MOLLUSKS. Mollusks exist in diverse forms, and although a mollusk is easily recognizable as such to a scientist who studies them, there is no obvious relationship between, say, an oyster and a flying squid. In fact people with no specialist knowledge are more likely to think of them as comprising separate groups: the gastropods (single shells), such as abalone or whelk, inhabiting single shells; the bivalves, such as clams and oysters, which have double shells; and the cephalopods (the name literally means "head-feet," referring to their strange configuration), which include cuttlefish, squid, and octopus.
The number of species in each of these groups is huge. The biodiversity of mollusks is far greater than that of fish and is exceeded only by the vast armies of insects. Three-quarters of the species of mollusks are gastropods, the category that is on the whole of least interest to human consumers. Next come bivalves. Third in numbers but greatest in size are the cephalopods. In relation to the human diet, the bivalves were probably the most important in prehistoric times, because most of them do not move around and many of them exist in the intertidal zones, or in very shallow waters, and are therefore easily gathered. Excavations at Skara Bray in the Shetlands have uncovered huge middens (a term used by archaeologists for a prehistoric refuse-heap of shells and bones) of bivalve shells, indicating very heavy consumption of them during the Stone Age. Evidence from coastal areas in many other parts of the world, including Japan, confirms this. Although consumption of clams and oysters and mussels and scallops is considerable today, especially in Europe and North America, it is the cephalopods that have become most important globally. The fishery for squid is conducted on a huge scale, and squid are a major source of protein for people in the Indo-Pacific area, as well as elsewhere.
In very ancient times, only people living near the coasts could benefit from eating marine mollusks ("marine" is specified in order to distinguish this group from terrestrial mollusks such as edible snails). Even if transport had been available to take mollusks far inland to other communities, the perishability of most of them (still a major factor today, despite the advent of refrigeration and freezing) would have ruled out such traffic. However, there may have been some exceptions. Preservation by drying is a method that is not applicable to many mollusks but can be used for cephalopods. The Greek practice of drying octopus is probably of great antiquity.
In developed countries where modern techniques are available, the transport of even delicate mollusks such as oysters (which have to be kept alive until consumption) is well assured and there is hardly anywhere in these countries where customers cannot enjoy the full range of mollusks. Availability accounts for increased demand, as does the dramatic increase in the size of human populations. However, the factor that has done most to make mollusks almost ubiquitous on dining tables is undoubtedly the great advances made in fishing techniques since medieval times. The huge resources of oceanic squid were simply not accessible in earlier times, whereas today there are few parts of any of the oceans where squid are relatively safe from capture. The sophistication of the equipment used by the vessels that fish for them, especially those from Japan, is extraordinary.
Before considering the three groups of mollusks in more detail, there is one question of nomenclature to consider, and another of classification.
The term "shellfish" is defined in the New Shorter Oxford English Dictionary as "any aquatic invertebrate animal whose outer covering is a shell, usually a mollusc (as an oyster, a winkle, a mussel, etc.) or a crustacean (as a crab, a prawn, a shrimp, etc.), especially one regarded as edible." The term is commonly used for crustaceans and for any mollusk living in an exterior shell. Thus it would apply to the single shells and bivalves but not to cephalopods, with one exception: the so-called chambered nautilus shells, which count as cephalopods but do inhabit shells. A few species of nautilus have value as food in the Indo-Pacific area. It is also relevant to point out that the other cephalopods tend to have what might be called "internal" shells, for example, the "cuttlebone" found in the cuttlefish. These constitute traces of external shells that have disappeared in the course of evolution. The chambered nautilus is, so to speak, poised to take a further step in evolution and abandon its shell, whereupon it would bear some resemblance to a small squid. The question of classification referred to above is this: does the term mollusk include miscellaneous sea creatures such as the sea cucumbers and sea anemones that are eaten in some parts of the world and that are neither fish nor crustaceans but, owing to their general appearance, might be taken to be mollusks? The answer is no; they belong to separate orders. For example, the sea cucumbers belong to the order Holothurian. Furthermore, if a creature is not a fish but does have a shell, must it be either a crustacean or a mollusk? Again the answer is negative. The sea urchin, whose ovaries are a prized delicacy, has what would normally be called a shell (covered usually with spines to repel predators), but it belongs to the phylum Echinodermata.
Having thus cleared what might otherwise be muddy waters for some readers it is time to look more closely at the three main groups of edible mollusks.
