A telltale trail of slime revealed where the slug had spent the early morning hours working its way from one side of the broccoli leaf to the other during the night. The past two days of rain opened the opportunity for slugs to emerge from their hiding places in the mulch and soil surface beneath my flowers and vegetables. Ordinarily they are most active at night, or not at all if the humidity is too low. Turning the leaf over, an inch-long slug was revealed. It was a grey garden slug (Deroceras reticulatum), a native of Europe and North Africa, one of eight introduced species found in Wisconsin.
This is the most common slug of Western Europe, considered to be a synanthrope, like dandelions and chipmunks: species that benefit from an association with people. It also is the most common slug in my garden, and a huge pest of ripening tomatoes late in the summer. Following rainy weather, I can rarely find a ripe tomato that doesn’t harbor one or more slugs that have eaten through the skin into the flesh, where they can feed leisurely, less concerned about dehydration.
In Wisconsin, we have about 180 species of slugs and snails, members of the Gastropod class of Mollusks. Over 100 are native land snails, and more than 65 are native aquatic snails, but there is only one native slug, the Carolina mantleslug (Philomycus carolinianus). The Carolina mantleslug is not likely to be found in your garden. It prefers forested habitat, where it hides under bark of trees or in rotting logs, and often is found feeding on older mushrooms or other fungi. You probably already know that slugs look a lot like snails without shells, and that’s pretty much the difference.
Slugs have a thickened area on their backs, called a mantle, which in snails develops into a shell made of calcium carbonate. Slugs also produce calcium carbonate, but not organized into a protective shell. The shell, of course, provides land snails much protection from dehydration, but not much protection from predation. Most predators of slugs will also eat snails, shell and all. In fact, terrestrial snails are an important source of calcium for many birds, used in producing their egg shells. Given their soft, moist bodies, it should be no surprise to learn than most mollusks are aquatic, and protected by shells, which, for many aquatic mollusks, are hard enough to deter many predators. Land snails and, especially, slugs are not only vulnerable to predation, but must avoid exposure to drying, and, therefore, keep their bodies covered by protective mucus. Actually slugs and snails produce two kinds of mucus. One kind is essentially the same slimy stuff we produce in our noses, and which protects our most sensitive tissues from drying. It performs the same protection against dehydration in gastropods. This mucus is also a barrier against the invasion of microorganisms that might be harmful. The other kind of mucus produced by gastropods is slippery, and allows them to slide their soft bodies over rough or dry surfaces. A slug can even cross the sharp edge of a razor without cutting itself, so moving over sandy soil or a bristly leaf surface is no big deal, as long as it is sliding on a thin carpet of mucus. It was a trail of this mucus that led me to find the grey garden slug on the broccoli leaf.
Many things in nature have provided the basis for myth and legend that was passed down from generation to generation in different cultures. Snails are no exception, and their mucus was part of what gave rise to some of the symbolic myths. Without the ability to produce mucus, land snails and slugs would be unable to move, except on cloudy days or at night, or over very wet surfaces. Some legends see the slime they produce as their means to self-motivation, the ability to move, to explore, to venture forth. Their protective mucus provides a means of girding against things that might harm or kill them, much as we must protect ourselves against those who would damage our reputations by harmful words, or our health by harmful agents. Snails and slugs, therefore, symbolically represent motivation, ambition, even adventure, as well as self-reliance and confidence. Some Native American cultures thought the mucus trail left by snails and slugs had healing properties, and some even would hold a slug in their mouth to ease a toothache. The Osage Indians traced their origin in myth to a “father snail.” It must be noted, however, that while some cultures considered the slow, careful movement of slugs and snails to be a virtue, taking time to experience life to its fullest, Christian mythology considered them to be symbolic of slothfulness.
As might be expected, the snail shell is also symbolic. The ability of the snail to fabricate its own house provides further symbolism of self-reliance. The snail produces its house, modifies it as needed, carries it where it goes, and withdraws into it when threatened. Marine snails even have an operculum, a door to close the opening of their shell. Can you think of any other species so marvelously equipped? Moreover, the spiral of the shell parallels the spiral found repeatedly throughout nature. Aztecs thought the spiral symbolized the moon and its cycles, or the fertility cycles of women. Overall, they saw the snail as symbolic of time, adaptability, and transition. In ancient Egypt the spiral, and the snail by association, was symbolic of the spiral of life such as we experience as we age, or perhaps as we see in evolution of form and function. Because snails emerge after rain, ancient cultures associated them with Earth’s fertility, and seasonal cycles of agriculture. Perhaps Dr. Seuss had the environmental symbolism of the snail in mind when he wrote The Lorax, first published in 1971, and still one of the most popular children’s books. Although this lyrical book warns of the environmental consequences of unrestrained consumption of natural resources, it continues to delight children as young as three or four. In the story, the now remorseful mythical Once-ler, whose actions precipitated the ecological disaster described in the book, shares his story with a young boy for the price of “15 cents, a nail, and the shell of a great-great-great-grandfather snail.”
