San Francisco State University
Department of Geography
Geography 316:  Biogeography

The Biogeography of the Banana Slug (Ariolimax columbianus)
by Jasmine Morrow, student in Geography 316, Fall 2000

"A slender banana slug in search of water."
Photographed by G. Donald Bain
The Geo-Images Project, Dept. of Geography
University of California, Berkeley

Taxonomic Classification:
Kingdom:  Animalia
   Phylum:   Mullusca
       Class:    Gastropoda
          Subclass:    Pulmonata
            Order:    Geophila
               Family:    Ariondae
                 Genus:     Ariolimax
                   Species:     Ariolimax columbianus

Description of Species:
     Augustus A. Gould first gave these soft bodied invertebrates a scientific name in 1851.  Some of the first studies were conducted along the Columbia River; hence the species name A. columbianus (Harper 1988).  Gastropoda describes the banana slugs stomach-foot or muscular-foot, which allows them to slowly crawl on a series of muscular waves (Denny 1980).  Pulmonata describes their small lung that opens to the outside with a pneumostome, the hole slugs breathe through.  The banana slug is the second largest slug in the world, growing up to 10 inches in length.  On average they range between 6-8 inches, with a life span of 1-7 years (Harper 1988).  Their coloring is sometimes bright yellow, occasionally with black spots, or solid greenish.  Individual slugs will change colors with alterations in food consumption, light exposure, and moisture levels.  Color may also indicate whether a slug is healthy, injured, or what age they are (Harper 1988).  Their coloring allows them to camouflage with leaves and other debris on the forest floor.  This serves as protection from such prey as beetles, raccoons, and even other banana slugs (Harper 1988).  That’s right, these funny critters are tertiary consumers.  They are forest floor scavengers that feed on small (already dead) animals, but they are mostly decomposers, feeding on detritus.  Banana slugs are considered general herbivores that eat all kinds of leaf litter, green plants and fungus (mushrooms are their favorite foods) and occasionally feed on animal feces and carcass (Rollo 1983).  In the process of debris munching, the A. columbianus disperse seeds and spores while excreting a nitrogen rich fertilizer (Gordon 1994).  Slugs, at times are also frugivores and may play a minor role in forest plant regeneration and seed dispersal (Gervais 1996).

    Ariolimax columbianus, commonly known as the banana slug, is endemic to the Pacific Northwest region of North America due to the relative stability and isolation of these areas.  They can be found in foggy moist forest habitats of southeast Alaska, British Columbia, Washington, Oregon, and California (Gordon 1994).    They are continuously distributed throughout this region where they seek shelter beneath coastal redwoods and detritus.  Within the primarily Mediterranean and Marine West Coast biomes, they remain active most of the year.  Banana slugs show arboreal tendencies but are commonly encountered on the ground or within creek banks and tree roots during the dry summer months (Ingram & Lotz 1950).  They usually partake in nocturnal activity due to their high moisture dependency, but during the cooler rainy winter months they are often seen during the day.  They are stenotopic, meaning they have very specific preferences and limited ecological tolerance.  Slugs are prone to desiccation, a complete or nearly complete drying out, such as may result in the formation of evaporites from bodies of water in an arid region.  At one time banana slugs were blamed for damage done to redwood seedlings.  A study later proved that a mutualistic relationship exists between the two.  Banana slugs would rather eat cardboard than eat the seedling of a Sequoia sempervirens (Harper 1988).  Instead, the slugs in the study ate other small plants which are redwood competitors for space, water, light, and nutrients.  Rollo and Wellington found that A. columbianus often return to their shelters even if only inhabited for one night.  Also, several slugs within their study alternated between to homes possibly displaying home range behavior.  It is suggested that this homing behavior is conducive to slug ofactory senses.  Slugs often disperse from sparse plant canopies to adjacent dense vegetation, but often will occupy less dense habitats to avoid competitors (Rollo & Wellington 1980).

"Banana Slug Crawling at Samuel P. Taylor Park."
Photo by Kirsten Leising

Natural History:
    Ariolimax columbianus begin their lives as 15-20 mm colorless eggs deposited by their hermaphroditic parent(s) who usually cross fertilize, ideally where ground water and humidity are high.  They are usually deposited in clumps of 30 eggs that will hatch in 3-8 weeks (Harper 1988).  In order to hatch, the eggs need sustained moisture, mild temperatures, and a good hiding place.  Once the eggs have hatched, it is up to the baby banana slugs to find their own food for survival.  Predators like to eat banana slugs at all stages of their lives.  A high number of eggs hopefully ensures the survival of the species, even though they have a high mortality rate.  Banana slugs have a high growth rate, which also helps to maintain a stable population.  Ariolimax columbianus are diurnal during late March to early July, then from late August to early November feeding mostly above ground on vegetation and sleeping on shrubs and trees at night (Richter 1980).
   When compared to other invertebrates, the life history characteristic of A. columbianus are strongly K-selected and probably evolved through intraspecific exploitative competition within predictable and stable forest habitats (Rollo 1983).  This means that they compete between other individuals of the same species (Ariolimax columbianus), depleting the same resources without directly interacting, except when mating.  Banana slugs tend to be loners who survive below the protective Western and Mountain hemlocks, coastal Redwoods, and Giant Sequoias of the Sierra Nevada west-face, as well as other various coniferous species (Walker 1999).  Hamilton and Wellington (1981) found Ariolimax columbianus mortality to be severely affected by population density which may be due to their non-aggressive tendencies.  They also concluded that banana slugs are able to endure longer periods of intense weather as compared to the Arion ater slug.  They attribute this to the large surface area to volume ratio which causes less evaporative stress.

