ANNALS OF THE UNNOTICED
Slimeballs
Liquid crystal mucus? Check. Apophallation? Check. Catching up with one of our world’s most astonishing little inhabitants.
Welcome to Planet Slug, a casually mind-blowing domain that bears scant resemblance to our own. These little guys (well, guys and dolls; we’ll discuss that shortly) may appear to be little more than slimy blobs, but those blobs possess a Cronenberg-esque natural history, complete with magic mucus, death-by-cheese grater, and, of course, penis chewing. And if you really like slime, you are really in the right place.
(Not) A Garden-Variety Pest
Ask any gardener and they’ll tell you that slugs can be a real problem. They feed in roving gangs by the light of the moon, and they eat everything from lilies to potatoes. There are ways to fight back, like a few cups of beer placed around your garden (slugs love to party; they also tend to fall in and drown). But we’re here to gain an appreciation for their deeply strange lives and loves, not plot their mass murder, so today we won’t dwell on ways to get rid of them.
Slug Bits: An Overview
Slugs are basically snails that have lost most or all of their shells in the course of their evolution. The general slug phenotype has arisen independently at different times, so the category “slug” is polyphyletic, meaning there are lots of what we call slugs scattered among widely different taxonomic groups. But no matter what family tree they spring from, most slugs have these parts in common:
- A foot. The slug uses its one foot for locomotion by creating rhythmic waves of muscle contraction. In this way they basically glide across surfaces.
- Four tentacles. These emerge from the head, and include two with eyespots on the end that can perceive light, and two that possess a sense of smell.
- Countless teeth in the form of a radula, a kind of a cheese grater thing that the slug uses to destroy your lettuce plants, and also…we’ll get into that later.
- A tail, but not much of one.
- Lots of water. Slugs, like you and me, are mostly water, and they tend to dry out pretty quickly, so unlike you and me (I hope) they keep themselves bathed in a nice thick layer of stay-moist mucus.
- Lots of muscle. If a slug feels threatened, it can do a whole-body flex and become hard and rubbery; it can also clamp its one foot onto wherever it’s sitting and excrete an extra layer of super-slimy mucus. In this state, it is both the snot and the doorknob.
Speaking of Mucus…
Yes, we’re speaking of mucus. It turns out that snail mucus is actually a liquid crystal, the same stuff you find in transitional lenses and mood rings. In the case of the slug, it’s an organic fluid organized in a crystalline form that can be either slippery or adhesive, depending on the molecular arrangement. If the slug wants to grab onto a surface, it excretes mucus dialed in to sticky mode; the slime produced is thick and gluey. If the slug wants to be slippery and hard to pick up, the slime it produces has the crystals shifted the other way. It’s the same stuff, with radically different form and function. Can your mucus do that?
It turns out that snail mucus is actually a liquid crystal, the same stuff you find in transitional lenses and mood rings.
Also speaking of mucus, that familiar shiny slime trail serves more purposes than meets the eye. Slug slime trails are actually a mode of communication, being loaded with pheromones that send messages to other slugs, including rivals and potential mates. And here is probably the least-surprising quality of slug slime: it tastes terrible. It’s thought that the nasty taste deters potential predators.
Literally Twisted
Let’s turn to that simple little snail’s inner workings. Like all gastropods, snails undergo something called “torsion,” which is just a fancy way of saying that at some point in each slug’s development its intestinal tract rotates 180 degrees so its anus is positioned directly above its face. That’s right: a slug’s butthole is on its forehead, and when they poop it all just runs down over their face. This adaptation allows gastropods like mullosks to live inside a shell without filling it up with excrement (if you have read my story about antlions, you’ll know that those little monsters have come up with a debatably better way to address much the same issue). Torsion in slugs appears to be one of Nature’s rare design flaws, since it sometimes results in contamination of their food by fecal matter. But torsion still happens, and in some species it happens in a matter of hours. What a special day for the young slug!
Here’s a sweet graphic of a gastropod’s guts going sideways:
By the way, slugs breathe through a little hole in one side of their body called a “pneumostome.” It’s always on the right side, because that’s just the way slugs are.
