(To complete all classifications ETI has added the Kingdom and the Phyla of all the different taxa treated on this DVD-ROM without higher classification descriptions. Texts from Lynn Margulis and Karlene V. Schwartz, Five Kingdoms. CD-ROM Copyright 2002 ETI / Freeman & Co Publishers)
From the jet-propelled (but very slow moving) chambered nautilus to the poison-producing cone shell, diversity within the molluscs is remarkable. Most molluscs have an internal or an external shell, a muscular foot, and an unsegmented, soft body. A mantle, which is a fold of the body wall, lines the external shell and secretes the calcium carbonate and protein of which the shell is made. A tubular extension of the mantle called the siphon, when present, directs cilia-generated water currents into the mantle cavity. The current carries food as well as dissolved gases to the gills of the mollusc. Some molluscs gather food by boring or scraping with a hard, chitinous ribbon called the radula, a unique molluscan structure. Other molluscan species are predators; gastropod Charonia consumes sea star Oreaster. Still other molluscs live as parasites or commensals with sea squirts (ascidians, Subphylum Urochordata), annelids (Phylum Annelida), echinoderms (Phylum Echinodermata), or crustaceans (Phylum Crustacea). Molluscs fill an extraordinary array of ecological niches: mud and sand flats, forests, soil, fresh water, deserts, driftwood, and the abyss of the sea.
About 50,000 species of molluscs have been described, making Mollusca the third largest phylum after Mandibulata (Phylum Mandibulata) and Chelicerata (Phylum Chelicerata). Mollusc shells come in a beautiful array of colors and patterns. Certain shell-less gastropods are given a green color by algae, from the chloroplasts of the marine algae on which they feed. One slug bioluminesces as a consequence of feeding on bioluminescent sea pansies (anthozoans, Phylum Cnidaria). Some molluscs produce ink, which in some cases is luminescent. Molluscs range in size from no bigger than a sand grain to the giant clam Tridacna, which can be 1.3 m wide. The giant squid Architeuthis, the largest invertebrate ever known, measured lengths approach 20 m, including 16 m long tentacles and a 4 m long body.
Molluscs are grouped into seven classes. Members of Class Monoplacophora have one cap-shaped shell over a flattened foot. They live in deep water off the west coast of the Americas, in the Gulf of Aden, in the South Atlantic, and off Antarctica. Monoplacophorans are better known from the fossil record—only about 10 living species have been described. Vema, the first of this class to be observed live, was taken from a submarine ridge off San Diego. Aplacophora, the second class—all wormlike and shell-less with external spicules of calcium carbonate—includes the deep-sea solenogasters (Chaetoderma and Neomenia). Class Polyplacophora includes chitons (Cryptochiton). Eight shell plates at least partly buried in the mantle overlie the large flat foot with which a chiton clings to its rock, often in heavy surf. Members of class Bivalvia (Pelecypoda) are the clams, mussels, scallops (Pecten), oysters, and wood-boring molluscs. A bivalve is usually sedentary and has two lateral shells called valves with a dorsal hinge. The foot in bivalves can be flattened like a snail’s foot, fingerlike, wedge shaped, or absent. They lack tentacles and heads. The largest class, Gastropoda, comprises abundant marine snails such as Busycon, sea slugs and their relations, and all terrestrial molluscs. If present, the gastropod shell is usually spirally coiled. Shell-less gastropods include the nudibranchs. The class Scaphopoda comprises the tooth shells (Dentalium), which have tusk-shaped shells open at both ends. They burrow into mud or sea sand foot first, leaving the narrow end of the shell exposed above the sand or mud. Class Cephalopoda includes octopods, which lack an internal shell; squids, which have an internal shell (pen) secreted by the mantle; and Nautilus, which has an external shell. A pen is chitinous and lacks calcium carbonate. The head and prehensile arms (tentacles) of a cephalopod encircles beaklike jaws.
Separate sexes that shed gametes into the water are common in most molluscs. But many terrestrial and freshwater species fertilize internally. Land snails, nudibranchs, and some bivalves usually cross-fertilize; they are monoecious. Certain species—all nudibranchs—are simultaneous hermaphrodites, a single individual simultaneously produces sperm and eggs. Others are protandric hermaphrodites; an individual male develops into a female as it ages. Oysters reverse their sex, from male to female and back again, several times in a season. After courtship, a male cephalopod releases packets of sperm (spermatophores) from his penis and transfers the spermatophore by using a specialized tentacle to the female’s mantle cavity.
