I recently photographed a banana slug for a short article I was writing about banana slug slime for the Santa Cruz (California) Hilltromper, a website for nature lovers in the area.
“Okay, now turn your head to the left… down a little… actually can you crawl up this rock a bit more for me? I’d really like to see some more of that slime,” I said, shoving my iPhone in this poor slug’s face.
After I finished the photoshoot, a passerby asked me what I had been photographing. When I told her, she said “They’re so yellow! They must not taste very good.”
I thought she was referring to the idea that brightly colored animals are often poisonous or don’t taste good. But having grown up in Northern California where these large slugs are beloved creatures, I couldn’t recall anything directly connecting their yellow color to self-defense. As part of my photography session, I had picked up the slug, and my hands were covered in gooey slime. I found myself wondering if the banana slug’s sliminess could make it hard to eat.
To answer my question, I talked to Christopher Viney, a professor of engineering at University of California, Merced who has studied the consistency of slippery materials like banana slug slime and giraffe saliva. It turns out that a protein called a mucin is key to banana slug self-defense. Mucins are a primary ingredient of the mucus that makes up banana slug slime. Mucin proteins form chains and have sugar molecules decorating their sides, like a wire brush, according to Viney.
Banana slugs excrete mucins as dry granules to make their slime. These granules expand when they meet up with nearby water, increasing their volume hundreds of times, Viney says. That’s like adding water to a grain of rice and seeing it expand to the volume of a tablespoon.
The mucins unfold in an organized pattern when they meet up with water. The sugar molecules network to form repeating patterns, like the patterns in a crystal. These patterns help link the mucus together, giving it an overall slimy feeling, according to Viney.
Banana slugs produce a lot of mucus in self-defense, causing their predators’ mouths to be filled with goop and a slippery slug.
Banana slug slime is also an anesthetic, meaning it will make a predator’s tongue or throat go numb.
Tongues aren’t the only things that are susceptible, says Viney. “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,” he says.
This numbing effect is unique to banana slug mucus, but mucins are unlikely to be involved.
In addition to self-defense, these slugs use mucins for a variety of other functions, including locomotion. Mucin patterns help slugs interact with rough surfaces. Mucins reorganize to coat sand or rocks, allowing slugs to cruise through the forest.
Interestingly, mucins play a similar role in giraffe saliva, which Viney thinks protects giraffes’ long tongues from pointy acacia tree thorns while these creatures munch on leaves.
Under a microscope, a grain of sand looks rough and spiky compared to pointy thorns, which look much more smooth and blunt, Viney says.
The mucins in banana slug slime have to form small networks to coat the tiny spikes of sand. The giraffe saliva mucins, however, organize into bigger patterns to maximize lubrication over the much larger—but still pokey—thorns, according to Viney.
Mucin patterns protect humans too. Our lungs contain mucus that catches rogue dust particles and pathogens such as viruses and bacteria. In healthy individuals, these contaminants get stuck in the mucus and specialized cells sweep the mucus away, allowing us to breathe easily.
When people have conditions like asthma or cystic fibrosis, the mucins don’t organize into the correct patterns, changing the mucus consistency. If the mucus is too watery or too sticky, then our cells can’t clear it and the contaminated mucus pools in our lungs, according to Viney.
He originally started researching banana slug slime to model cystic fibrosis patient mucus. He wanted to learn what properties determined the stickiness of the mucus.
Banana slug slime doesn’t totally protect slugs from getting eaten. Many predators still find banana slugs to be a tasty snack. Animals like raccoons will roll banana slugs around in the dirt to try to avoid being slimed.
Humans also eat banana slugs. A quick Google search will lead you to a variety of recipes that detail how to make your own deep fried banana slug. They even have step-by-step pictures.
These recipes all agree on one point: make sure you remove the slime.