The life story of a slug.

News and commentary about environmental issues.
April 17 2009 7:03 AM

Feeling Sluggish

From a continuing series on revolting creatures.

More revolting creatures: the vulture, the tick, and the jellyfish.

A slug.
An Arion lusitanicus slug eating in the garden on July 24, 2005

Slugs don't sting; they don't suck our blood. Yet when the subject of repulsive creatures comes up, slugs are often the first ones that crawl to mind.

The slug physique is not appealing—one broad muscular foot topped with a gut (thus the class name gastropod). But what makes slugs so uniquely unattractive is their slime—a soft, slippery mucous coating that allows the animals to undulate along the ground with wavelike contractions. Anyone who has picked up slugs in the garden without gloves knows how persistent and clingy the stuff is, nearly impossible to wash off.

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The slug creates its slime by secreting a mixture of proteins and sugars through its foot and combining it with water. The stuff turns into something that manages to be a liquid while the slug is moving and stiffens like drying rubber cement when the slug stands still. (MIT scientists are still trying to figure out just how that works.) In a moderately sticky form, the slime gives the creature the ability to travel upside down or up your screen door. Slime at any viscosity serves to discourage the slug's predators—birds, box turtles, and snakes as well as small mammals.

Snails have similar slime but seem less loathsome because the shell covers some of the goo. Snails, by the way, came first. On the backs of many slugs, under a Sherlock Holmes-like cape of skin called the mantle, you can see a slim vestigial shell. The slug probably lost the shell in order to maneuver into small spaces. (It may also give them some extra speed: Slugs move almost twice as fast as snails.) The drawback to ditching the hard outer covering is that slugs are highly vulnerable to dehydration, so they do their foraging at night or on cloudy days. On very hot days, slugs will often huddle together in the shade of a piece of wood or a rock, flank to flank. Scientists say it's to stay cool, not to socialize. (But how do they know?)

The most inhumane of our tactics in the war on slugs is to sprinkle salt on their backs. People are driven to this brutal method because slugs are maddeningly destructive in the garden or field. Their mouthparts work like miniature chainsaws, grinding and shredding the most succulent, tender, and hopeful green shoots. Close up you can see their little heads, topped with two pairs of antennae, swinging from side to side like kindergarteners imitating elephants. A farmer's field of soybean seedlings can be lost to slugs in a night; whole cabbage heads are rendered inedible. In a recently published volume of Samuel Beckett's letters, the glum playwright remarks, "I'm depressed the way a slug-ridden cabbage might be expected to be."

But for all our gloom and disgust, the slug has its special charms and talents. The love life of a slug—or at least its sex life—might have caused even Beckett to marvel. ("I can't go on. I'll go on.") When a slug goes into reproductive mode, it drops a chemical into its slime trail that sends the message of readiness. It can take a while for two slugs in heat to find each other, but once that happens, the pair might engage in foreplay for hours, sampling chemical secretions on each other's surfaces. The nibbling often leads to biting and tail lashing. Most slugs mate on the ground, but Limax maximus, aka the leopard slug, produces a strong cable of slime by which the two lovers dangle from a tree.

In most kinds of slug, the penis is about half the length of its body. ("Is that a Kalashnikov in your pocket or are you just glad to see me?") It's not clear why such anatomical grandeur might be important for survival: Slug experts discount the idea that the oversize penis functions like a peacock tail, as a display of virility—they point out that the chemical signaling and seduction occurs before the magisterial organs even come into view. In any event, the chosen mate isn't likely to be impressed, since most slugs possess both male and female sexual organs. During a single coupling, slugs can mate reciprocally—with each partner inseminating and being inseminated—or one can serve as the recipient.

With afterglow, the less enviable aspect of slug sex can occur. Here's one major drawback to being both large and adhesive: The sexual organs sometimes get stuck. In this case one slug gnaws off the other's penis in a process called apophallation. (The dispossessed slug then goes on with life as a female.)

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