Little Lives of Earth and Form
No Sluggards, These!
By Tamia Nelson
October 28, 2008
Even the most dedicated paddler has to get out and walk from time to time. This isn’t a bad thing. Any walk along a riverside trail in a misty drizzle is a feast for all the senses. In the diffused light and humid air, familiar colors are suddenly, strikingly vivid. Commonplace scents become richer and more evocative. And the small sounds of foraging wildlife carry further. So when a recent day dawned warm and wet, I grabbed my camera and took a hike along The River. I wasn’t disappointed. Families of geese shot touch-and-goes on the Flow, honking all the time, while the forested slopes resounded with the hammering of woodpeckers, dining out lavishly on dead trees. Nearby, a troupe of chickadees chattered companionably in the hemlocks. A deer—I couldn’t tell if it was a doe or a buck—crashed through a stand of young birch, as chipmunks toiled busily in the fallen leaves and red squirrels scolded passers-by from the very tops of the tallest pines. Yet despite all this, my attention was elsewhere, riveted on the ground under my feet. The forest floor was alive with slugs.
Of course, not everyone—or every paddler, for that matter—shares my passion for these shell-less snails. Slugs don’t get much respect, and that’s too bad. Like leeches, another oft-reviled invertebrate, slugs play their part in nature’s grand scheme. But it wasn’t abstract ecology that had me kneeling stiffly on the wet ground to take pictures of slugs. Nope, it was something altogether different. It was their…
Hard to believe? OK. I won’t press the point. The old saw is right. You know the one I mean: that thing about beauty being in the eye of the beholder. But before you dismiss my infatuation out of hand, take a close look at the next slug to cross your path. A small magnifier will bring him/her—slugs do away with much of the awkwardness and uncertainty of courtship by bundling the machinery of both sexes together in one package—close enough to appreciate his subtle coloration and elegant form. You don’t have a magnifier? Then use your camera’s macro setting to transport yourself into the world of the slug.
Here’s an example. The setting? A graveled portion of a fire road. A big guy stretches out to make time down the trail, while my fingernail provides scale.
But while a picture may be worth a thousand words, it can’t compare with Being There. So I recommend finding your own invertebrate collaborator. Unless you live in a desert, or unless snow is already falling where you are, it shouldn’t be too hard. There are a lot of slugs around, and they’re a colorful tribe, ranging across the spectrum from the recently discovered white “ghost slug” of Wales to the European black slug, and including such showy species as the brilliant yellow banana slug of the Pacific Northwest. On the other hand, it may not be easy to identify your find(s). I’ve yet to discover a good slug key. For the moment, therefore, I’m relying on simple color labels. The chestnut brown beauty in the photo above is typical of most slugs I see here in the Adirondack foothills, with second place going to a pale khaki species sporting longitudinal brown stripes, seen below. The maple leaf provides scale.
On first inspection, slugs seem almost featureless. But look again and you’ll start to distinguish a number of characteristic “landmarks,” beginning with the mantle, the shield-shaped saddle on a slug’s back. Here’s a quick guide to the external anatomy of these fascinating creatures:
The mantle hints at the link between slugs and their closest kin, land snails. Snails’ characteristic shells grow out of their mantles, but in slugs the shell is either altogether absent or much reduced. It’s a classic evolutionary trade-off: what slugs lose in protection against desiccation and predators, they gain in maneuverability. Now look closer at a nearby slug. With luck, you’ll see that the mantle is pierced by a single opening, the breathing pore, or “pneumostome.” It’s usually on the right-hand side, and it’s only open some of the time, so luck does play a role. And luck deserted me on the day I took the photos for this article. In the next shot, you can see my chestnut slug running for cover. He’s moving fast—for a slug—but he’s not breathing hard: his pneumostome is shut tight.
Slugs can take shelter in nooks and crannies that are off-limits to big-bodied snails, but maneuverability is useless if you can’t tell where you’re going. So slugs possess several handy aides to navigation. To find their way around, they rely on two sets of sensory tentacles. The upper pair are eye stalks (see the black eye spots in the photo below?), while the shorter pair are olfactory sensors—the slug’s nose, if you will. Both sets of tentacles can be withdrawn at will. This is very useful. Slugs don’t need safety glasses!
Let’s come back to the subject of maneuverability for a minute. Just how do slugs get around? On foot, of course. A slug, like a snail, has a single muscular foot, which undulates rhythmically—and, yes, sure-footedly—over just about any surface you care to name. Here’s a close-up, taken when I inadvertently dislodged the little khaki-colored guy from his perch on the maple leaf. As you can see, he didn’t lose any time getting back on his feet…er…foot. He was almost there before I had time to snap this photo:
The shiny stuff in the picture is slime (aka mucus). Slugs secrete two kinds. One is slippery; the other, sticky. Lacking a shell, and having a body that’s mostly water, slugs need all the help they can get to keep from drying out. Slime does the job, trapping a slug’s life-giving fluids inside, where they belong. It’s also vitally important in helping the slug in…
Slip Slidin’ Away
The gray streaks on the wooden beam in the photo below are dry “slime trails,” records of passage left behind by trekking slugs. (The wide smear of fresh mucus just beneath the pictured slug—he’s the same slug shown in the photo above—is the imprint left by his body after he righted himself.)
