Mollusca

The animal Phylum Mollusca (mollusks) is the second largest animal group on earth, after insects. It is also the most numerically dominant taxon in the sea, where the vast majority of molluscan species live. The mollusks include a wide variety of forms, most of which are generally recognized by a hard external shell. Snails (Class Gastropoda) are the largest group, with an estimated 80,000 living species, ranging from periwinkles and cowries, to garden snails and slugs, and to the shell-less nudi-branchs or sea-slugs. Clams (Class Bivalvia, formerly Lamellibranchia or Pelecypoda) are the second largest, with about 20,000 living species of scallops, freshwater mussels, giant clams, oysters, and their relatives. Next in terms of living species is Cephalopoda (squids, octopuses, nautilus, and cuttlefish), with approximately 650 species; Polyplacophora or chitons (600 species); Scaphopoda or tusk-shells (350 species); and two smaller groups—vermiform aplacophorans (divided into Caudofoveata and Aplacophora, with a total of300 species); and the primitive, serially organized limpets called Monoplacophora (11 species).

Mollusks occupy most of the recognized ecological niches, spanning marine, freshwater, and terrestrial biomes, at nearly all elevations and ocean depths. Most mollusks are

Closeup of a sectioned chambered nautilus shell (Bettmann/Corbis)

dependent upon aqueous environments; apla-cophorans, monoplacophorans, chitons, scaphopods, and cephalopods are all exclusively marine dwellers. Even land-adapted terrestrial snails largely require moist environments, and they have developed elaborate methods of preventing the desiccation of their bodies and spawn. Most mollusks are free-living, but numerous examples of commensal and parasitic species also exist. Some have antipredator toxins in their bodies (often signaled by bright colors), while others use toxins to subdue their prey. The feeding modes of mollusks include all of the major invertebrate types (herbivores, carnivores, detritivores, and filter feeders), but dietary specialization is common, with representatives that, for example, feed on a single type of algae or cast mucus nets into the water to trap planktonic particles. Although mollusks are famous for slow "creeping," some are modified for more unique modes of locomotion, such as active swimming, passive drifting, or boring into wood or coral. Reproductively, mollusks show examples of dioecity (separate sexes), hermaphroditism (both sexes in the same individual), broadcast spawning, copulation, brooding, and the construction of spermatophores (packets of sperm exchanged between partners) and elaborate egg cases. Mollusks range in size from microscopic to massive: the adults of many snails barely exceed 1 mm, while the giant squid

(Architeuthis) is the largest living invertebrate, attaining 20 m in length. Because the hard external of shell of mollusks fossilizes so well, the paleontological record of mollusks is extensive, with nearly 100,000 extinct species recorded from as far back as the Cambrian Era (530 million years ago). This remarkable diversity of form and function, plus a nearly continuous geographical and temporal record, makes the Phylum Mollusca an ideal group for studying almost any aspect of biological diversity.

Although mollusks are best recognized by their calcified external shells, the shell itself is not the synapomorphy (shared derived character) of the phylum. It is rather the shell-forming tissue, called the mantle, that is recognized in that role. The mantle forms a sheath around the body, enclosing a pallial cavity that houses the gills (or, in the case of land snails, lungs); the openings of the reproductive, excretory, and digestive systems; as well as a variety of chemosensory organs. When a shell is present, it is the cells of the mantle epithelium that extract calcium from the environment and secrete new shell material along a growing edge. A shell is absent (lost) in some members of nearly every class, most notably so in octopuses, garden slugs, and sea-slugs.

Other mollusks, such as squids and various close relatives of the sea slugs, have rudimentary or internalized shells that may not be fully calcified. Most molluscan shells (of gastropods, cephalopods, scaphopods, and mono-placophorans) are univalved. Gastropod shells are characteristically coiled, a form retained in the larvae even in those taxa whose adult shell is secondarily uncoiled. The shells of clams two usually subequal valves, held together with hinge teeth, an elastic ligament, and a pair of adductor muscles. The cephalo-pod shell is typically an internal, chitinous or lightly calcified rod, but the chambered nau tilus has a coiled external shell equipped with gas-filled chambers used to control buoyancy. The shell of a chiton consists of eight (rarely seven) plates surrounded by a muscular girdle impregnated with chitinous scales or spines. The aplacophoran shell is represented solely by calcareous scales or spicules embedded in the integument. Monoplacophoran shells resemble those of gastropod limpets but are distinguished by a series of muscle scars on the inner surface. All molluscan shells retain a record of their growth, in the form of the larval shell and a series of growth lines.