     The banana slug is a mollusk, which came from the sea.  Their stages of evolution can be seen in other various species of land and sea mollusks.  Therefore, slugs are believed to have evolved from marine snails and to have once been protected by a large shell (Harper 1988).  The tremendous adaptation variation in slug traits such as size and color, make their fossils challenging to classify.  Not to mention the challenge of identifying their soft bodies, which become distorted and faded during fossilization (Runham 1970).  A “slug” is actually a word describing their type of body.  Divergent evolution is evident in the variation between internal and external shells.  Studies of slug anatomy have determined that the current slug shape has evolved through parallel evolution both aquatically and over land, deriving from snails (Harper 1988).  Pulmonate species of mollusks in the intermediate stages of land evolution maintain shells that are too small to actually protect and contain their bodies effectively (Gordon 1994).  Other’s, like the banana slug, conceal remnants of their shell within their small bodies.  The caudal mucus plug covers the caudal pit which serves as a defense mechanism, and may also allow banana slugs greater access to nutrients through arboreal means (Richter 1980).  Land snails have limited themselves to conditions of high moisture, the need for protection from light and heat, but have moved away from their dependency on calcium rich soils due to the shedding of their external shell.  In retrospect, they also lost their prized protective barrier, allowing predators easy access to a terrestrial treat.  In return, banana slugs are able to live in a wider range of environments.  They have also become quite mobile with their elastic sleek bodies and gained the ability to live below ground for extended periods of time, as deemed necessary as in places like southern Alaska (Harper 1988).
    The Cambrian Period of the geological time scale is the first division of the Paleozoic era in which the ancestral wormlike mollusca existed.  It was probably during this time span (~500-570 million years ago) that large aquatic invertebrates existed with skeletons and hard shells of chitin or lime.  Their shells have preserved their fossils much better than their slug decendents (Funk & Wagnall 2000).  During the Devonian period (~ 395 million years ago) invertebrates began to leave the ocean, probably due to increased solar radiation and warmer wet conditions on land.  It can only be hypothesized that this is when various snails left the ocean for land, some eventually losing most of their shell and becoming slug-like.

Source:  Gordon 1994

    Limiting abiotic factors such as moisture, temperature, and the need for heavy ground cover make the forest habitats of the west coast the perfect homes for the terrestrial slugs.  The Ariolimax columbianus are most likely paleoendemics restricted to a range that extends as far north as Sitka, Alaska, to the west side of the Cascade range in British Columbia, on the west coast of Washington and Oregon, and finally into California, primarily on the coast and as far south as Salinas Valley (Harper 1988).  In 1992, The Veliger reported the first documented siting of A. columbianus in Plumas County, California on the western slopes of the Sierra Nevada's (Groves 1992).  This is also the highest elevation (1,097 meters) the banana slugs have ever been documented at.  A biotic factor that restricts the  distribution range of the banana slug has been studied by Mark Denny who attributes their high cost of mucus (slime) locomotion (1980).  Two disadvantages of adhesive crawling of gastropods, according to Denny are that the adhesive power of the slime must be overcome by the animal, and that the mucus must be produced to replace the expended slime during movement.  These factors limit the distance to which slugs will crawl in search of food or mates.  Banana slug slime also serves as defense from predators, helps them retain moisture, reproduce, and may also serve as nutrition if necessary (Harper 1988).

Source:  Harper 1988

Other interesting information:


"Banana slug makes its way home."
Photo by Elise Willett (2000)


Bain, G. Donald.  "The Geo-Images Project."  (October 26, 2000)

Denny, M.  June 13, 1980.  "Locomotion:  The Cost of Gastropod Crawling."  Science Vol. 208.

Funk and Wagnalls.  "Funk and Wagnalls Knowledge Center."  (November 19, 2000)

Gervais, Jennifer, Anna Traveset and Mary F. Willson.  July 1998.  "The Potential for Seed Dispersal by the Banana Slug (Ariolimax columbianus)."  American Midland Naturalist.  140(1), 103-110.

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

Groves, Lindsay T.  April 1, 1992.  "New Range Information for the Banana Slug Ariolimax columbianus." The Veliger.  35(2): 157-159.

Hamilton, Peter and W.G. Wellington.  1981.  "The effects of food and density on the nocturnal behaviour of Arion ater and Ariolimax columbianus."  Researches on Population Ecology (Kyoto).  23(2), 309-317.

Harper, Alice B.  The Banana Slug.  Aptos, California:  Bay Leaves Press, 1988.

Ingram, William and Carol Lotz.  June 1950.  "Land Mollusks of the San Francisco Bay Counties." Journal of Entomology and Zoology.  Vol. 42, #2.

Richter, K.O.  1980.  "Movement, reproduction, defenswe and nutrition as functions of the caudal mucul in Ariolimax columbianus."  The Veliger.  23(1), 43-47.

Rollo, David C. and W.G. Wellington. 1981.  "Environmental orientation by terrestrial Mollusca with particular reference to homing behavior."  Canadian Journal of Zoology.  59(2), 225-239.

Rollo, David C. 1983.  "Consequences of Competition on the Reproduction and Mortality of three species of Terrestrial Slugs." Researches on Population Ecology.  25, 20-43.

Runham N. and P. Hunter.  Terrestrial Slugs.  London: Hutchinson and Co, 1970.

Walker, Laurence.  The North American Forests.  United States: CRC Press, 1999.

Willet, Elise. 2000. Department of Geography.   San Francisco State University.

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