Molluscs generally deposit eggs in gel clusters, bubble rafts, horny capsules, or sand collars. Female octopuses clean, aerate, and defend their eggs. The cephalopod Argonauta female produces a delicate paper case in which she lives while brooding her embryos. Free-swimming planktonic trochophore larvae hatch from fertilized mollusc eggs. In gastropods, bivalves, and scaphopods, the trochophore larvae develop into veliger larvae before metamorphosis to the adult form. Cephalopod species often have a juvenile stage called a paralarva between the trochophore larval stage and adulthood. Some freshwater clams and chitons brood young. Some periwinkles even “give birth” to live young. The eggs of terrestrial molluscs develop directly into adults without a free-living larval stage.
Bivalves and some snails are sedentary filter feeders; their mucus-covered gills entangle food particles and convey them to ciliated flaps, called palps, where particles are sorted, and into the nearby mouth. The hypobranchial gland produces mucus that binds particles transported by the gills. On its head, Vema has a tiny mouth with a U-shaped anterior lip, having a radula within. Many large molluscs feed on crustaceans, fish, echinoderms, annelids, and other molluscs; for example, Busycon levers its foot between the shells of clams and scrapes away its prey’s tissues with its toothed radula. A firm flap, the operculum, closes the shell of some retracted molluscs. The ciliated tentacles of scaphopods reach into sand for foraminiferans (Phylum Granuloreticulosa). Chitons, snails, and slugs project their toothed radulae to rasp algae and then retract the radulae into their mouths. These teeth are capped with magnetite, an iron oxide, in some chitons. In many bivalves and gastropods, a crystalline style releases digestive enzymes and is rotated by cilia to pull food-laden mucus strings into the stomach. The style is inside the digestive tract and is a habitat for bacteria such as the giant spirochete (Phylum Spirochaeta). Food is moved through the mollusc gut mainly by the action of cilia, except in cephalopods, and digestion is intracellular. The hepatopancreas secretes enzymes. Solid waste is discharged from the anus into the mantle cavity, from which waste leaves with the excurrent flow. The digestive tract of certain bivalves from sulfide-rich habitats is reduced, suggesting that the chemolithoautotrophic bacteria in their gills are contributing to the nutrition of the host mollusc.
Aquatic molluscs exchange gases through the body surface. Gills (ctenidia) usually lie within the mantle cavity between mantle and viscera, function in gas exchange, and may also sort food, depending on the species. Monoplacophorans have multiples of gills, multiple auricles and ventricles (heart chambers) and multiple nephridia—features that relate these segmented molluscs to annelids. The spectacular dorsal tufts of nudibranchs exchange respiratory gases and contain extensions of the digestive tract. The moist mantle cavity of some garden and freshwater snails functions as an air-breathing, highly vascularized lung. In most molluscs, a dorsal heart pumps blood through vessels and an extensive hemocoel. Cephalopods have a completely closed circulation with capillaries, arteries, and veins; squids have accessory hearts as well, and these accessory hearts pump blood to the gills in series with the main heart. Scaphopods lack hearts, gills, and blood vessels; the scaphopod mantle cavity is the respiratory surface with foot contractions moving blood through sinuses—spaces in the tissues. The mollusc kidneys (metanephridia) collect soluble waste from the coelom and discharge it into the mantle cavity through the renal pore.
The mollusc nervous system generally—except for that of cephalopods—consists of three to five pairs of ganglia with connections between them. Balance organs called statocysts are found in pelagic (ocean-dwelling) molluscs. Incoming water passes a chemoreceptor called the osphradium (Greek osphra, a smell) before it reaches gills. The cephalopod brain is more complex than that of any other invertebrate; cephalopods have a complex memory and learning behavior, as well as image-forming, focusable eyes with cornea, lens, iris, and retina. Giant nerve fibers of squid synchronize jet-propelled escape responses.