The gastropods, or single shell mollusks, have contributed less to human nutrition than either the bivalves or the cephalods. This is not because the single shells are too small. Some, such as whelks, attain a considerable size, up to 90 cm (35 inches) in the case of the species Melongena pugilina, which is eaten in Malaysia and the Philippines. Large whelk, often called conch, are eaten in the Caribbean, where they are known locally as lambis or lambie. One speciality is the conch stew of Martinique and Guadaloupe while another is soused conch, (lambie souse ). In Life and Food, Cristine Mackie describes this specialty as well as other food of the region, and makes one particularly interesting observation. She believes that the native inhabitants, who are known to have consumed conch in large quantities and whose experience preparing it stretches back over many centuries, probably showed early white settlers how to extract the meat and clean it, a special skill requiring instruction.
Even very small single shells are eaten, for example, the little top-shell of the Mediterranean (Monodonta turbinata, of the family Trochidae) or the equally small periwinkle (Littorina littorea, family Lacunidae), known locally as winkles and found on North Atlantic shores, both east and west, but mostly appreciated in Europe. In general, however, the appetite for single shells has diminished in many parts of the world, largely because they are fished locally and few of the edible species have more than minimal gastronomic merit.
Nevertheless, one family among the single shells, Haliotidae, to which the abalone belong, certainly does merit attention for human consumption. There are species all round the world. In California, for example, the red abalone (H. rufescens), is probably the best known although H. tuberculata has been famous since classical times in the Mediterranean and on the European Atlantic coast as far north as the Channel Islands (where it is known as ormer in English and ormeau in French). However, supplies are not abundant. Indeed, along much of the northwest coast of the United States the fishery is either closed outright or subject to severe restrictions. In Japan there is a tradition that stretches back to antiquity of husband-and-wife teams fishing for abalone; the wife dives while the husband tends the boat and the lifeline. Depending on the quality of the various species, the Japanese may eat them raw, diced and iced and furnished with a dipping sauce, or grilled and steamed. Generally, abalone is tough and must be tenderized before being cooked.
Although some abalone can reach a size of up to 25 cm (10–11 in.), they may be regarded as a sophisticated descendant of the ordinary limpet. Limpets, seen clinging tenaciously to seaside rocks, are much smaller and biologically less complicated creatures, but are edible and utilized in interesting local recipes; for example, in some parts of Scotland people were known to mix limpet juice with oatmeal.
The aristocrat of bivalves, in the western world, is the oyster. This is odd because in the nineteenth century oysters were so plentiful and cheap that they were considered to be a food of the poor. Today virtually all the oysters brought to market are cultured. In France especially, there are complex systems followed by oyster farmers, from the initial seeding (planting on special tiles) of the spat of existing oysters through various changes of environment designed to afford protection from predators and to encourage growth. Oysters thrive in the "parks" created for them, and are carefully graded before being transported live to markets. The district of Marennes-Oléron accounts for well over half the French production, but other place names such as Arcachon indicate other famous oyster areas. In England the oysters of Colchester in Essex and of Whitstable in Kent were once of great renown, but nowadays most of the oysters reaching British markets come from the south of Ireland.
What is said above relates in part to the European oyster, Ostrea edulis. However, populations of this species have been very seriously depleted, in some places to vanishing point, and 90 percent of the oysters now consumed in Europe belong to the species Crassostrea angulata, popularly known as the Portuguese oyster. It is a native of Portugal and Spain and also known in the Indo-Pacific as the Giant Pacific oyster.
In North America, the American oyster, Crassostrea virginica, holds sway. Like the Portuguese, it is larger than the European. American oysters are marketed under many names, indicating the place of origin, for example, Cape oysters from Cape Cod (notably Wellfleet and Chatham); Long Island (Bluepoint, Gardiners Bay), and the Chesapeake Bay area (Chincoteague Bay). Of the other American species of oyster, the best is probably the Olympia oyster, a subspecies of the Californian oyster, Ostrea lurida.
Australasian oysters include the Sydney rock oyster, Crassostrea commercialis, which is perhaps the most esteemed of all seafoods for Australians.