Snails and slugs are primitive creatures that haven’t changed greatly in millions of years. Snail shells, especially those in the marine environment, leave good fossil records that date to at least 500 million years ago. It is a good bet that slugs have been around as long, but they leave very little fossil evidence. The earliest snails (and slugs, if they were present) were aquatic, living in the oceans during the Cambrian period. Perhaps the best example is the chambered nautilus, valued for its beauty to the point of being collected to near-extinction, and little changed in 500 million years. There were no multicellular organisms on the land at that time, as far as we know. Snails glided (some could swim) around the shallow ocean floor, grazing on immobile algae or other primitive plants and animals. During this period, the digestive tract of snails developed into a flow-through system, with food taken in at the mouth and wastes released at the other end of the evolving gut. Gills evolved. Fertilization, which in primitive gastropods occurred in the external environment when eggs and sperm were released into the water, similar to fish and amphibians, became more sophisticated, occurring internally. This would become quite important for later adaptation to land, especially for an organism that could not move very fast or far to reach the water needed to aid its reproduction. Land adaptation by gastropods began about 350 million years ago, during the Carboniferous period. At this time, the land was dominated by primitive plants such as ferns and horsetails, many much larger than their relatives today. Warm, tropical conditions in the mid-latitudes, with accumulating organic matter that would later become coal beds, led to high oxygen levels in the atmosphere, and evolution of larger animals than found today, including insects, arachnids, and amphibians. Somewhat later, amphibians would give rise to the first reptiles, but dinosaurs would not appear for another 100 million years. By this time, warm, shallow seas supported extensive and elaborate marine ecosystems with great diversity, including sharks, and a rich diversity of mollusks, including a great many gastropods.
Terrestrial gastropods preceded mammals and birds by over 200 million years. Significantly, the ferocious dinosaurs that once terrorized Earth’s terrestrial fauna did not survive, but the lowly snail and slug have somehow managed very well. The total number of extant gastropods is estimated at 100,000, perhaps more, a diversity second only to insects. Most are aquatic, and are found primarily in salt water, but many have adapted to freshwater as well as to land. Presently, there are 62,000 species described. Compare that, for example, to only 10,000 known species of birds, and less than 6,000 species of mammals. What allows land snails and slugs to be so successful, some even found in deserts?
Although snails and, even more so slugs, appear quite limited and vulnerable to predation and dehydration, they are among the very first land animals that still exist today, along with amphibians, and arthropods such as insects, spiders, and millipedes. Some are amphibious, able to exist on land and in either salt- or fresh water part of the time, depending on the species. It seems likely that early adaptation to land occurred in the intertidal zone, where the ability to survive from one high tide to the next opened vast new resource opportunities. Most land snails and slugs have no gills, which, of course, would be useless out of water. Instead, they have evolved a lung for gas exchange. But ability to produce specialized mucus is certainly one of the most critical characteristics that enabled development of terrestrial gastropods.
Many of the anatomical advances in snails and slugs occurred while they were still confined to their aquatic environments. Adaptations included a kidney, a stomach, and reproductive structures, as well as eyes and a primitive brain. At your next opportunity, carefully observe a snail or slug. You will see two prominent tentacles, each with a dark spot at the tip. Those are the snail’s or slug’s eyes. The most primitive eye is a cup-like depression in the outer skin with light-sensitive cells at the bottom. As eyes evolved, the opening of the cup became narrower, allowing sharper focusing, like a pinhole camera, and eventually was completely closed with a lens. By moving its tentacles, the snail can “see” in black and white a vague outline of things near it. If you touch a tentacle, the snail or slug will withdraw it quickly, a good indication that nerves, muscle, and tactile sensitivity are part of the gastropod’s adaptability. Nerve cells connect the eyes and other sensitive cells to a primitive brain. Aquatic snails and slugs also evolved eyes, but they are at the base of the tentacles.
Most terrestrial snails and slugs have a second pair of tentacles that are beneath the eye stalks, and point down. As with the longer tentacles, the shorter ones can be retracted quickly. While these shorter tentacles are sensitive to the texture of the surface, they may be more useful in detecting odors. They have cells that absorb volatile chemicals from the air, giving the snail or slug the ability to smell, which is useful in search of food or mates.