Other primitive molluscan characteristics include bilateral symmetry; protostomous, schizocoelous embryonic development (with spiral cleavage, then splitting the mesoderm to form the body cavity, or coelom); reduction of the coelom; cephalization of the nervous and sensory systems into a well-developed head; a muscular foot for locomotion; a chiti-nous toothed ribbon (radula) for rasping and transporting food particles; an open circulatory system of haemocoels supplied by a three-chambered heart and blood vessels; kidneys (metanephridia) for waste excretion; a sophisticated nervous system of ganglia, nerves, and sense organs; and dioecious reproduction with free-swimming aquatic larval stages (tro-chophore and veliger). Almost all of these characteristics have been lost or radically modified in at least one major lineage. Cilia and mucus secreted from numerous glands are used for a wide variety of physiological functions, including locomotion; water circulation; gathering, sorting, and transport of food particles; egg formation and deposition; and chemosensation.

The anatomical modifications associated with feeding and reproduction are particularly character-rich and have proven especially useful in molluscan taxonomy and phy-logenetic analyses. The molluscan radula provides an excellent example in this regard: impregnated with iron in chitons to facilitate scraping algae from rock surfaces; developed into a hollow dart in cone shells to deliver a dose of toxin to prey; or into a stylet in some shell-less gastropods for piercing individual algal cells and suck the liquid sap. Cephalopods have elaborate nervous systems, including the most complex brain of any invertebrate; eyes that are astonishingly similar (but not homologous) to vertebrate eyes; and chromatophores that make possible rapid color changes and complex behaviors. Several mollusks, notably the squid (Loligo) and sea-hare (Aplysia), are useful models for neurological research because of the simplicity yet accessibility of their nervous systems.

Gastropods and bivalves are the two most successful molluscan classes, in terms of extant species and ecological diversity. Gastropods are characterized by torsion, a 180-degree twist in the internal anatomy relative to the foot that occurs during larval development. Although the coiled shell might seem to reflect torsion, it does not: the coil is a strategy for increasing body size, while maintaining stability. The characteristics most illustrative of torsion are the anteriorly placed mantle cavity, the U-shaped digestive system (with anus adjacent to mouth), and the figure-eight-shaped nerve cords crossing at midbody (streptoneurous). Many derived gastropods are "detorted" or untwisted (euthyneurous), achieved through concentration of the nervous system or differential growth of the mantle. Part of the success of gastropods is measured by their escape from the aquatic environment, made possible by development of a "lung" from the mantle cavity, along with the ability to avoid desiccation through aestivation (sealing the shell during hibernation) and through development of air-tight egg cases.

Bivalves have entirely lost the head during the course of evolution, and along with it, the characteristically molluscan radular feeding apparatus and the cephalic sense organs (eyes, tentacles). In its place functionally, the bivalve gill has become a filter-feeding organ, complete with ciliated fields to sort particle sizes plus a food groove leading particles to the mouth. Bivalves have most successfully invaded soft sediments, with shell ridges and processes presumably to stabilize their position in the sediment. Another uniquely bivalve characteristic is the byssus, a bundle of elastic protein fibers secreted by the foot that can attach to hard objects as an "anchor." Some bivalves, such as mussels, form extensive beds of individuals attached to rocks and to each other using byssal threads. Other bivalves, such as oysters, cement their shells to hard surfaces and to each other. These assemblages frequently serve as crevice-rich substrata for a myriad of other invertebrates.

Many species of mollusks have had an economic impact on human populations, either beneficially as food sources, laboratory research animals, and ornamentals, or detrimentally as disease vectors, agricultural pests, and bio-foulers. Alien molluscan species have played a significant role in the latter regard. In U.S. freshwaters, the Asian clam (Corbicula manilen-sis) spread quickly across the continent in the 1960s, establishing itself as a permanent resident. Then in the 1980s, two species of zebra mussels (Dreissena polymorpha and D. bugen-sis) entered North America through the Great Lakes and spread cross-continent even faster. Much has been made of the water-cleansing abilities of zebra mussels. Nevertheless, they create severe economic problems and seriously impact native freshwater mussels (Unionoidea), whose center of worldwide biodiversity lies in eastern North America.

