SEA SLUGS, SNAILS, AND LIMPETS: Gastropoda

PHYSICAL CHARACTERISTICS

The most conspicuous feature of many gastropods is the shell. Although sometimes flattened and caplike, most shells are cone-shaped shelters into which they can completely withdraw their bodies. The single, lopsided shells are usually made up of spiraled tubes called whorls (worlz). The shells are lopsided because whorls form below one another, instead of around each other. In spite of the fact that they are lopsided, the shell is carried over the back so that its weight is evenly balanced over the body. But not all gastropods have lopsided shells. In sea snails, known as cowries, the last whorl completely covers all the others and appears to be symmetrical. The left and right sides of symmetrical objects are the same size and shape, giving them a balanced, rather than lopsided, look. Young limpets have a distinctly lopsided and coiled shell, but as they develop into adults, the shell becomes smooth and symmetrical, resembling a Chinese hat.

The whorls form around a central line, or axis. Inside the shell, the whorls turn around a central column of shell called the columella (kol-yuh-MEL-uh). The smallest whorls are the oldest and were made while the gastropod was still in the larval stage. The last and largest whorl is the newest and ends at the opening of the shell, where the head and foot stick out. The spiraled stack of whorls above the opening is called the spire. If the whorls develop counterclockwise, the shell is said to be left-handed, while clockwise whorls are right-handed. To determine if a shell is left- or right-handed, stand the shell up so that the spire is pointed up and the opening faces toward you. If the shell opens to the right of the spire, it is said to be right-handed; if the opening is on the left, it is left-handed. Most gastropods have right-handed shells, while some species are left-handed. A few species have individuals that are either right- or left-handed. Some marine and land snails have a flat, horny disc above the back of their foot called the operculum (o-PUHR-kye-lem). The operculum is usually made of the same tough material that covers the outside of the shell. When the head and foot are withdrawn into the shell, the operculum follows to form a tight cover over the shell's opening.

In species with shieldlike shells, such as keyhole limpets and abalones, there is a notch of one or more holes in the shell that allow a current of oxygen-carrying water to reach the body and eggs, sperm, and wastes to be carried away. These animals live on rocks in the pounding surf, and their low shells and muscular feet help to keep them from being knocked off and washed away by the tides.

Gastropod shells, if present at all, come in a wide array of colors, patterns, and surface sculptures. The surface of the shell is sometimes pearly or has a highly polished, porcelainlike quality. A thin or thick covering usually protects the outer part of the shell. The shells of cowries lack this protective coating and are instead covered by the mantle. In sea slugs, the shell is small, thin, and found either inside or outside the body, if there is a shell at all. The mantle is the fleshy organ located between the body and the shell. It makes the shell by producing a hard mineral called calcium carbonate.

Like other mollusks, the bodies of gastropods are soft and fleshy. The bodies of sea slugs are often brilliantly colored and patterned and sometimes covered with fleshy, stinging outgrowths. In snails, the head and foot are withdrawn into the shell by a powerful muscle that is attached inside to the columella. The heads of all gastropods are more distinctive than in most other mollusks and may or may not have eyes and one or two pairs of tentacles. The mouthparts include a radula (RAE-jeh-leh). The radula is a tonguelike structure with rows of extremely hard teeth that are used to scrape food off rocks or pull and tear at flesh. The muscular foot has been changed in some groups to help with swimming or burrowing. The mantle forms a cavity that lies in front or to the right of the muscular foot. Inside the mantle cavity is a comblike gill used for breathing. The digestive tract is u-shaped. Both it and the nervous system are twisted. Gasropods have one or two kidneylike organs that filter out wastes from the blood. These organs, along with the anus, open into the mantle cavity above or near the head. The anus is the opening at the end of the digestive tract where solid wastes leave the body.

GEOGRAPHIC RANGE

Gastropods are found on all continents and in all oceans.

HABITAT

Gastropods live in a wide variety of habitats in the ocean, on land, and in bodies of fresh water.

DIET

Gastropods eat many kinds of foods. Some species filter out bits of plants, animals, and other organisms floating in the water. Many scrape algae (AL-jee) or crustlike animals off rocks in tide pools and elsewhere on the ocean bottom. Others prey on all kinds of freshwater or marine animals. Most species living on land eat both living and dead plants. Some land snails prey on earthworms or other snails.

