Creature Feature: The Banana Slug is Living Proof that a Slimy Little Gastropod Mollusk Can be Loaded with Charisma
Can a small, slimy, shell-less, forest-dwelling gastropod whose diet includes animal droppings and other dead stuff develop an enthusiastic fan following? You’re darn right. Consider these facts about the banana slug:
• It’s the star of several community celebrations, including the nationally–publicized Russian River Banana Slug Festival.
• It’s the official sports mascot of UC-Santa Cruz, and ESPN Sports Travel has named it one of the ten best team nicknames in college basketball.
• It’s the name of an environmental musical group, the Banana Slug String Band.
• It provides the raison d’etre for the International Slugfest, an organized campaign to locate and document exceptionally large banana slugs.
• It’s one of the leading contenders (along with the geoduck and the giant Pacific octupus) for designation as the official state mollusk of Washington. Only a gubenatorial veto prevented it from becoming California's official state mollusk in the late 1980s.
We could go on, but the point is made. Banana slugs have charisma – loads of it. That said, it remains that they are odd creatures that most Americans know little about.
The Pacific banana slug, a cousin of the snail, is a shell-less gastropod mollusk belonging to the genus Ariolomax. This genus includes three main species -- columbianus, dolichophallus, and californicus – as well as two known subspecies. To the unschooled eye, of course, these differences count for naught.
Banana slugs grow six to ten inches in length -- making them the second-largest slugs in the world (after the Limax genus in Europe) -- and can live for as long as seven years. They are named for their roughly cylindrical shape and characteristic golden yellow color (often with dark spots). Banana slugs do come in other colors, including greenish-brown, nearly black, and even white. Though the less common colors may reflect the influences of diet, available light, moisture, age, health, and other factors, the basic coloration evolved to blend well with detritus and help slugs avoid detection by salamanders, garter snakes, raccoons, foxes, porcupines, crows, ducks, beetles, and other predators. Some predators avoid slugs so they won’t have to deal with the mucus coating (as by rolling the slug in the dirt).
Banana slugs are stenotopic, meaning that they can withstand only a limited range of variations in environmental conditions. The climate has to be reasonably mild because severe winter cold will kill them. They need moist environments because severe desiccation can kill them. Since detritus and related organic matter provides most of their food, rotting plant and animal material must be abundant. Mushrooms are a preferred food, but they’ll consume lichens, algae, fruit, seeds, and even animal droppings and carcasses
All things considered, it’s easy to understand why nearly all banana slugs live in the floor of temperate coniferous rainforests and similar rainy-foggy-damp habitats within the long, narrow, mountain-backed Marine West Coast climate zone that stretches along the North Pacific Coast. This encompasses a huge area extending from the Salinas Valley of central California northward through coastal Oregon, Washington, British Columbia (west of the Cascades), and southeastern Alaska. Only in a few places does this range extend inland more than a couple of hundred miles.
You can look for banana slugs in a number of national parks in the North Pacific Coast region, including Muir Woods National Monument, Point Reyes National Seashore, Redwood National and State Parks, Olympic National Park, the Fort Clatsop National Memorial unit of Lewis and Clark National Historical Park, and Sitka National Historical Park. The latter park is situated in the Alaskan panhandle near the northernmost outpost of banana slugdom.
Banana slugs are generally nocturnal, but they aren’t exclusively creatures of the night. You can often see them out and about in the daytime during chilly spells and the cooler rainy winter months. If it’s a relatively dry time of year, check out detritus near creeks, in tree root tangles, and in other moist places. You’re not likely to see slugs in their usual haunts during bad dry spells though, because they wait out those periods by covering themselves with mucus, leaves, and soil.
Wherever you find banana slugs, you’ll recognize them when you see them. Nothing in the forest looks quite like a banana slug. (See the accompanying photo.)
The two distinctive sets of tentacles on the slug’s head are superbly designed sensory organs. The shorter set is for feeling and smelling, and the longer set is for seeing. The dark dots at the end of the longer tentacles are the animal’s eyes. If you watch those eyestems while the slug searches for food or copes with obstacles, you can see them functioning like a periscope as they stretch up and down and turn in all directions. The eyestems can even be retracted in the blink of an eye. The mouth, which is situated between the lower tentacles, is equipped with a radula, a tongue-like organ covered with a seemingly countless number of tiny teeth.
The mucus coating that most folks call slime is certainly a signature attribute. It actually serves many useful purposes. In addition to helping the slug avoid dehydration, the mucus helps the slug slide along the ground on the muscular foot covering its lower body, protects the animal’s soft body from sharp rocks, twigs, and other hazards, and discourages predators with its foul taste and mouth-numbing anesthetic effects.
The slime also contains pheremones that help slugs find mates. Since the banana slug is a hermaphrodite -- each animal having both male and female reproductive organs – the task is not especially difficult.
The more we learn about banana slugs, the more we come to appreciate that their presence enriches and helps to stabilize the ecosystems they inhabit. By feeding on detritus, they help to recycle nutrients and make them available for new growth. Slug excretions not only provide nitrogen-rich fertilizer, but also help to disperse spores and seeds needed for forest plant regeneration. Since the banana slug’s ecological niche is not yet fully understood, we shouldn’t be surprised to learn that there are other praiseworthy contributions.
In the words of proud UC-Santa Cruz sports fans – and slug fans everywhere -- Go Slugs!
Postscript: Let’s be brutally honest here. Banana slugs are not universally loved, nor even appreciated, by many of the people who deal with them. During the rainy season they may squish under your feet on trails and walkways, invade yards, gardens, sheds, garages, and houses, and leave glistening slime trails all over the place. They‘ll get into the darndest places too, since they can climb walls, move upside down, and squeeze into small holes. BTW, if you want to hear some really over-the-top remarks about the species, just ask somebody who is not Bear Grylls to describe a banana slug's distinctive taste. Some people do eat them at festivals, you know, although only the Lord knows why.