Snails
Snails
Snails are mollusks typically with a coiled, more or less helical, shell as their most conspicuous external feature. When active, snails creep on a broad muscular foot, and display a head with eyes and sensory tentacles. Inside the shell is an asymmetrical visceral mass and one or more gills or lungs used for respiration. Beneath the head is a mouth equipped with a radula, a spiky, long, rasping tongue like organ used to scrape algae off rocks or to bore holes in the shells of other mollusks. The shell of snails is secreted by an enveloping layer of tissue called the mantle. Some snails, such as the tiny caecums of salt marshes, may be only 0.08 in (2 mm) in height, while other species, such as the horse conch of southern Florida, may grow to 23.6 in (60 cm).
The degree of coiling of the shells is highly variable from one species to another. Limpets exhibit very little coiling, and abalones have a shell that is broad and flat, with scarcely two-and-a-half turns or whorls. In terebrids, there may be as many as 25 coils with a spire so sharp that it is difficult to count the smaller whorls. In a peculiar snail called Vermicularia, the turns lose contact as the shell grows, and a process of uncoiling occurs, resulting in a shell that looks like certain calcareous worm tubes. The coiling of the shells of snails may be right-handed or left-handed. Among the oldest fossils the types were of roughly equal frequency, but most living species are right-handed. If one holds a snail shell with the central axis vertical and the spire on top, the opening, from which emerge the head and foot, is usually on the right. In the whelk Busycon perversum, the aperture is on the left. Many snails have a partly mineralized, leathery operculum that closes the door on predators when the soft parts are withdrawn inside the shell.
Snails, slugs, and nudibranchs are classified in the class Gastropoda (meaning stomach-foot) in the phylum Molluska. There are more species of gastropods than species of the other five classes of mollusks combined. The exact number is uncertain, but estimates range from a total of 55,000 to 100,000 species of mollusks.
Snails are grouped into subclasses according to the position of the gills: for example, the Prosobranchia have gills in front of the heart and other viscera, while the Opisthobranchia have gills behind. Associated with this anatomical difference, the prosobranchs have the auricle of the heart anterior to the ventricle and the visceral nerve cord in a figure eight, while opisthobranchs have auricle posterior to ventricle, and an oval nerve loop. The
Prosobranchia, which are entirely marine, are further divided into order Archaeogastropoda, with paired gills and numerous teeth in the radula, and the order Caenogastropoda, with a single set of gills and few teeth. The prefixes mean ancient and recent, respectively, suggesting that the first set of traits evolved earlier. The Nudibranchia (sea slugs) lack a shell and have atypical gills as adults, although the young look very much like other snails. The third subclass, the Pulmonata, contains all snails with a lung rather than gills, and includes most of the terrestrial snails and many of the freshwater snails.
The common names whelk and conch refer to large snails. Whelk, derived from an old English word, is reserved for members of a single family (the Buccinidae), containing animals up to 6 in (15.2 cm) in height, which are predators and scavengers of the northern Atlantic littoral zone. Their empty shells are often inhabited by hermit crabs.
Conch comes from the Latin and Spanish concha, meaning shell. Conchs are the largest snails, with enough meat in the foot to make them popular in stews and salads. The species names of conchs indicate their size. Strombus gigas, the queen conch (family Strombidae), has a massive shell with a pink lining, up to 11.8 in (30 cm) high. Pleuroploca gigantea, the Florida horse conch (family Fasciolaridae), has an even larger but somewhat thinner shell up to 23.6 in (60 cm) high. The area around Key West, Florida, has been called the Conch Republic, and people there are known as “conchs.”
Both bivalves and gastropods are found as fossils in early Cambrian rocks, which contain the first abundant animal fossils. In geological terms, many forms appeared abruptly, giving rise to the expression “Cambrian explosion” to signify the metazoan radiation of 550 million years ago. The Burgess Shale, an exceptionally well-preserved record of animal life of the mid-Cambrian, contains slit-shells, snails similar to modern species of Pleurotomaria. The slit-shells are assigned to the order Archaeogastropoda, having two long gill plumes, regarded as a primitive feature. The right gill is absent in the Caenogastropoda.
The consensus among zoologists is that the mol-lusks evolved from a worm-like ancestor, because patterns of early development, very conservative traits, are similar to those of living worms. Most frequently mentioned are sipunculids, but polychaetes, and echiurid worms are also good candidates for the living worms most resembling the presumed ancestor of mol-lusks. These groups share spiral cleavage and determinate development. When the fertilized egg begins to divide into 2, 4, 8, 16, etc. cells, division is not at right angles to the previous plane of cleavage but in an oblique direction, so that the new cells from a spiral pattern, quite unlike the orthoradial cleavage pattern seen in echinoderms, for example.
Determinate development means that each part of the surface of the egg leads to a definite structure of the embryo, such as gut, head, limbs, and so on. In other words, the fates of the cells produced in early divisions are fixed. This is a feature of development in arthropods, annelids, and mollusks, taken to indicate that the phyla are related. With the fossil evidence so ambiguous and DNA data relatively sparse, relations among the phyla have had to depend heavily on features of early development Spiral cleavage, in a way, foreshadows not only the later coiling of the shell but another type of twisting that occurs during development, known as torsion.
