Families: Amphibolidae, Ellobiidae, Enidae
The order Pulmonata is very different from other Gastropods. Unlike other Gastropods, pulmonates lack gills and instead have a vascular sack within their shell. This vascular sack is like a lung in which the muscles in the mantle cavity expand and contract. This lung gives them a unique advantage of being able to live on land. In Greek the word Pulmonata refers to the lung, this can be found in words such as pulmonary. Pulmonates are the land snail that we unfortunately step on. However, certain slugs and freshwater snails are included in the order Pulmonata (see figure 1).
Land snail, Helix pomatia
Land slug, Limacus flavus
Pulmonates have been documented as far back as the Carboniferous period. A cool thing about pulmonates is that they don’t have an operculum like other Gastropods. Pulmonates can be found anywhere in the deserts or in moist places such as swamps or creek beds. The marine pulmonates can be found in muddy freshwater streams.
As you can see in this picture, the lung cavity is located in the mantle. The snail is hermaphroditic and has one genital pore. There is a respiratory pore on the top of the snail which is used for the intake and outtake of air. There is a salivary gland and duct that is used during consumption of food. Details of consumption, reproduction, and excretion are found in section III.
III. FORM AND FUNCTION
All pulmonates use a radula to eat. A radula (see figure 3) is a protrusable chitinous structure that contains microscopic ‘teeth’ called cuticulae. Most pulmonates are herbivores and their diet includes: fruit, stems, vegetables, soft bark, fungi, and algae. It is said that one can faintly hear a snail ‘munching’ on its food – meaning that you can hear the snail’s radula breaking and scraping apart food.
A radula of the mud snail
A few pulmonates are carnivores and they prey upon earthworms and other snails. See figure 4.
A rosy wolf snail eating a Bradybaena similaris
Digestion is extracellular and nutrition is carried to the cells by the circulatory system.
Most pulmonates are monoecious and can produce both spermatozoa and ova. Pulmonates reach sexual maturity at the age of six weeks to five years depending on the species. Pulmonates perform courtship before mating (see figure 5). Courtship can last anywhere from two to twelve hours! Once they approve of each other, one snail inseminates the other. The inseminator will insert his penis into the genital pore of the other. Fertilization occurs internally. Pulmonates will bury their eggs (a brood can be as many as 100) in shallow topsoil or under a log where the environment is moist and warm. The incubation is two to four weeks if the weather is good. A juvenile snail will hatch and the cycle begins all over again.
Courtship between two Helicid.
Two pulmonates mating
Two Helix aspersa mating.
Pulmonates are born with a very thin, transparent shell. The shell is weak because it lacks calcium. So the first thing a newborn baby snail does is consume its egg. There are reports of baby snails eating other eggs around it, even if the other eggs contains baby snails. As the snail matures, color will slowly develop in the snail until the juvenile snail resembles the adult (see figure 6). The shell grows with the snail. The snail secretes calcium carbonate along the edges and inner side of the shell. Snails need a good supply of calcium in order to keep their shell strong. Low calcium levels along with low pH levels in the snail’s environment can cause a snail’s shell to be thin and cracked. The snail has many predators ranging from birds, snakes, insects, small mammals, and even other snails.
Awww! A baby pulmonate.
Pulmonates have a single nephridium (kidney) which helps regulate waste. They also have a well developed circulatory system which transports waste from the cells. Pulmonates digest extracellularly and they have a stomach, intestine, liver, and kidney. Food that is unable to be processed is excreted in a solid form through the excretory pore. One thing that makes pulmonates special is that their excretions are solid unlike most of their Gastropod counterparts. Since most pulmonates are terrestrial, they need to conserve as much water as possible (see figure 7).
Helix aspersa defecating
IV. IMPACT ON WORLD/HUMANITY
Snails have had an impact on humans for thousands of years. They have been seen as the deadly sin of sloth in the Judeo-Christian culture, a symbol of the moon and rebirth in the Aztec culture, and as a representation of one’s self in dreams by Carl Jung and his followers. Snails have been a dish for people all around the world. For many people the snail is a delicacy and a good source of protein. Snail consumption can be found all over Europe, Africa, and parts of North America (see figure 8). Snails can also be pests to farmers. Snails eat plants that can be valuable to agriculture and the market. If the snail population grows out of control then that can spell disaster for the farmers.
An escargot dish
This journal article is about the effects of dopamine, ergometrine (a dopamine blocker), and serotonin on the crawling speed of a Helix lucorum. These three chemicals seemed to have directly influenced the snails’ ability to crawl and its speed. Dopamine caused the snail to slow down or stop. Ergometrine caused a speed up in movement along with an extension of the sole. Serotonin stimulated movement without any change in the sole. A direct link to the journal article: http://jeb.biologists.org/cgi/content/abstract/204/9/1625
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