The sex lives of snails and slugs are unusually sensual, and in some, rather bizarre. Most species of terrestrial gastropods are hermaphroditic, being fully functional both as a male and a female. Sex among snails and slugs is slow, cautious, and sometimes rough. When completely successful, each partner inseminates the other. The rough part is during courtship, which can last for hours. Some species can shoot their partner with one or more “love darts.” The love dart is made of calcium carbonate, the same material as the shell, coated, of course, with mucus. The dart, or darts, is delivered before mating. In what can be regarded only as a violent act, a snail or slug will thrust its love dart into its partner, where the dart penetrates and remains embedded in the soft flesh. Evidence indicates that this is painful to the recipient. The dart is not the penis. While its purpose is not completely understood, the dart appears to deliver hormones through the mucus coating that causes the snail’s or slug’s eggs to be receptive only to the sperm of the dart thrower. Because snails and slugs often mate with multiple partners, this gives the dart thrower a huge paternity advantage, but it is not something that benefits the partner that gets darted. In fact, once darted, its life is shortened.
In part because of love darts, snail and slug sex is cautious, with several tentative advances, touching, withdrawal, and advancing, always slow and slimy. If each finds the other acceptable, their two moist bodies eventually unite in a remarkably intimate embrace. It is during this courtship that a love dart might be delivered. Love darts aside, while intimate, giving and receiving during sex, as with heterosexual partners, is not equal. Sperm is cheap and there is an advantage in delivering it to as many partners as possible to advance the male’s genetic lineage. Eggs and the nurturing of offspring, however, are costly, so there is reason for females to be selective in the qualities of the sperm donor to ensure that the best genes are passed to her offspring. This give-and-take negotiation with slugs and snails can go either way, with either or both being dart throwers and sperm donors. In mating of slugs and snails, the slowness of the copulation may stem from the maneuvering of each partner to deliver a love dart and its sperm without necessarily opening itself to receiving sperm from the other. This foreplay can go on for hours. The stakes are even more increased because gastropods so engaged are more vulnerable to predation.
Mating typically occurs during warm, moist weather. If all goes well, the snails or slugs will maneuver their reproductive pores together, penetrating each other with their penises to deliver sperm to the other’s vagina. Both the penis and the vagina are within the same sexual pore, so attempting to deliver sperm without receiving same must be very tricky. As many as 100 eggs may be fertilized. There is nothing very uncommon here, including hermaphroditic anatomy, which occurs in many worms and arthropods, including earthworms. Some species of snails and slugs, however, can fertilize their own eggs, perhaps if a mate is not handy, much as some plants can pollinate their own flowers if cross-pollination does not occur. Eggs of terrestrial gastropods are usually buried in the soil surface or litter, and adults may mate again in a few weeks, most likely with a different partner. If weather is favorable, eggs can develop in two to four weeks. Snail shells develop even before eggs hatch. Tiny snails and slugs can develop rapidly, but in many species, they won’t look much like an adult for several weeks, and some species may require three or more years to reach maturity. Large species of snail can live in the wild for ten or more years. In captivity, some large snails have lived for 25 years. As with many of their characteristics, longevity of gastropods is surprising.
Terrestrial snails and slugs are mostly vegetarian. Most are generalists, but some specialize on consumption of algae, bryophytes, fungi, leaves, fruits, or other plant structures. In my garden, I’ve plucked them from tomatoes, melons, cucumbers, peas, beans, and even potato tubers. Many feed on decaying organic matter, and on fungi associated with decomposition. Some species are carnivorous, although most that favor a non-vegetarian diet are found in fresh- or salt water, and some, including a few terrestrial species, will eat various invertebrates as well as plant tissue. Some eat other snails and slugs, some prefer earthworms, or carrion of almost any origin. It should be no surprise that any diverse class of organisms that has been around for even half as long as gastropods will have evolved to fill a wide array of niches, everything from specialized predators to generalists that consume nearly anything organic. Speaking of specialization, some slugs and snails are involved in some pretty complex predator-prey and host-parasite relationships. One of the most bizarre involves moose.
A nematode parasite of white-tailed deer, usually called “brainworm,” lays its eggs in the meningeal tissues in the central nervous system. Most white-tails are strongly resistant to the nematode and show few, if any, symptoms other than perhaps mild meningitis. The brainworm may reside in a white-tail for years. As the nematode eggs hatch, the minute larvae enter the bloodstream, travel to the lungs, then up the respiratory track, and are swallowed. Passing through the stomach, the larvae are released with feces. Slugs and snails feeding on the feces will become infected by the nematode larvae, which then develop through their second and third stages, apparently causing relatively minor problems in the gastropods. If the slug or snail moves onto vegetation to feed, it may be consumed by deer or another ungulate such as moose, elk, or mule deer. If consumed by a white-tailed deer, the larvae pass through the stomach wall into the central nervous symptom, eventually maturing and completing their life cycle by laying eggs in meningeal tissue, again without harm to the white-tail. If ingested by a different ungulate, especially moose, which have little resistance to the nematode, the larvae move from the stomach to the brain, where severe neurological symptoms result. These include extreme weakness, partial or total blindness, loss of fear, and nearly 100 percent mortality. Nematode infection of susceptible ungulates occurs only when their ranges significantly overlap those of white-tails which are the primary host. As white-tail range extends northward into moose range, brainworm is a growing problem for moose. Interestingly, it has been suggested that because red-bellied snakes are important predators of slugs and snails, the more red-bellied snakes, whose range extends into the southern areas of moose range, the fewer brainworm-infected moose.
