fringes wear off.

Tiny representatives of the family Valvatidae, such as Valvata cristata (Fig. 78, c), also inhabit the rivers, lakes, and ponds of Ukraine.

Large apple snails (family Ampullariidae), familiar to aquarists worldwide, inhabit tropical freshwaters. Their mantle cavity is divided by a partition into right

and left sections: one contains a gill serving for aquatic Respiration, while the other functions as a lung. Remaining near the water's surface, the apple snail extends a long siphon formed by the folded edge of the mantle to breathe atmospheric air; when diving deeper, it switches to gill respiration.

Order Stenoglossa. This order comprises marine inhabitants. The vast majority of them are predators, and some species feed on carrion. Their shells lack a nacreous (mother-of-pearl) layer. The anterior part of the head is elongated into a proboscis capable of retracting into a sheath. Some species possess additional salivary glands, the secretion of which helps them bore through the shells of bivalve mollusks upon which they prey. In all stenoglossans, the left edge of the mantle is drawn out into a gutter-like or tubular siphon that lies within the siphonal canal of the shell. When these mollusks burrow into the substrate, the siphon is extended outward. Most species possess a horny operculum. Stenoglossans have a single ctenidium, a single atrium, and one kidney. All stenoglossans are dioecious, and their larvae possess a well-developed velum with long lobes.

This group includes a large number of families whose representatives possess fairly large shells well-known to collectors.

Large, spirally coiled shells are characteristic of members of the whelk family (Buccinidae). Among them, species of the genera Buccinium (see Fig. 56, a) and Neptunea inhabit shallow depths in the northern PARTS OF THE Atlantic and Pacific Oceans. The edible whelk Buccinum is commercially harvested in England, Scotland, and Ireland.

The family Muricidae includes the so-called rock shells or purples, whose shells are covered with ridges and spines, and the lower edge of the aperture is frequently extended into a long siphonal canal (Fig. 79, a). Species of the genus Murex were harvested in ancient times to obtain a valuable dye—Tyrian purple—which is produced by their hypobranchial gland. This family also includes the veined rapa whelk (Rapana thomasiana, Fig. 79, b), a predator that devastates oyster banks in the Sea of Japan. In 1947, Rapana was first discovered in the Black Sea, where its egg masses had been accidentally transported on the hulls of ships. Within a short period, it spread throughout the entire Black Sea and now poses a threat to oyster mariculture.

Fig. 79. Order Stenoglossa:

а — Murex; b — Rapana thomasiana; c — Conus textile

Among gastropods with attractive shells, notable Examples include representatives of the families Olividae (genera Oliva and Nagra), Mitridae (episcopal mitre, Mitra episcopalis), and Conidae (genus Conus; Fig. 79, c). These are predominantly tropical marine species. Among them, cone snails possess venom glands whose secretion fills the radular teeth. In humans, the venom of these mollusks causes severe skin inflammation, and fatal cases have been documented.