Whereas oysters are always visible, many bivalves are not. They burrow into the sand and all one can see is perhaps their "siphon" protruding, or a little hole left by the siphon. Some species are remarkably adept at burying themselves quickly and deeply. The razor shells (socalled because they resemble old cut-throat razors) are among the champions in this art. They are known in Orkney as "spoots," and "spooting" by hand is a pastime that calls for great expertise. There are many other clams in both hemispheres that live closer to the surface of the sand and are gathered more easily. Consumption is highest in North America, where they play a leading role in the traditional clambake, which is an important feature of the seafood cultures of many coastal areas, especially New England. Kathy Neustadt explains the cultural and social importance of clams in We Gather Together: Food and Festival in American Life. A purely practical description is found in the classic cookbook by Mrs. Lincoln, Mrs. Lincoln's Boston Cookbook (1891).
Mussels dominate the European market, at least in terms of quantities sold. The waters surrounding Galicia in the north of Spain include bays that are ideal for the culture of mussels on big ropes suspended from the surface of the sea. By the end of the twentieth century, Spanish exports of mussels had grown to such an extent that they dominated the market, although there is a smaller but substantial industry in the Netherlands, providing mussels mainly for consumption in Belgium. Mussels with french fries (moules et frites ) is counted by some as the national dish of the Belgians; it enjoys popularity there which is without a parallel anywhere else in the world. However, there are many other ways of preparing mussels including the famous French dish moules à la marinière (mussels steamed open in a large covered pot with chopped shallots, herbs, white wine, vinegar, and butter). Mussels are also a useful ingredient in seafood stews and kindred dishes. Mussels can be steamed or fried, and it is also possible to dry mussels (after a boiling). In Thailand dried mussels are coated with sugar and then fried, producing an intriguing dish that might seem strange to western palates.
Those familiar with Irish culture know the song about Sweet Molly Malone who, in the streets of Dublin (where she is commemorated by a charming statue), would cry her wares: "Cockles and mussels alive alive o'." Cockles constitute a large and important group of bivalves, with the European Cerastoderma edule being the most important. In some places it is quite remarkably abundant; densities of over ten thousand individuals per square meter have been recorded. With a maximum measurement around 6 cm (2 inches), this is not the largest cockle; that distinction goes to the spiny cockle of the Mediterranean, Acanthocardia aculeata, whose body inside the shell is blood red, may reach 10 cm (4 in.). One of the cockles of the Pacific coast of North America, Clinocardium nuttalli, may be slightly larger still. Cockles resemble clams in their burrowing down into the sand.
Like oysters, mussels are visible wherever they grow. Another visible bivalve of gastronomic importance is the scallop, who for most of its life is not attached to anything but swims freely, using the rapid opening and shutting of its two shells as a means of propulsion. The muscle connecting the two shells is therefore particularly large and strong, a feature welcome to consumers since this white muscle is the principal edible part. (The orange-yellow coral is also eaten and the "mantle" or "frill" more rarely.)
Of the many species, Pecten maximus, the Great Scallop, and Pecten jacobaeus, the Pilgrim scallop, are the best known in Europe. The former may measure 16 cm (6 in.) across, while the latter is smaller. It is, however, the latter which has a special religious significance, since its shell has for very many centuries been the badge worn by pilgrims to the shrine of St. James at Santiago de Compostela in Galicia. Indeed, the French name Coquille Saint-Jacques is sometimes applied to scallops in a more general way, as in the famous dish Coquilles Saint-Jacques à la provençale. Besides being a badge for pilgrims, scallops have a cultural significance in many other contexts, a point that is well brought out by Cox (1957).
The so-called bay scallop, Argopecten irradians, is the common commercial scallop of the American Atlantic coast. Its muscle, which is usually the only part sold, is a great delicacy. If really fresh, it may be eaten raw, flavored by its own juices. There is also a growing North American fishery for the Atlantic deep-sea scallop, Placopecten magellicanus.
The scallop is well provided with eyes. About fifty of these, green ringed prettily with blue, are set in the frill. These do not show the scallops where they are going, since they are necessarily always going in the other direction, but they do warn them of any danger approaching from behind.
Edible bivalves can be very small, such as the little wedge shells. Other bivalves, of which parts only can be eaten, are huge, notably the giant clam of the Indo-Pacific, Tridacna gigas, which can measure 1 meter (40 in.) across and weigh several hundred kilos; the shells from one of these can provide two washbasins or church fonts.
Because bivalves have two shells joined together, they symbolize in Chinese and other cultures a married couple. Although many of them are plain in color, some have very striking patterns on the outside of their shells, such as zig-zag markings.