Below you can see a slime trail in the making. The mucus is secreted by the slug’s foot. It contains tiny fibers that increase traction in the same way that high-country skiers’ climbing skins do, permitting slugs to travel up smooth, near-vertical surfaces.
They can even hang upside down, as the next picture shows. A bonus: you can also see the elusive pneumostome.
And if all this weren’t enough, a slug who needs to get down from a high place in a hurry can descend on a zip-line made of slime. Slip slidin’ away, indeed!
But that’s just the beginning. Slime serves other purposes beyond hydration and locomotion. Slugs use slime trails to identify prospective mates. Even hermaphrodites need someone to love, it seems. (More prosaically, cross-fertilization conveys distinct evolutionary advantages.) Unfortunately, many predators—including predatory slugs—also use the same trails to track potential prey. In nature, love and death are often closely intertwined. Once again, however, slime comes up trumps. Not only is a slime-covered meal mighty hard for many predators to hang on to, but slime apparently has a foul taste. (No, I haven’t put this to the test myself. But at least one intrepid biologist has.) That said, not all predators are fussy eaters. Turtles consume slugs without any apparent qualms. So do frogs and toads, not to mention snakes, salamanders, birds, and many small mammals. Chipmunks, in particular, seem to think slugs a great delicacy, almost as if they were nature’s own bratwurst.
Still, the supply of such treats outstrips demand, and enough slugs escape becoming meals to insure their continued abundance. That’s a good thing, because biology turns the familiar computer-science catch-phrase on its head. In the wild, it’s…
Garbage In, Treasure Out
And slugs are among nature’s most accomplished recyclers, gobbling up everything from dead leaves and dropped fruit (and discarded banana peels) to fragments of offal left behind by carnivores. If you’ve ever spent time in a city where the trash haulers have gone on strike, you know what happens when the garbage piles up. (Just ask anyone who lives in Naples.) It’s not much different in the wild world. If dead animals and plants simply lay where they fell, the nutrient cycles on which all living things depend would quickly grind to a halt. Luckily, slugs are always there to carry the can—along with a number of able allies, from bacteria and fungi to coyotes and skunks. The happy consequence? Nothing is wasted.
Of course, slugs won’t turn up their noses at living plants, as well, and many gardeners wage an endless (and largely fruitless) campaign against them. You can also see the evidence of slugs’ hearty appetites on wild mushrooms. The next time you go walking in the woods, look carefully at the mushrooms at your feet. Some will show obvious signs that they’ve been nibbled by squirrels or mice, but closer inspection will often reveal more subtle traces of slugs’ recent meals. The two small, white spots in the cap of the topmost mushroom in the photo above were made by a grazing slug. Don’t wait too long to go exploring for slug sign, however. Nature’s about to bring down the curtain on the slug’s year, at least in Canoe Country. It’s all about…
The Fall of the Leaf
As unlikely as it might seem, paddlers and slugs have a lot in common, beginning with a taste for wild places. We both thrive in well-watered environments, too. And—let’s be honest now—no matter how quick and agile we are, neither slugs nor paddlers are likely to set any world speed records. Most importantly, though, we’re both “birds of passage,” doomed to give ground before the approach of General Winter. Canoeists and kayakers have more options, of course. We can fly away to more temperate climes or retreat indoors, contenting ourselves with maps and dreams till spring sets the waters free again. Slugs aren’t so lucky. Retreat’s not an option for them. Some die at year’s end. Others have to meet General Winter on his own terms. How do they survive? In a word—antifreeze. Slugs burrow down deep into leaf litter, until the sticky mucus sheath that covers their bodies is studded with a protective armor of dirt and detritus. Then they curl up and doze away through the long winter months, safeguarded by a sort of circulating antifreeze till the returning sun once again calls them back to life.
Hmm… That’s not too different from many padders I know, come to think of it—right down to the antifreeze! That’s another thing we have in common with slugs, I guess.
Getting Ready for Winter
Wilderness isn’t what it seems. The wild woods are carefully tended by regiments of tireless caretakers, workers whose importance transcends their small size and seeming insignificance. They’re essential operatives in the natural world’s nutrient recycling service. No sluggards, these! Hard workers all. So the next time you’re out and about on a wet day, take a few minutes to stop, squat down, and pay homage to the slug crawling at your feet. You owe this little guy a lot.
Copyright © 2008 by Verloren Hoop Productions. All rights reserved.