Molluscs are preyed on by many animals—toothed whales, haddock, walrus, cod, and the sea star Asterias among them—and hundreds of mollusc species are sources of food for humans. The first recorded aquaculturist, Sergius Orata, a Roman, cultured oysters in the first century B.C. Tools, trumpets, sacred and decorative objects, cameos, and buttons are carved from mollusc shells. Freshwater mussels and pearl oysters secrete pearl over irritating particles. The internal calcareous shell (pen) of a cephalopod, the cuttlefish Sepia, is used as the cuttlebone on which caged birds con- dition their beaks; the cuttlefish is benthic (bottom dwelling) and uses its cuttlebone to regulate buoyancy. Sepia, a brown pigment used by artists, is prepared from cuttlefish ink. A yellow secretion of Murex, a marine gastropod, was the source of the dye royal, or Tyrrhenian, purple used by Phoenicians. Tusk shells served as wampum for native Americans near the West Coast. East Coast native Americans drilled “roanoke,” a hard substance used as money, from cockles. Mercenaria, the familiar edible clam, or quahog, was the source of purple wampum. Abalone shells were so widely used for ornaments, including bracelets, that trade routes have been traced by their distribution from California into the interior.
Some freshwater snails are intermediate hosts of Schistosoma—a trematode parasite—giving rise to the disease schistosomiasis (bilharzia). Oyster drills and rock- and wood-boring bivalves, such as shipworms, riddle pilings and wooden ships. Dreissena, zebra mussel, carried to the United States from Europe in ship ballast water, clogs water pipes. Land slugs and snails can be garden pests.
Because shells preserve well, molluscs are abundantly documented in the fossil record. Monoplacophoran shells are abundant in the middle Cambrian period. Bivalves appear by the middle Cambrian; chitons, cephalopods, and gastropods by the Upper Cambrian. The oldest tooth-shell fossils appear in the middle Ordovician, about 450 million years before the present. Octopuses appear in the fossil record rather recently—about 65 million years ago, in the Cretaceous. Squids, the most modern cephalopods, appear later, in the Tertiary.
The relationship of molluscs to other phyla is debated. Molluscs have a true coelom, reduced to the small pericardial cavity around the heart and gonad. Although adult annelids, flatworms, and molluscs appear very different from one another, most marine molluscs and annelids pass through a swimming, ciliated trochophore larval stage; for this reason, molluscs are believed by some zoologists to share ancestors with annelids (Phylum Annelida) and flatworms (Phylum Platyhelminthes). Digestion within cells rather than in a stomach cavity, the plan of the nervous system, external cilia, and gliding and creeping with ventral waves relate molluscs more strongly to flatworms than to annelids. The monoplacophoran Vema and the closely related Neopilina are thought by researchers to closely resemble ancestral middle Cambrian fossil molluscs. Others believe that the segmentation of living monoplacophorans derived from secondary replication of body parts and that ancestral molluscs may have been soft-bodied organisms like their trochophore larvae. The intriguing theory that molluscs may be closely related to sipunculans (Phylum Sipuncula) is discussed in the sipunculan essay.
Literature
¥ Abbott, R. T., American seashells: The marine Mollusca of the Atlantic and Pacific coasts of North America, 2d ed. Van Nostrand Reinhold; New York; 1974.
¥ Brooks, W. K., The oyster. Johns Hopkins University Press; Baltimore; 1996.
¥ Lane, F. W., Kingdom of the octopus: The life history of the Cephalopoda. Jarrolds; London; 1960. Sheridan House; New York; 1960.
¥ Morris, P. A., A field guide to the shells of the Atlantic and Gulf coasts and the West Indies, 3d ed. Houghton Mifflin; Boston; 1973.
¥ Morton, J. E., Molluscs, 4th ed. Hutchinson University Library; London; 1967.
¥ Page, H. M., C. R. Fisher, and J. J. Childress, “Role of filter-feeding in the nutritional biology of a deep-sea mussel with methanotrophic symbionts.” Marine Biology 104:251–257; 1990.
¥ Scheltema, A. H., “Aplacophora as progenetic aculiferans and the coelomate origin of mollusks as the sister taxon of sipuncula.” Biological Bulletin 184:57–78; 1993.
¥ Solem, A., The shell makers. Wiley (Interscience); New York; 1974.
¥ Vogel, S., “Flow-assisted mantle cavity refilling in jetting squid.” Biological Bulletin 172:61–68; 1987.
¥ Wilbur, K. M., ed., The Mollusca, 12 vols. Academic Press; New York; 1983–1988.
¥ Yonge, C. M., Oysters, 2d ed. Collins; London; 1966.