A similar example can involve people. The rat lungworm, also a nematode, infects the lung tissue of rats. Larvae are ingested and passed with droppings that may then come into contact with slugs and snails, where the larvae develop. When slugs and snails are consumed by rats, the larvae invade the rats’ lungs, where the larvae mature. If people eat the infected slug or snail, they can become infected, although recovery usually occurs without treatment.
An important disease of people, called “schistosomiasis,” also known as “snail fever,” is caused by a parasitic flatworm. The symptoms of infection include abdominal pain, bloody stool or urine, and diarrhea. Untreated, the flatworm can cause kidney failure, or bladder cancer. The flatworms live in freshwater snails, and larvae are released into water. Those bathing in the water may become infected through their skin, with initial symptoms, if any, of inching and inflammation. The larvae move through the body to the stomach and lower digestive track. Symptoms of schistosomiasis may not develop for several weeks or months. It is estimated that as many as 100,000 people or more die each year from the disease, which occurs in tropical regions of Africa, Asia, and South America.
Slugs and snails are host to dozens of other diseases and parasites, including viruses, bacteria, and different species of nematodes and other roundworms, and various parasitic flies, but most cannot infect people. Indeed, based on archeological evidence, slugs and snails have been consumed by people for at least 6,000 years. Relatively few species of snails are commonly eaten by people today, and nearly all are raised on snail farms. The escargot industry in United States is worth an estimated $300 million a year. In France, Spain, and Portugal escargot farming is an even bigger industry.
When you see a slug or snail, remember that you are seeing a very successful descendant of an ancient lineage of organisms, “older than the hills,” with a lineage longer than any other organism in your back yard. They were among the first organisms to adapt to the terrestrial environment, even before insects or amphibians. They witnessed the rise and fall of dinosaurs, and the advance and retreat of several great ice sheets. Perhaps most remarkably, they survived at least five “mass extinctions” since gastropods first crawled across the floor of ancient seas. It is estimated that up to 90 percent of all species that lived on Earth in the last 500 million years have gone extinct, but the diversity of slugs and snails has grown! In the last hundred years, they have endured the latest great extinction that continues today: the unprecedented loss of species resulting from human activities, which seems likely to be the greatest insult life on Earth has ever experienced. If history is any indication, the slugs and snails will outlast humans. What a story they have to tell, if the snail in your backyard could talk. Wouldn’t you like to hear about its great, great, great grandfather? Oliver Wendell Holmes tells part of the story in his epic poem.
The Chambered Nautilus
This is the ship of pearl, which, poets feign,
Sails the unshadowed main,—
The venturous bark that flings
On the sweet summer wind its purpled wings
In gulfs enchanted, where the Siren sings,
And coral reefs lie bare,
Where the cold sea-maids rise to sun their streaming hair.
Its webs of living gauze no more unfurl;
Wrecked is the ship of pearl!
And every chambered cell,
Where its dim dreaming life was wont to dwell,
As the frail tenant shaped his growing shell,
Before thee lies revealed,—
Its irised ceiling rent, its sunless crypt unsealed!
Year after year beheld the silent toil
That spread his lustrous coil;
Still, as the spiral grew,
He left the past year’s dwelling for the new,
Stole with soft step its shining archway through,
Built up its idle door,
Stretched in his last-found home, and knew the old no more.
Thanks for the heavenly message brought by thee,
Child of the wandering sea,
Cast from her lap, forlorn!
From thy dead lips a clearer note is born
Than ever Triton blew from wreathèd horn!
While on mine ear it rings,
Through the deep caves of thought I hear a voice that sings:—
Build thee more stately mansions, O my soul,
As the swift seasons roll!
Leave thy low-vaulted past!
Let each new temple, nobler than the last,
Shut thee from heaven with a dome more vast,
Till thou at length art free,
Leaving thine outgrown shell by life’s unresting sea!
© 2016 Alan Haney and http://www.StarBimaging.wordpress.com All rights reserved.