A comprehensive reference book published by the Food and Agriculture Organization, or FAO (by Clyde F. E. Roper, Michael J. Sweeney, and Cornelia E. Nauen), provides a good overview of the cephalopod fisheries, offering information on over two hundred species. Although confined to species "of interest to fisheries," this work does include some which are utilized at the subsistence and artisanal levels only and some which at present have only a potential value in commerce. A few cephalopods are of outstanding importance in commerce: squid of the genus Loligo and Todarodes pacificus, the Japanese flying squid, are outstanding examples. Squid account for approximately 70 percent of the world catch, while cuttlefish represent between 10 and 15 percent and octopus between 10 and 20 percent.
The "flying squid" do not really fly but can propel themselves out of the water and glide. They have longer and thinner bodies than other squid, which makes them less suitable for being stuffed. All squid have eight short and two long tentacles. The long ones can be shot out to catch prey. The size of adult squid varies greatly from little more than 20 cm (8 in.) to 20 m (67 ft.) overall.
Cuttlefish also have eight short and two long tentacles, but they are more compact than the squids, having a broader body. Their "ink," like that of squid, is contained in sacs but may be expelled in large clouds to facilitate evasive action. Cuttlefish ink was used historically to make the color sepia, and the Chinese have called the cuttlefish "the clerk of the sea-gods," in a reference to the ink (Read, 1939). Generally, Chinese names for cephalopods are far more descriptive than English ones. For example, the Chinese call one small squid "shallow water soft fish," indicating where it is found, while the cuttlefish may be known as "tiger-blotched black thief." This highly specific nomenclature is in line with the fact that cephalopods play a larger part in food culture in China than in most other countries. While it is true that cephalopods, especially squid, have now reached the market in almost all important countries of the world, it is only in certain regions, including the whole of the Mediterranean, that there is a long tradition of eating them. Full acceptance in North America is still to come, although the influence of Americans from a Mediterranean background has done much to show the way to others.
Those repelled by the appearance of cephalopods might be especially upset by the octopus. The name octopus refers to its eight arms, each armed with suckers for grasping prey. The best octopus for eating have arms with a twin row of suckers on each. The most common octopus of the Mediterranean, Octopus vulgaris, has been important since classical times in many of the cultures of the region, and has figured frequently in art, as on Greek vases of the late classical period.
The flesh of the octopus is notorious for being tough and requiring treatment to soften it before cooking. This does not apply to tiny baby octopus, but the larger specimens are beaten against rocks by fishermen, or struck with mallets, to tenderize them. Delicious and flavorful octopus dishes include the Spanish pulpos con papas, the French poulpe à la niçoise, and the Neapolitan polpetielli alla Luciana.
In connection with the last-named dish, a Neapolitan author, Signora Jeanne Caròla Francesconi (1965), has given a vivid description of methods of fishing based on the fact that the true octopus (the kind with two rows of suckers on each arm) is especially attracted by the color white. Thus a piece of white rag may be placed in the center of a five-pronged hook and lowered down to ensnare the octopus. "It is also fished with a pottery amphora (called a 'mummarella') which is likewise painted white and contains white stones; this is lowered to the bottom on the end of a rope, near a rock. The 'true' octopus, if he sees it, will empty the pebbles out and instal himself inside as though in a nest. The fisherman, alerted by seeing the white pebbles scattered outside the amphora, pulls it up and thus catches the octopus." Although the octopus is thus outwitted, it is fair to add that of all mollusks it possesses the most intelligence. Experiments conducted at an aquarium in Naples established that an octopus is capable of learning, for example, how to move from one tank to another, when there are several apparent exits of which only one allows passage.
Clark, Eleanor. The Oysters of Locmariaquer. New York: Pantheon, 1964.
Cox, Ian, ed. The Scallop: Studies of a Shell and Its Influences on Humankind. London: Shell Transport and Trading Co., 1957.
Francesconi, Jeanne Caròla. La Cucina Napoletana. Naples: Fausto Fiorentino Editore, 1965.
Lincoln, Mrs. D. A. Mrs. Lincoln's Boston Cook Book. Boston: Roberts Brothers, 1891.
Mackie, Cristine. Life and Food in the Caribbean. London: Weidenfeld & Nicolson, 1991.