BEHAVIOR AND REPRODUCTION

Like most animals, gastropods must feed, fight, flee, and mate. To do all this they have developed many different behaviors. Most species sense their world through the presence of certain kinds of chemicals produced by their foods and other members of their own species. Aquatic species regularly move up and down in the water at certain times of the day in search of food and mates. Depending on the species, they may become active during the day or at night, whenever they are least likely to be attacked by predators.

Gastropods usually require both males and females to reproduce, although some species are hermaphroditic (her-MAE-fro-DIH-tik). Hermaphroditic individuals either have the reproductive organs of both sexes at the same time or start out as males and later become females. In most species, the males transfer sperm, or packets of sperm, directly into the female's reproductive system. The sperm is stored in a special sac. Fertilization takes place only when the eggs are laid. The eggs are laid or released individually or in groups. The eggs of species living in water usually hatch into larvae (LAR-vee) known as veligers (VEL-ih-jerz). The veliger has a special, round feeding organ lined with hairlike cilia (SIH-lee-uh) that it also uses for swimming. It also has a foot, shell, and other adult features. During this stage a single, powerful muscle, usually permanently twists the body and shell, if present. In some snails, including those living on land, the veliger stage takes place inside the egg, which then hatches into a tiny version of the adult.

GASTROPODS AND PEOPLE

For centuries, gastropod shells, especially cowries, have been used as money. The flesh of some gastropods, especially the muscular foot, is often considered a very tasty treat. People also use the shells of some gastropods to create works of art, as well as bowls, fishhooks, buttons, beads, and other forms of jewelry. Shell collecting has been popular for centuries, with some species commanding high prices.

GASTROPODS DO THE TWIST

Gastropods are the only animals that twist as they develop. The mantle and body of the veliger rotates 90° to 180° in relation to the foot. This rotation twists the digestive tract into a u-shape and turns part of the nervous system into an imperfect figure eight. The twisting of the body and its developing shell is referred to as torsion (TOR-shen). Torsion in gastropods is caused by the uneven pull of a single muscle inside the body.

CONSERVATION STATUS

One thousand one hundred eighty-two (1,182) species of gastropods are listed by the World Conservation Union (IUCN) as threatened or endangered.

NO COMMON NAME (Corolla spectabilis): SPECIES ACCOUNTS

Physical characteristics: The body of this small, jellylike species is transparent. The dark contents of its digestive tract are clearly visible from the outside. It does not have a shell. Large, winglike plates stick out from the sides of the body. Adults measure up to 3 inches (80 millimeters) long, while the wing plates span 6 inches (160 millimeters) across.

Geographic range: This species is found in the Atlantic and Pacific Oceans, between the latitudes of 40° North and 5° South.

Habitat: This species is found near the surface of the open sea.

Diet: Corolla spectabilis eats tiny animals floating in the water.

Behavior and reproduction: Corolla spectabilis sometimes live in large numbers at the water's surface. Food is captured with sheets of a sticky material made by special glands along the edges of the wings. When frightened they can swim away very quickly.

The species is hermaphroditic. Adults mature as males and then later become females. Their mating behavior is unknown. Eggs are produced in long, sticky strings measuring up to 1.6 feet (0.5 meters) long.

Corolla spectabilis and people: This species is not known to impact people or their activities.

Conservation status: This species is not considered threatened or endangered. ∎

ROMAN SNAIL (Helix pomatia): SPECIES ACCOUNTS

Physical characteristics: This species is the largest snail in Europe. Its ball-like shell is creamy white with spirals of brown bands. It measures up to 2 inches (50 millimeters) across. The body is gray with paler bumps.

Geographic range: Originally from Central and Southern Europe, this species now also lives in the United Kingdom, Scandinavia, and Spain.

Habitat: They live in woods, hedges, and weeds up to elevations of 6,500 feet (2,000 meters).

Diet: This species eats living plants.

Behavior and reproduction: Roman snails hibernate in shallow holes during the winter. They have the ability to forage as far as 150 to 300 feet (50 to 100 meters) and still find their way back.

