What About Banana Slugs?

An olive-colored banana slug. The dot on the right side toward the front of the slug is the closed entrance to the single lung (pneumostome). (Photo by Ken Bevis, DNR)

 “We’d be up to our eyeballs in (organic) debris if those guys weren’t at work!”

Richard Zabel, Executive Director of the Western Forestry and Conservation Association

“Dr.” Zabel is remarkably insightful in his commentary on the role of decomposers; those amazing organisms that break down material in ecosystems. Forests need a lot of these, acting continuously, for our forests contain an enormous amount of organic material ultimately produced by photosynthesis. A normal west side forest in the Pacific Northwest contains somewhere between 330 and 790 tons/ acre of standing biomass**. That’s a lot. And all of this material must eventually decompose into foundational elements, feeding the nutrient cycles and ecology of our forests.

Wow. Just wow. Good thing this stuff breaks down.

Ever hear of “charismatic mega-fauna”? These are creatures that easily capture our attention; critters like grizzly bear, elk, mountain goats or cougars. These animals usually function at high levels in the food chain, eating plants or being eaten out on the Serengeti of our imaginations.

Now, back to decomposition. It is one of the most important ecological functions going, keeping our nutrient cycling going and feeding the plants and fungi of the world. The animal the esteemed Zabel was referring to is:

Ariolimax columbianus

Our own banana slug! Now that’s charismatic mega-fauna.

They are the second largest slug in the world, growing up to 12 inches long — but most are between 4 and 6 inches long. They come in a variety of colors, such as olive green, gray with black spots, yellow, even white. Local areas may have similar color patterns*, which could be adaptation in action. They occur in moist forests all along the Pacific coast of North America.

A handsome banana slug with tentacles extended. (Photo by Ken Bevis, DNR)
A handsome banana slug with tentacles extended. (Photo by Ken Bevis, DNR)

There are four funky stalks on their head. The upper ones are eye stalks for light reception, and the lower ones are chemical receptors used to “taste” the environment. Racing slugs at the University of California Santa Cruz (home of the Fighting Banana Slugs) open their breathing hole wide when competing****. It is on the right side of the animal, called the pneumostome, and allows the single lung to open and gather oxygen when the slug is working hard. (Yes, even slugs hurry sometimes.) Otherwise, in normal relaxed slug mode, they get enough oxygen through their wet, mucous-covered skin.

Speaking of slime, banana slugs have a magnificent tool in their mucous coating. They have glands all over their body that provide this slick and slimy multi-purpose coating. It protects them from dehydration, and allows them to cover themselves in a ball of the gooey stuff to hole up during dry spells. (That’s why you don’t see them out and about in the heat of summer.)

Ever wonder how slugs cruise along so gracefully? They don’t actually crawl across the forest floor at all. They lay down a trail of perfect slime to slide over, and that let’s them move with a certain undulating grace. It is at once slippery, and sticky, and allows them to climb vertical surfaces. And the slime is full of chemical signals telling other slugs which way they went, and whether they might be a potential mate. They even eat mucous to replenish their own supplies.

Banana slugs, despite their savory name and appearance, have very few predators thanks to this mucous. It apparently tastes bad and few animals have developed a taste for it. Raccoons will sometimes roll them in dirt to cover the slug (and the flavor?) and then have slug sushi, but mostly, they are left alone. Slug slime is a miracle, multi-purpose substance!

Banana slugs never have to worry about getting a date either: Not because they are so wonderfully handsome/beautiful (although they are in their own mollusk-y way), but because they are hermaphrodites. Yes, slugs are both boy and girl at the same time. They do look for a mate during the wet spring, and exchange sperm in an amazing mating ritual involving hanging by slime threads, exuding their enormous sex organs, and intertwining in, well, a rather sensuous manner (check out the you tube of Richard Attenborough watching their European cousins, the Leopard slugs). And afterwards, Romance? Commitment? Nope. Each slug goes off separately and lays eggs in moist, rotting wood, leaving their kids to their own fates. Slugs don’t do family or child care, so the hatched-out miniature slugs are on their own from day one. No divorces or day care bills for banana slugs!

Most significant to us, banana slugs eat detritus (rotting plant material) and mushrooms. They are important players in the forest ecosystem as nutrient and material recyclers, breaking complex plant matter down into basic components that can further move in the ecosystem. ***

So next time you see a groovy, big banana slug cruising along in your forest, treat it with a little respect and admiration. They are on duty for all of us, doing critical ecosystem functions with little fanfare and appreciation.

And when I searched on Slug Songs, there’s even a video of dancing slugs. Who’d a thunk it!

Sammy the Slug: Now that’s a mascot! (Bing images)
Sammy the Slug: Now that’s a mascot! (Bing images)

Send me photos and stories about the wonders of wildlife, and your own Encounters of the Slug kind, in your forests!

*Source: Slater Museum blog post, “The Pacific Northwest is Slug Country”, July 2016.

**Stewart T. Schultz (1990).  The Northwest Coast: A Natural History.  Portland, Oregon, Timber Press.

**Waring, RH, and JF Franklin. (1979).  Evergreen coniferous forests of the Pacific Northwest.  Science 204: 1380-1386.

**Waring, RH, (1982) Land of the giant conifers.  Natural History 91(10):54-63.

*** Wikipedia and various other sources for cool facts about slugs.

**** I’m not sure if they really do this but it seems like a good idea and probably has happened down there sometime!

Ken Bevis, DNR Stewardship Biologist, Ken.Bevis@dnr.wa.gov