Neustadt, Kathy. "'Born among the Shells': The Quakers of Allen's Neck and Their Clambake." In We Gather Together: Food and Festival in American Life, edited by Theodore C. Humphrey and Lin T. Humphey. Ann Arbor: UMI Research Press, 1988.
Read, Bernard E. Chinese Materia Medica: Fish Drugs. Peking: Peking Natural History Bulletin, 1939.
A mollusk is a soft-bodied invertebrate (an animal without a backbone) that is often protected by a hard shell. Most mollusks live in water and make up the second largest group of invertebrates (next to arthropods). Mollusks are represented by such diverse invertebrates as clams, slugs, snails, and octopuses.
There are about 100,000 species of mollusks, most of which live in the water. All are soft-bodied, nonsegmented, and usually enclosed in some sort of covering made of calcium carbonate. Whether a land snail or an underwater clam, all mollusks share certain traits. They all have a "mantle," or a covering of tissue that protects their internal organs. Many species secrete a substance that forms a hard shell, protecting them from predators. Their digestive systems are made up of a mouth, throat, gullet, stomach, intestine, and anus. Mollusks that live underwater use gills to get their oxygen, while land-dwelling mollusks have lungs. All have a two-chambered heart that pumps blood through vessels that branch throughout their bodies. The mollusk nervous system consists of two pairs of nerve cords. Some mollusks have eyes and other sense organs. Most mollusks also have a muscular "foot" that is used for slow crawling or digging. In an octopus, this foot is divided into arms or tentacles lined with suction cups.
There are at least eight species of mollusks known, but most fit into three main groups—gastropods, bivalves, and cephalopods—that are grouped according to the shape of their muscular foot. The name gastropod means "stomach-foot" and describes those mollusks, like snails, that
have a sucker-like foot. This is the largest group of mollusks and is best represented by the common garden snail. Most gastropods, excluding slugs, have a single shell often shaped like a spiral, or coil. They also have a large, flat foot that they use to slide along on top of the mucus they secrete. This is why they leave a visible, slimy trail behind them. A gastropod has a distinct head with tentacles that can move up and down like a periscope and act as eyes or sense organs. A snail also has rows of tiny teeth. Not just confined to land, there are many ocean-dwelling gastropods like periwinkles, abalones, and sea slugs.
A bivalve is a mollusk that has a two-part shell joined by a hinge. Bivalves are water animals and are best represented by clams, oysters, and scallops. All bivalves are filter feeders. They filter water through large gills that catch bits of food and absorb oxygen. To do this, however, they must keep their shells open, and as a result, expose their vulnerable soft body. A mollusk, like a clam, moves on the ocean's bottom by thrusting its large, hatchet-shaped, muscular foot between the open shells, thus pushing it along slowly. Unlike gastropods, bivalves have no head, and their sense organs are not well-developed. Bivalves do have a powerful adductor muscle that they use to clamp their shells tightly together.
Members of the cephalopod group of mollusks, like the octopus and squid, are free-swimming and have no shell. Unlike typical mollusks, they are fast-moving hunters and prey on other animals. An octopus has a sharp beak, like a parrot's, that it uses to rip its food and break open shells. In squid and octopuses, the mollusk foot has evolved into long arms, or tentacles, around the head. They use these tentacles to capture fish. In some ways, cephalopods resemble vertebrates because they have a solid internal support structure, like a skeleton, and a large head with eyes, as well as a good-sized brain. Cephalopods move by a form of jet propulsion in which they forcefully squirt water. They also can eject a cloud of dark-colored chemicals that covers their escape as they zoom away. A giant squid may extend its tentacles as far as 60 feet (18.29 meters) and has the largest eyes in the animal kingdom—up to 1 foot (0.3 meters) wide. Some squid and octopuses can change color like a chameleon. The only cephalopod with a shell on the outside of its body is the nautilus whose shell is divided into chambers. It lives in the largest chamber and adds another when it grows. It fills the empty chambers with a gas that makes it easier to swim. All three groups of mollusks reproduce sexually through the union of male sperm and female eggs. Unlike gastropods and cephalopods, however, bivalves are not necessarily either male or female. Instead they may contain both male and female organs.
