Names, Classification, Range, and Habitat
The common name, “banana slug,” describes eight species of terrestrial slugs found in damp, temperate, coniferous forests along the west coast, from California to Alaska. Isolated populations occur along the coast as far south as San Diego County, and also on some moist western slopes of the Sierras. California, especially the Santa Cruz Mountains, is a species hot spot.
Often bright yellow, banana slugs can resemble their namesake fruit in various stages of ripeness or decay, shading from white to greenish browns and almost black. The Pacific banana slug (A. columbianus), which is sometimes spotty, is the most widespread, and is found outside California. Edgewood’s denizens could be one of two other species, A. californicus, or A. dolichophallus. Externally they all appear very similar, and experts use internal differences, mainly in the genital structures, to distinguish the species. Modern molecular biology suggests new classifications.
Slugs and snails are gastropods (“stomach-foot”), probably the largest class within the very large and diverse phylum of Mollusca. Technically, “slug” refers only to a body type, not to a closely related animal grouping, because slugs evolved from snails multiple times, in different lineages. That means a sluggy body must offer advantages. A shell certainly affords protection, but it’s also bulky, and requires a higher calcium diet. Streamlined slugs gained access to microenvironments where snails would never fit, but lost protection against dehydration. Slugs require reliable moisture. When it’s too hot or dry to come out, they hunker down in a damp, shady shelter. Save your banana slug searches for cool, moist days, and shady locations that tend to stay damp and cool even on sunny days.
What to Look for and Notice
Banana slugs are the largest land slug in North American and second largest in world. They can reach 10 inches and 4 ounces to quarter-pounders, but are more typically about 6 to 8 in. long. (One European slug reaches 12 in.)
Four tentacles extend from the head. Sticking up like periscopes, the larger, upper two are optical tentacles; the small black dots at the ends detect light intensity. The lower sensory pair feel and smell. All tentacles move independently and can quickly retract to protect the valuable sensors. If a tentacle is lost, it regenerates. Below the tentacles, the mouth contains the radula (L. scraper) a vaguely tongue-like ribbon covered with rows of tiny, replaceable backward-pointing teeth, which scrape food into the esophagus. (It’s also used in slug fights.)
To eat, the slug extends its odontophore, which supports the radula, as seen in this video by Kevin Dick.
What appears to be draped over the back and head is a leathery covering called the mantle. It’s what secretes the shell in snails, but provides some protection for slugs too. An opening on the right side (pneumostome), which the animal can open or close, leads to an inner cavity like a lung. The mantle conceals two more nearby openings: the genital opening is forward of, and the anus is to the rear of the pneumostome. A keel-like ridge, or carina runs down the back to the tail. The appearance and color of an individual can vary with its circumstances, such as level of hydration, diet, health, age or injury.
Slime and Locomotion
Serving many vital purposes, slime (or mucus) is essential and slugs secrete it from pretty much everywhere, using different “recipes” in different body parts and circumstances. Slime helps prevent dehydration, and allows gas exchange for respiration. (As in amphibians, the “lung” doesn’t do all the work; wet skin helps.) When temperatures rise and humidity drops, slugs coat themselves in thick slime and duff, roll up in a safe place and go dormant (estivate) until conditions improve.
Slime also facilitates movement and protects against sharp edges. Gastropods move by contracting muscles in the foot, in multiple places at the same time, resulting in a series of waves that look like small arcs rippling along the body’s length. (You can see this if you watch through a glass pane.) Each arc is a muscular contraction lifting up and lunging forward, and they collectively propel the slug. Slime helps slugs adhere to surfaces, and also increases suction, so they can glide along vertical surfaces, or even upside down. If they trek up a tree too far, they can shorten the return trip by exuding a slime cord and lowering themselves back down. Slime can’t be reeled back in, but is often eaten, so is not wasted. Estimates vary, but banana slugs typically sail along at about 3–4 inches/minute, with a maximum speed of over 6 inches/minute reported. Although their slime helps them move, they’re also constantly working against it because it adheres to surfaces.
Slime also carries chemical messages, important to homing behavior and mating, and helps repel predators—more on these topics below. Finally, although you hear about folks who handle and even kiss banana slugs, it’s best not to touch banana slugs—for their sake. They’re used to what they encounter on the forest floor, but not to the sweat, soap and oils that might be on our hands.
Life and Behavior
Banana slugs lead solitary lives, cruising the forest by night for food, and by day too when humidity allows. They find shelter sites to wait out drier times, and can return to them, apparently “homing” to preferred shelter sites by scent, which may include their own slime trail.
In their temperate range, mating occurs any time of year, and often several times. Pheromones in their slime advertise readiness and help attract mates. Banana slugs are simultaneous hermaphrodites, meaning they have both female and male organs, and although they can self-fertilize, they usually cross-mate, with both slugs accepting and delivering sperm during an encounter. Being hermaphroditic doubles the chances of reproducing, which is helpful when population density is low. They can also store sperm for many weeks, to fertilize eggs that mature after mating. Both slugs lay clutches of 20–30 translucent eggs (~1/4 inch), protected under logs or leaves. Eggs hatch a month or so later. Baby slugs, an inch at most, are on their own once they emerge from the eggs. They can live up to seven years.