Let’s Get It On
Slug reproduction is no less fraught than their digestive tracts. Every little slug you see is a true hermaphrodite, possessing both a penis and a vagina. When slugs mate, they do it twice, simultaneously: they sidle up to each other, line up their reproductive organs and go at it, leaving the encounter with a load of each other’s sperm to fertilize each other’s eggs. There are other organisms, worms included, that employ this convenient arrangement, and it works fine, except when it doesn’t. When it doesn’t, it generally means someone is losing a penis. This is has been extensively documented in a few species of banana slugs, those ginormous yellow monsters that are common in the Pacific Northwest.
If you were like me and had never seen one before — didn’t even know such a thing existed in this world — and you stumbled across a half-foot-long, DayGlo-yellow banana slug during a hike in a redwood forest, you might, as I did, acquire a new and profound respect for nature’s vast comedic genius. No, you’re not dreaming, and yes, it’s freaking alive! At that point being ignorant of the fantastic beast’s sex life was probably a good thing, since the brain can only handle so much innovation at once.
The banana slug has a penis at least as long as its own body, 7–8 inches or so. If a banana slug doesn’t have a penis, as about 5% of them don’t, it’s probably because another banana slug recently chewed it off while they were mating, retaining the organ inside the chewer’s vagina. This is called “apophallation,” and researchers think it’s either (A) an effort to block the vaginal opening so no other slugs will have a chance to mate, or (B) a reaction to feeling threatened and needing to disengage as quickly as possible in order to make an escape. Either way, one banana slug is no longer the hermaphrodite it once was, and the other one now owns two 8-inch penises.
Slugs Give Chase!
There are countless kinds of slugs, and they have countless feeding habits. Most of them find and consume plant material, whether decaying dead plants or the happy healthy leaves of your spinach seedlings. However, they are not all vegetarians; there are scavenger slugs that eat dead and decaying animal matter, and there are also predaceous “hunter slugs” that chase down, kill and eat other organisms. We’re talking here about prey that can’t outrun a slug, and if you’re thinking “other slugs,” you’re absolutely correct. There are large, predatory slugs out there, chasing down and devouring their own kind. It’s your classic low speed chase scenario.
So now that we’re okay with carnivorous, predatory slugs as a concept, and we can live knowing that they hunt, kill and eat other slugs, let me throw you a bit of a curve: there are also giant hunter slugs that eat baby birds, in the nest, alive. According to researcher Katarzyna Turzańska at the University of Wroclaw in Poland, it isn’t all that unusual, although the actual moment of slugs predating on nestlings isn’t easy to observe: “You are more likely to come across the traces of the ‘tragedy’: dead or alive nestlings with heavy injuries, covered in slime — and often slugs’ droppings found nearby.” Recall that slugs don’t have mouths or teeth as such; they have a toothed rasping organ, a radula, that they use for scraping holes in leaves, and, as it turns out, also holes in defenseless baby birds. Maybe we should go back to talking about apophallation.
Slugs Get Theirs
If you’re still feeling bad about the baby birds, it might help to reflect on the fact that nearly everything eats slugs, including, appropriately, birds. Slugs are also preyed upon by reptiles, insects, spiders, mammals, amphibians and fish, to name a few (if you’re wondering how a fish gets the chance to eat a slug, apparently it’s a fisherman thing). But if you’re a slug, being gobbled up by a hungry starling is easy compared to enduring an attack by one of the zillions of little beasties hanging around waiting for a chance to eat you up from the inside out. Here we’re talking about parasites and parasitoids — insects, worms, and other organisms that live part or all of their lives inside another organism, feeding off the tissues and organs of the host. The number and variety of slug parasites is breathtaking. Here are a few highlights:
- The nematode worm Agfa flexilis live in the slug’s salivary glands.
- A related species, Angiostoma limacis, lives in the slug’s rectum (and I bet that nearly killed him).
- Many, many species of flies parasitize slugs, which means their maggots live in the slug’s guts and eat it alive from the inside out, until it’s time to chew their way to the surface and drop to the ground and pupate.
- Some meningitis-causing nematode worms have larval stages that can only live in mollusks, including slugs. In a few rare cases, humans have developed fatal parasite-induced meningitis from eating raw slugs. This includes banana slugs, which we will just say need to be thoroughly cooked before you eat them.
Now Leaving Slug World
We could stay all day, but other topics beckon. We have carnivorous caterpillars to watch out for, and parthenogenetic gall wasps to consult. As always, keep the little ones in your mind, and watch where you’re walking!