[See alsoInvertebrate ]
Mollusks (phylum Mollusca) are the second largest group of invertebrates (arthropods are the largest), with over 100,000 species. They are characterized by a head with sense organs and mouth, a muscular foot, a hump containing the digestive and reproductive organs, and a covering of tissue (the mantle) that usually secretes a hard, protective shell. Among the more familiar mollusks are snails, whelks, conchs, clams, mussels, scallops, oysters, squid, and octopuses. Less conspicuous, but also common, are chitons, cuttlefish, limpets, nudibranchs, and slugs.
The largest number of species of mollusks are in the class Gastropoda, which includes snails with a coiled shell, and others lacking a shell. The next largest group consists of the bivalves (class Bivalvia), the chitons (class Amphineura), and octopus and squid, (class Cephalopoda). The other classes of mollusks are the class Scaphopoda (consisting of a few species of small mollusks with a tapered, tubular shell) and the class Monoplacophora, a class once regarded as extinct, but now known to have a few living species restricted to the ocean depths.
Mollusks are ancient. Fossil shells recognizable as gastropods and bivalves are present in rocks from the Cambrian period, about 570 million years ago. Present classifications based on the evolutionary relationships of mollusks are derived from studies of embryonic development, comparative anatomy, and genetic sequences. The findings suggest that mollusks are related to sipunculid, annelid, and echiurid worms.
Mollusks provide a clear example of a phenomenon called adaptive radiation (adaptation followed by spread in the particular niche). The gastropods and bivalves that were originally marine, subsequently radiated into freshwater habitats. Without much change in gross appearance, these animals developed physiological mechanisms to retain salts within their cells and prevent excessive swelling from water intake in freshwater. Several groups of freshwater snails then produced species adapted to life on land. Gills adapted for the extraction of oxygen from water were transformed in land snails into lungs that extract oxygen from air, and the ammonia excretion typical of aquatic mollusks became uric acid excretion typical of birds and reptiles. A small squid, Onycoteuthis, moves so rapidly through the water that it often becomes airborne. It does not nest in trees, but it may help to explain why some authors ascribe to the squid Loligo a “parrot beak” and a “gizzard.” Restaurant menus often include bivalve mollusks (oysters on the half-shell, steamed mussels, fried clams), cephalopod mollusks (fried squid), or gastropod snails (escargots).
Gilpin, Daniel. Snails, Shellfish, and other Mollusks. Mankato, MN: Compass Point Books, 2006.
Miller, Ruth. Mollusks. Orlando: Raintree, 2005.
Sturm, C.F., T.A. Pierce, and A. Valdes. The Mollusks: A Guide to Their Study, Collection, and Preservation. Parkland, FL: Universal Publishers, 2006.
C. S. Hammen
The Mollusca (mollusks) are a large phylum of animals that includes the snails, slugs, clams, squids, and octopi, among others. Most are marine, many are freshwater, and some snails and slugs are terrestrial. The phylum name refers to their soft, pulpy bodies (mollis means "soft"). In many cases, the body is protected by a hard shell of calcium carbonate—the seashells familiar to beachcombers and "half shells" familiar to oyster lovers.
The shell is secreted by a membrane called the mantle that envelops the body like a cloak. In species without an external shell, such as octopi, the mantle forms an outermost skinlike body covering. The mantle encloses a space, the mantle cavity, which usually contains comblike gills for respiration. In some seemingly shell-less species—squids and cuttlefish—the shell is embedded in the mantle and can be found only by dissection. Most mollusks also have a radula. In snails, this is a tonguelike belt equipped with a few hundred to thousands of chitinous teeth, used to scrape food from surfaces such as rocks.
The most behaviorally sophisticated of all invertebrate animals are the cephalopod mollusks: the octopi, squids, cuttlefish, and nautilus. Cephalopods have long, flexible arms, equipped in most cases with suckers for prey capture. They are active swimmers; some have eyes remarkably similar to human eyes; they have more complex brains than any other invertebrate; and, correspondingly, they exhibit remarkably subtle social behaviors and learning capabilities.
Kenneth S. Saladin
Pechenik, Jan A. Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill, 2000.
Rupert, E. E., and R. D. Barnes. Invertebrate Zoology, 6th ed. Philadelphia, PA: W. B. Saunders, Co., 1994.