Banana slug mating behavior is … remarkable. Foreplay begins with laying down a copious slime blanket, but can be quite violent and takes hours. Once they maneuver themselves into a curved yin-yang position with right sides together, they remain pressed together for several more hours. And when they’re ready to separate, troubles often arise because huge male organs have evolved … and they get stuck. So, yes, they chew them off. It’s not clear whether they can regenerate the lost organ, but they can apparently carry on as females. This state of affairs may best be framed in evolutionary terms, probably as a result of sexual conflict and partner manipulation tactics.
What They Eat and Who Eats Them
Banana slugs eat pretty much anything, but do avoid some potential foods, while preferring others. They eat many types of living, dead or decaying plant matter and lichens, as well as animal droppings and carrion. They seem to be fond of mushrooms, which are abundant in their damp domain.
They don’t make an easy meal themselves, but do show up on a variety of predators’ menus. One defensive action is to change body shape by contracting into more of a ball, becoming too big a mouthful, while upping slime production. A shrew or mole snatching an inviting baby banana slug can end up wrestling more of an unmanageable melon instead. For predators who don’t mind a balled-up slug, slime provides another two-pronged defense. The sticky consistency alone repels some predators, and those who get a slug to the moist skin of their mouths discover the tingling, anesthetic properties of slime. Some raccoons roll slugs in dirt or duff prior to dining. Salamanders, newts, garter snakes, foxes, porcupines, crows, ducks, beetles and millipedes have all been observed eating banana slugs.
A further note on the bright yellow color—is it camouflage/mimicry, or a warning? Both or neither? Sometimes a bright color warns predators, “Beware of inedible yuckiness!” and that’s somewhat true with banana slugs. Then again they might be harder to see than we think, for instance when gliding (slowly!) past bay trees, willows, or coffee berries, which drop bright yellow, slug-shaped leaves. Also, consider that the slugs are most active at night, and nocturnal predators don’t see the same as we do.
Roles in Ecosystem
Banana slugs have been called the clean-up crew for the forest floor. These detritovores speed decomposition, thus helping recycle nutrients and enriching soil. As they drift from one meal to the next, they help spread seeds and spores. A study to determine what was damaging coast redwood seedlings found that confined banana slugs would eat their cardboard enclosure or starve rather than nibble Sequoia sempervirons seedlings. They exhibited no such restraint for other seedlings that sprouted, however, thus eliminating competition and turning it into fertilizer for redwoods. The great trees, in turn, provide deep shade, and comb foggy skies for moisture so critical to the slugs.
Banana Slug Fun Facts
In 1986, amid controversy, the banana slug became the official mascot of the University of California, Santa Cruz campus. Two years later, with prompting from local Peninsula children, the State Legislature voted to elevate the creature to State Mollusk, but then-Gov. Deukmejian vetoed the bill. California still has no official state mollusk. Late in 2014, UCSC alumni received email from “Sammy the Banana Slug,” part of a successful crowd-sourced campaign to fund the sequencing of the banana slug genome. Students will undertake the project in 2015, UCSC’s 50th anniversary. Ariolimax dolichophallus could become the first terrestrial slug to have its genome mapped, and scientists can use the information to reveal more about banana slug evolution and biology.
Where and when at Edgewood am I likely to see banana slugs?
In Edgewood’s woodlands, watch for banana slugs during the day in the rainy season. As habitat drys in the spring, look carefully at waterways that stay damp. Hikers most often see banana slugs near the small waterfall on the Sylvan Trail, around culvert marker SYS3C. Also look around the redwood trees in the Day Camp, near picnic area.
Banana Slugs. Wikipedia.
Bauer, C. 2011. Banana Slugs Unpeeled. KQED Quest.
Cassidy, J. and Pickett, M. 2015. Banana Slugs: Secret of the Slime. KQED Science.
Dick, K. Banana Slugs Video.
Harper, A.B. 1988. The Banana Slug: A Close Look at a Giant Forest Slug of Western North America. Bay Leaves Press.
Janiskee, B. 2010. Creature Feature: The Banana Slug is Living Proof that a Slimy Little Gastropod Mollusk Can be Loaded with Charisma. National Park Traveler.
Miller, B.L.W. 2006. Sexual Conflict in Banana Slugs.
Skene, J. 2011. Banana Slugs “Do The Wave” with Slime. KQED Quest.
By Carolyn J. Strange. Science writer Carolyn J. Strange has written hundreds of articles. She became an Edgewood neighbor in 1998, a Friends of Edgewood docent in 2003, and has served the Friends of Edgewood in various ways ever since.