Males and females have elaborate courtship behavior lasting several hours. Batches of up to forty eggs are laid in the ground during the spring and summer. They take from three to five weeks to hatch. Snails reach adulthood in three or four years and live up to ten years.

Roman snails and people: They are eaten by people, especially in France, but are considered pests on grapes grown to make wine.

Conservation status: Roman snails are not considered threatened or endangered. ∎

SHIELD LIMPET (Lottia pelta): SPECIES ACCOUNTS

Physical characteristics: The surface of the hatlike shell has fine growth rings and ribs radiating out from the center like spokes on a wheel, with a high point slightly toward the front. The color varies from blue black to light brown, with or without white markings. Adult shells reach up to 2.3 inches (60 millimeters) across.

Geographic range: They are found in the Northern Pacific Ocean, from Honshu, Japan, across to Baja California, Mexico.

Habitat: They live along rocky coastlines, in the middle of the area affected by high and low tides.

Diet: Shiled limpets eat a variety of kinds of algae.

Behavior and reproduction: This species is active at night. The color and surface texture of individual shells gradually change as the limpet moves onto different surfaces.

Males and females larger than 0.39 inches (10 millimeters) reproduce in spring. Smaller individuals, as well as those living in the water just beyond the tide action, reproduce throughout the year. Eggs and sperm are released into the water where fertilization takes place.

Shield limpets and people: This species was once an important food source for ancient peoples living along the coast.

Conservation status: Shield limpets are not considered threatened or endangered. ∎

TOP SHELL (Trochus niloticus): SPECIES ACCOUNTS

Physical characteristics: This species has a large, cone-shaped shell with a wide base measuring 1.6 to 3.9 inches (40 to 100 millimeters) in height. The shell is marked with wavy, purplish pink lines These markings are sometimes covered by crustlike growths of algae.

Geographic range: This species is found in the warm, tropical waters of the Indo-Pacific.

Habitat: They live on coral reefs and in lagoons.

Diet: This species eats threadlike algae and diatoms.

Behavior and reproduction: Top shells are active at night.

Eggs and sperm are united in the water, where the produced larvae spend only a brief period of time.

Top shells and people: The flesh is eaten, and the shells are used for decoration.

Conservation status: Top shells are not considered threatened or endangered. ∎

GEOGRAPHY CONE SHELL (Conus geographus): SPECIES ACCOUNTS

Physical characteristics: The shell spire is flat with knobby whorls. The shell opening is long and slightly expanded toward the front. The outer surface is marked with light, gold brown markings.

Geographic range: This species is found in the warm, tropical waters of the Indo-Pacific.

Habitat: They live around coral reefs, in sand, and on chunks of broken reefs.

Diet: They eat fishes, worms, and other snails.

Behavior and reproduction: They hunt at night. The radula has a few poison-injecting teeth that are used like harpoons and shot from the end of a long, trunklike proboscis. Fishes and other prey are hooked by the teeth and then pulled in.

The eggs are fertilized inside the female's body.

Geography cone shells and people: The venom from their bite is very toxic and can be fatal.

Conservation status: Geography cone shells are not considered threatened or endangered. ∎

FOR MORE INFORMATION

Books:

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 2003.

Gordon, D. G. Field Guide to the Slug. Seattle, WA: Sasquatch Books, 1994.

Meinkoth, N. A. National Audubon Society Field Guide to North American Sea Shore Creatures. New York: Alfred A. Knopf, 1981.

Periodicals:

Davidson, T. "Tree Snails. Gems of the Everglades." National Geographic (March 1967): 372-387.

Hamner, W. M. "Blue-water Plankton." National Geographic (October 1974): 530-545.

Web sites:

"Abalone." http://seafood.ucdavis.edu/pubs/abalone.htm (accessed on May 2, 2005).

"Class Gastropoda (Gastropods, Slugs, and Snails)." http://animaldiversity.ummz.umich.edu/site/accounts/information/Gastropoda.html (accessed on May 2, 2005).

"Gastropods." http://www.mesa.edu.au/friends/seashores/gastropods.html (accessed on May 2, 2005).

Hardy's Internet Guide to Marine Gastropods.http://www.gastropods.com/ (accessed on May 2, 2005).