In addition to competing for physical space and particulate food in the water column, zebra mussels physically attach to unionoids, effectively suffocating them. As a result of these impacts, coupled with the adverse effects of human development (damming, dredging/ filling, chemical pollution, eutrophication, overcollecting, and so forth), thirty-five species of freshwater mussels are now extinct in North America. Another sixty-nine are formally listed as endangered or threatened, leading one knowledgeable source to estimate that 67 percent of all freshwater mussels are at risk of extinction. In recent years, alien species in marine localities have become more common; examples in the United States are green mussels (Perna spp.) in Texas and Florida, and Rapa whelks (Rapana venosa) in Chesapeake Bay. The impacts to both human populations and local ecology in these areas are comparable to those experienced in freshwaters, and control measures are proving equally difficult.

Conservation efforts are prominent in the field of malacology, usually by promoting reserve areas of critical habitat, regulating live collecting, and tracking critically endangered species. Organizations with active conservation groups include the American Malacological Society and the World Conservation Union. Marine species are comparatively unaffected by human activities (although there are a few recently extinct marine species, and local extirpations are common). However, freshwater and terrestrial species, whose home ranges are often extremely limited, have been strongly influenced by anthropogenic factors. The two most severely affected groups of mol-lusks are the unionoid freshwater mussels discussed above, and the landsnails (especially Achatinella spp.) of Hawaii. Unionoids have a specialized larval form, called a glochidium, which must attach to a fish before metamorphosing into a benthic juvenile. For that reason, their survival relies upon suitable ecological conditions for fish as well as for mollusks. Hawaiian landsnails have experienced the additional problem of unsuccessful biological control. In the 1950s, the carnivorous rosy wolfsnail (Euglandina rosea) was intentionally introduced to Hawaii to combat the alien giant African snail (Achatina fulica). Regrettably, Euglandina found the native land-snails more palatable than the intended Achatina, and contributed significantly toward eradicating fifteen to twenty endemic Achatinella species.

Molluscan systematics is far from well resolved, with new species and revised classifications appearing regularly in the published literature. Several important groups have been described only recently: for example, living monoplacophorans were discovered in 1952, although their shells had long been recognized as Paleozoic fossils. In the past twenty years, a resurgence of anatomical studies, plus the application of molecular and phylogenetic (cladistic) methods, has significantly advanced our understanding of natural groups and dramatically changed molluscan classification. One of the best examples is in Gastropoda, where the traditional classification of Proso-branchia (familiar snails such as conchs, whelks, limpets, periwinkles, and cowries), Opisthobranchia (sea-slugs and relatives), and Pulmonata (landsnails and slugs) has been transformed into a large series of equally ranked clades. Likewise, the traditional prosobranch classification of Archaeo-, Meso-, and Neogas-tropoda has been replaced by a newer, cladis-tically based taxonomy. Similar changes are occurring throughout Mollusca, as current research reworks long-held dogmatic ideas.

—Paula M. Mikkelsen

See also: Alien Species; Phylogeny Bibliography

Beesley, Pam L., Graham J. B. Ross, and Alice Wells, eds. 1998. Mollusca: The Southern Synthesis: Fauna of Australia. Vol. 5. Melbourne: CSIRO; Carlton, James

T. 1992. "Introduced Marine and Estuarine Mollusks of North America: An End-of-the-20th-century Perspective." Journal of Shellfish Research 11, no. 2: 489-505; Cowie, Robert H. 1992. "Evolution and Extinction of Partulidae, Endemic Pacific Island Land Snails." Philosophical Transactions of the Royal Society of London, Series B 335:167-191; Parmalee, Paul W., and Arthur E. Bogan. 1998. The Freshwater Mussels of Tennessee. Knoxville: University of Tennessee Press; Ponder, Winston F., and David R. Lindberg. 1997. "Towards a Phylogeny of Gastropod Molluscs: An Analysis Using Morphological Characters." Zoological Journal of the Linnean Society 119:83-265; Taylor, John, ed. 1996. Origin and Evolutionary Radiation of the Mollusca. Oxford: Oxford University Press; Van Bruggen, Adolf Cornelis, Sue M. Wells, and Theo C. M. Kemperman, eds. 1995. Biodiversity and Conservation of the Mollusca. Oegstgeest-Leiden, The Netherlands: Backhuys.

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