Banana slugs are the celebrities of the slug world. They're huge, reaching nearly 10 inches in length and weighing over four ounces. They're bright yellow, which makes them much more interesting to look at than a brown, garden-variety slug. And they're so beloved that they're even a mascot for University of California, Santa Cruz. However, their best attribute isn't their pretty looks. It's their slime. Truly, banana slug slime is something to marvel at, once you've gotten over the ick factor.
Their slime starts out as dry granules
It would take a vast amount of water to constantly produce slime, so the banana slug has a novel adaptation that makes the slug's environment to do much of the heavy lifting. Banana slugs dispense dry granules of mucus which then absorb surrounding water. A granule can absorb several hundreds of times its volume in water, helping the slug create maximum lubrication with minimum exertion.
"That’s like adding water to a grain of rice and seeing it expand to the volume of a tablespoon," writes Sarah McQuate at Scientific American.
This is why banana slugs need to be in moist environments. All that water around them is critical for keeping them on the move.
Built-in bungee cord of slime
A banana slug can use slime for more than sliding across the ground from point A to point B. It also uses it for acrobatics. According to Bay Nature, "The banana slug has a mucus plug at the end of its tail. It can generate a slime cord from this plug, which it then uses like a rappelling line to descend slowly from high places. The banana slug’s relative Limax maximus, the leopard slug, mates in midair while hanging suspended by its mucus strand."
When you're thinking about a big slug that can weigh as much as four ounces, that's an impressive feat of strength for slime!
Slime will make a predator's tongue numb
The slime covering a banana slug helps deter would-be predators, and not just because of the stickiness. Along with increasing the production of slime to create a sticky mouthful, the slime also contains chemicals that act as an anesthetic, numbing the tongue and throat of an animal that tries to eat it. It only takes one attempt to figure out that banana slugs aren't worth the trouble as a snack.
You might also notice this effect if you try to handle a banana slug. According to Christopher Viney, a professor of engineering at University of California, Merced, "If you don't wear gloves when you pick up banana slugs, you will find that your fingertips start feeling numb after a short while."
Meanwhile, that same slime helps provide a meal for the slug. As plant matter and debris clings to the slug, the slime helps to slowly slide it all down to the end of its body. You may have noticed this in action if you've come across a banana slug with a wad of pine needles and leaf fragments at its tail. As the collection grows, the snail can simply turn around and feast on what's been gathering at its rear end.
Slime is simultaneously a lubricant and adhesive
Slime is simultaneously a liquid and solid, or rather, a substance somewhere in between the two. Slug slime is a liquid crystal, organizing molecules in a structured but flexible way.
As Ask Nature points out:
snail and slug mucus is apparently used to both lubricate the snail’s movement over surfaces and to facilitate the transfer of adhesive force to the surface. Only a "viscoelastic" fluid could have both properties since ordinary fluids are either viscous or elastic.
Researchers are looking into how to harness this dual power for methods of locomotion.
Slime is a long chemical message to others
As we noted earlier, slime contains a lot of interesting properties and chemicals — and no one knows this better than the banana slugs themselves. As they travel along and leave behind a trail of slime, they're also laying down notes to one another. Other slugs can read the messages and can follow the trails.
Curious to learn more about banana slugs and their amazing slime? KQED Deep Look has a great short video on the subject, which you can view above.