Mollusks (phylum Mollusca) are the second largest group of invertebrates (the arthropods being the largest), with over 100,000 species . They are characterized by a head with sense organs and mouth, a muscular foot, a visceral hump containing the digestive and reproductive organs, and an envelope of tissue (the mantle) that usually secretes a hard, protective shell. Practically all of the shells found on beaches and prized by collectors belong to mollusks. Among the more familiar mollusks are snails , whelks, conchs, clams, mussels, scallops, oysters, squid , and octopuses. Less conspicuous, but also common, are chitons , cuttlefish , limpets , nudibranchs, and slugs .
The largest number of species of mollusks are in the class Gastropoda, which includes snails with a coiled shell, and others lacking a shell. The next largest group are the bivalves (class Bivalvia), the chitons (class Amphineura), and octopus and squid, (class Cephalopoda). The other classes of mollusks are the class Scaphopoda (consisting of a few species of small mollusks with a tapered, tubular shell) and the class Monoplacophora, a class once regarded as extinct, but now known to have a few living species restricted to the ocean depths.
Fossil shells recognizable as gastropods and bivalves are present in rocks from the Cambrian period, about 570 million years ago. Present classifications based on the evolutionary relationships of mollusks are derived from studies of embryonic development, comparative anatomy, and RNA nucleotide sequences . The findings suggest affinities of mollusks with sipunculid, annelid, and echiurid worms.
Mollusks provide a clear example of adaptive radiation . The gastropods and bivalves which were originally marine, subsequently radiated into freshwater habitats. Without much change in gross appearance, these animals developed physiological mechanisms to retain salts within their cells and prevent excessive swelling from water intake in freshwater. Several groups of freshwater snails then produced species adapted to life on land. Gills adapted for the extraction of oxygen from water were transformed in land snails into lungs which extract oxygen from air, and the ammonia excretion typical of aquatic mollusks became uric acid excretion typical of birds and reptiles . A small squid, Onycoteuthis, moves so rapidly through the water that it often becomes airborne. It does not nest in trees, but it may help to explain why some authors ascribe to the squid Loligo a "parrot beak" and a "gizzard." Restaurant menus often include bivalve mollusks (oysters on the half-shell, steamed mussels, fried clams), cephalopod mollusks (fried squid), or gastropod snails (escargots).
"Mollusca." Chemical Zoology. vol. 7. edited by M. Florkin and B T. Scheer. New York: Academic Press, 1972.
Morton, J.E. Molluscs. New York: Harper, 1960.
Mollusks belong to the phylum Mollusca and make up the second largest group of invertebrates (animals lacking backbones) after the arthropods. Over 100,000 species of mollusks have been identified. Restaurant menus often include a variety of mollusk dishes, such as oysters on the half-shell, steamed mussels, fried clams, fried squid, or escargots.
Mollusks have certain characteristic features, including a head with sense organs and a mouth, a muscular foot, a hump containing the digestive and reproductive organs, and an envelope of tissue (called the mantle) that usually secretes a hard, protective shell. Practically all of the shells found on beaches and prized by collectors belong to mollusks. Among the more familiar mollusks are snails, whelks, conchs, clams, mussels, scallops, oysters, squid, and octopuses. Less noticeable, but also common, are chitons, cuttlefish, limpets, nudibranchs, and slugs.
Classes of mollusks
The largest number of species of mollusks are in the class Gastropoda, which includes snails with a coiled shell and others lacking a shell. The next largest group are the bivalves (class Bivalvia), the chitons (class Amphineura), and octopus and squid, (class Cephalopoda). Other classes of mollusks are the class Scaphopoda, consisting of a few species of small mollusks with a tapered, tubular shell, and the class Monoplacophora. The last of these classes was once thought to be extinct, but a few living species have been found in the ocean depths. Some fossil shells recognizable as gastropods and bivalves have been found in rocks 570 million years old.
Mollusks provide a clear example of adaptive radiation. Adaptive radiation is the process by which closely related organisms gradually evolve in different directions in order to take advantage of specialized parts of the environment. The gastropods and bivalves were originally marine organisms, living in salt water. They subsequently evolved to take advantage of freshwater habitats. Without much change in their outward appearance, these animals developed physiological mechanisms to retain salts within their cells, a problem they did not face as marine organisms. This new development prevented excessive swelling of their bodies from intake of freshwater.
Several groups of freshwater snails then produced species adapted to life on land. The gills they originally used for the extraction of oxygen
from water were transformed in land snails into lungs, which extract oxygen from air. Similarly, the excretion of ammonia typical of aquatic mollusks evolved into uric acid excretion typical of birds and reptiles.