As the young snail grows, the whole visceral sac rotates about a longitudinal axis 180° or half a turn to the right. With respect to the head and foot, the midgut and anus are at first situated ventro-posteri-orly, and after torsion they are displaced dorsal and to the right. This puts the end of the gut in the mantle cavity, above the head. The gonad and digestive gland lie in the hind end of the animal, which is quite isolated from the outside, inside the spire in coni-spiral forms. The result of torsion is an embryo that looks symmetrical externally, but is twisted inside. Gastropod torsion is a morphogenic event that enables the veliger larva to retract head and foot completely and seal the opening of the shell with the operculum. The condition persists in most juvenile and adult snails, although some opisthobranchs undergo de-torsion. It seems reasonable, but the evidence fails to support the hypothesis that torsion was the result of selective pressure to improve defense against predation. This idea was tested experimentally: planktonic predators devoured pre- and post-torsion veligers with equal frequency.
It seems probable that prosobranchs with shells coiled in one plane were first in evolution, and that the piling up of whorls to make sharply pointed shells occurred in several lines. Opisthobranchs show loss of gill on one side and loss of shell in family Aplysidae and order Nudibranchia, indicating a more recent origin. Finally, the pulmonates probably derived from opisthobranchs by development of a lung from the mantle cavity when the snails invaded the land in the Mesozoic Era.
Regarding the biology of reproduction, snails are generally of two sexes. They mate, the female receives sperm from the male, and lays fertilized eggs, which develop into swimming larvae. The pulmonates, a large group that includes terrestrial snails and many that live in lakes and ponds, have a different method. They are hermaphroditic, each individual is both male and female, and when they mate each snail fertilizes the eggs of the other. Then each animal deposits a jelly-coated mass of developing eggs in a place selected to avoid drying out or predation. A number of gastropods, such as limpets, are sequential hermaphrodites, the same individual is male at first maturity, and later becomes female. Female snails are usually larger than males.
Snails have occupied practically every type of habitat that supports animal life. Dehydration appears to be the greatest danger for terrestrial snails, while predation is the greatest danger for marine snails. Bieler has estimated that 53% of all snail species are prosobranchs, largely marine, 4% opisthobranchs, entirely marine, and the remaining 43% pulmonates, terrestrial and freshwater. In intertidal zones, numbers of proso-branchs such as the common periwinkle Littorina lit-torea is immense. According to Abbott, Littorina probably reached North America from Europe on driftwood “before the time of the Vikings” (about AD 1000) and gradually extended its range from Newfoundland to Ocean City, Maryland. In exchange, about 100 years ago northern Europe was invaded by the common slipper shell Crepidula fornicata, which has proliferated to the point of being a pest of English oyster beds.
Shell collecting has been a popular hobby for about 200 years, and the most attractive and valuable shells are those of snails. Visitors to the beaches of southwest Florida can hardly avoid becoming collectors, the shells are so varied and abundant. Malacologists have mixed feelings about shell collecting. No matter how rare or how beautiful, a shell that lacks a label specifying date, place, conditions, and name of collector is scientifically worthless.
A number of snails are of culinary interest, especially in France and in French restaurants worldwide. Escargot are usually the large land snails Helix pomatia or Helix aspersa, both often the subjects of biochemical studies. Helix aspersa, from the Mediterranean, has escaped and multiplied in Charleston, South Carolina and other southern towns. Called the speckled garden snail, these animals can be prevented from destroying garden plants by using them as a table delicacy. In Burgundy, France, snails are served with garlic butter and much discussion of the proper wine to accompany them.
Marine snails are edible also, although not as popular as marine bivalves such as scallops and oysters. Abalones are also called ormers, and furnish a kind of seafood steak in coastal regions. After eliminating the visceral mass, the meat is tenderized with a wooden hammer (“pas d’ormeau sans marteau”), and is often fried or en blanquette, a white stew. The foot of the whelk Buccinum undatum is cooked and served either cold or warm in a white wine sauce.
Resources
BOOKS
Brusca, Richard C., Gary J. Brusca, and Nancy J. Haver. Invertebrates. Sunderland, MA: Sinauer Associates, Inc., 2003.
Chase, Ronald. Behavior and Its Neural Control in Gastropod Molluscs. New York, NY: Oxford University Press, 2002.
PERIODICALS
Bieler, R. “Gastropod Phylogeny and Systematics.” Annual Review of Ecological Systematics 23 (1992): 311-338.
OTHER
The Academy of Natural Sciences Philadelphia. “Malacolog: A database of Western Atlantic Marine Mollusca.” 2005. <http://www.malacolog.org/> (accessed November 27, 2006).
The American Malacological Society. “Home Page.” <http://www.malacological.org/> (accessed November 27, 2006).
Carl S. Hammen