SLUGS. The mere word causes some to shiver in disgust. These creepy, slimy creatures are constantly labeled as repulsive, gross, and frighteningly horrible pests. Throughout my research, these observations and thoughts were found frequently. However, once I understood the biology and ecology of the land slug, I revered these mollusks for their greater scientific importance. Land slugs are a truly remarkable animal deserving our respect, not disgust.
When I was younger, I can distinctly remember them crawling out of the tiny crevices of our brick patio, slowly inching their way around my backyard. Night after night, in the damp, dark Maryland springtime they would emerge in our backyard. My family would be talking outside after dinner, the candles lit as the day dimmed. Suddenly, the peace would be broken by a piercing "Ahhhhhhhhhhhhhhh!" A terrorizing scream would be heard. The slugs had emerged.
My brother and I were mystified by the creatures and we performed our own scientific observations. We wondered how during the day not a single slug could be seen, but at night it was hard not to step on one. We followed the slithering creatures as they crawled sluggishly out of the cracks of the patio walls. Their slimy, dark bodies glistened when our flashlights shown upon them. The sizes of the slug in our yard varied from under an inch to four inches in length. We sought to examine the largest specimen we could find. The two tiny tentacles on the slug's head searched about as we neared. Sometimes we dared each other to pick one up. The slimy sensation of holding the slug's oozing skin of mucus was not pleasant. My brother and I would usually drop the 'icky' creatures in disgust.
Long before I learned of the scientific biology or ecological importance of slugs, I learned that salt would make slugs shrivel up. My doubting mind naturally questioned this fact and I decided to perform a scientific experiment. I was unaware that the slug's body was composed of soft substances, mostly water. I was also unaware that the salt would cause the slug's body to dehydrate and deteriorate, making the slug a hissing mass of desiccating fluids (2). So, one night I took the large container of salt, isolated a slug, and poured at least a cup of the tiny white crystals over the slimy organism. My eyes widened as the slug was transformed into an oozing lump of fluids. I ran away in disbelief.
While slugs were observed as interesting by my brother and me, we were well aware that others considered them gross. One book on slugs explains it best, "Many people are offended by slugs and snails. They find them ugly and dislike their cold, clammy bodies and the unpleasant slime they leave behind"(7). During my research I found that this concept was widely held. More than half of the resources I discovered pertaining to slugs dealt with their removal. From salting to stomping to drowning in beer to trapping to buying a duck, I learned how to rid the world of the slug. Cecil Adams, a newspaper columnist, stated in his column, "slugs are a leading cause of death in Seattle, owing to the effect that so many people are grossed out of existence" (2). The statement that slugs cause death is, of course, false, but the fact that slugs are perceived as repulsive is very common. One book on pest removal suggested that one should "crush them [snails or slugs] against the sides of a pot with a stick and replace the pot without removing the bodies. Crushed snails and slugs make the pots attractive to other snails and increase your catch" (8). Statements like this horrified me. I began to believe the slug was, indeed, a sick, gross, revolting, terrorizing, frightening pest that should become extinct.
While many resources dealt with slug removal, I was able to find some positive information about the land slug. This increased my knowledge and appreciation for the organism. To understand the slug's benefits, one must first understand the slug's biology. Slugs are members of the kingdom Animalia, thus they are eukaryotic, multi-cellar, and heterotrophic. Slugs are members of the phylum Molluska, thus they are invertebrates with soft bodies and circulatory, nervous, and digestive systems. Further classification of the slug is class Gastropoda, subclass Pulmonata, order Stylommatophora, suborder Sigmurethra, and a variety of families (4). Thousands of kinds of land slugs exist in the world, varying in size, shape, and color (3). Some slugs are extremely small, less than one inch in length, While others are over ten inches. Black, brown, white, yellowish, and gray are all hues that are attached to certain slugs. Slugs are extremely similar to snails except slugs do not possess the large shell that is attached to snails. Slugs do, however, have a shell that is hidden by the fleshy mantle on their backs (8).
The slug's body is a very interesting system that allows the organism to thrive in its environment. Slugs are able to move slowly with their soft, but muscular foot on their bellies. The waves of muscular contractions allow the slug to travel along a path of secreted mucus which the foot produces. Joined to this foot is the slug's head where two pairs of tentacles are located. One pair serves as optical tentacles. These "eyes" allow the slug to determine day or night and see dim shapes (3). The other pair of tentacles serves as sensory tentacles which feel the environment of the slug (4). Also on the head of the slug is the mouth. The mouth contains a tongue-like radula that can move back and forth carrying food to be eaten by the slug's tiny teeth (3). The rest of the slug's body is a lump covered by the mantle, a thin strip of skin. The mantle cavity lies between the mantle and the main body. This cavity allows slugs to take in water and air for respiratory purposes and excrete wastes. Slugs are covered with a tough, slippery skin that is not waterproof and must be kept wet for survival. Slugs prevent themselves from drying out by secreting mucus to protect their bodies (3). The slug is an excellent example of an organism that has been able to successfully adapt for survival (6).
Slugs live in a variety of environments but, in general, the environment must be damp and cool. As noted in my childhood observations, slugs are nocturnal. During the day slugs hide in damp, shady places - under stones, logs, rotting leaves, or between crevices. At night, when the environment is cool and damp, and there is no danger of their bodies drying out, slugs leave their hiding places to forage for food. In very cold weather, like other animals, slugs hibernate underground until warmer weather returns (3).
Slugs have an important place in the ecosystem of the world. Most slugs are herbivores, eating fungi, lichens, green plants, shoots, roots, leaves, fruits, vegetables, and flowers. Slugs are also known for being scavengers; eating decaying vegetation, animal feces, and carrion. Slugs are not as much in danger of being prey as other organisms are. Many potential predators of the slug do not eat it because of its sticky, slimy mucus (3). A German writer once questioned this fact and licked his slug-slimmed hand. His appetite was lost for days, proving that slugs do not taste good (7). Predators do, however, exist. They include the hedgehog, badger, shrew, mole, mouse, frog, toad, snake, carnivorous beetle, some birds, and some other mammals (9). Thus, the slug has an important place in the ecosystem and the food web of the environments in which it lives. Certain organisms depend upon the slug for its food, and the slug, in turn, depends upon other organisms.
Although some may think that the extinction of the slug would be "good riddance," the presence of the slug is very important. Ecologist Tina Teearu states, "Each and every being on earth has its place (a 'niche') in the ecosystem and together it works"(12). If the slug was to suddenly disappear, the ecosystems and food webs that contained the slug would be drastically affected. Ecologist William H. Amos explains, "the removal of any single species of animal from a food web has effects throughout the web" (1). Predators of the slug would have to search for new food sources. Others, too, would be affected by the disappearance of the slug. Wildlife ecologist Karl Studenroth states, "In ecosystems they [slugs] are important in breaking down decomposing materials faster and helping to release nutrients back into the overall system quicker" (11). Without slugs recycling decaying and fecal matter, soils would lose important nutrients, thus plants and crops would not grow as well. This, of course, would create a domino effect, affecting the entire ecosystem and food web of organisms.
Tina Teearu clarifies an important point, "Slugs may seem slimy and horrible to us, but we need them just as much as we need cows for milk or chickens for eggs." While many people think that the removal of the land slug species form the earth would be beneficial, the disappearance of slugs would have a negative impact on humans. In addition to making up the biodiversity of this world, slugs play important roles in the ecosystem that directly impact humans. For example, if the slug did not perform its important role as a decomposer, there would be a domino effect which would reach humans. As Karl Studenroth explains "nutrients would be locked up in dead organisms longer and unavailable to living organisms [without the slug's presence]." This "locking up of nutrients" would effect the fertileness of soil. Fertile soil is very important to human existence, as Tina Teearu states, "at the end of the end, absolutely everything we humans eat can be tracked back to soil." While gardeners and farmers may think that the disappearance of slugs would improve their gardens and crops from being destroyed by the pesky slug, they are wrong. If slugs were to become extinct, soils would be less fertile and, as William H. Amos states, "other species would be obliged to compensate for their absence." Meaning, other organisms deemed as "pests" would fill in for the slugs. Even master gardener Monica Strom, DTR admits that "slugs have a very important purpose" (10).
Slugs are also important to humans in lesser known ways. Currently the slug is being studied at the University of Washington. The biochemical properties and cellular mechanisms of the slug's mucus are under investigation. Researchers at the University believe that learning more about slug mucus could help in treating or curing human deficiencies involving mucus. Professor Deyrup-Olsen of the University of Washington explains, "This [the slug's mucus chemical make-up] has a potential relationship to human diseases in which mucus formation is abnormal. An example is the genetic based disease cystic fibrosis" (5). If the slug were to become extinct, it would halt this ongoing research.
In conclusion, land slugs are an important species on the Earth. They play significant roles in their ecosystems and serve humans in many ways. Through my investigation of the slug, I learned a major lesson about biodiversity on Earth: although an organism may appear disgusting and insignificant, every organism plays an extremely important role on Earth. Extinction of one life form will affect many life forms and everyone must realize this and strive to support biodiversity. It is important to remember the phrase "nothing in nature is wasted."
1. Amos, William H., Ecologist. e-mail: bilamos@)rr2.connriver.net
2. Chicago Reader "Dear Cecil," activated through America Online's "Straight Dope," 1988-1997.
3. Coldrey, Jennifer. Discovering Slugs and Snails. New York: The Bookwright Press, 1987.
4. The Connoissuese of Slugs WEB Page http://www.teleport.com/(jleon/slugs.html
5. Deyrup-Olsen, Ingrith, Professor Emeritus in the Department of Zoology, University of Washington. e-mail: firstname.lastname@example.org
6. Grolier Interactive Inc., "Slug," activated through America Online.
7. Jacobsin, Morris K. and David R. Franz. Wonders of Snails & Slugs. New York: Dodd, Mead & Company, 1980.
8. Olkowski, William, Shelia Daar, and Helga Olkowski. Common Sense Pest Control. Newtown: The Taunton Press, 1991.
9. "Slugs: The Organism WEB Page." http://users.aol.com/fmkamad2/Biology/bioslug.html
10. Strom, Monica, DTR Master Gardener Granite Falls, Washington e-mail: SeWBizzy@aol.com
11. Studenroth, Karl, Ecologist. Grand Bay, Alabama e-mail: Kstu1111@aol.com
12. Teearu, Tina, Ecologist Shrewsbury, Shropshire, UK, e-mail: Teearu@aol.com
Less than 1 period.
Supplement a study of biology with an activity drawn from this winning student essay.