When we humans have a big old glob of snot trailing from our noses, it can be… kind of gross.

But snot is just our way of getting rid of dust, pollen, viruses or other nasties that may get into our noses. And in the animal world, slimy secretions can take lots of forms and do other, really useful things.

Slime doesn’t have a strict scientific definition, but a lot of it has the same basic chemical ingredients: proteins, carbohydrates, and lots of water. In most slime, these ingredients are arranged in long, flexible polymers called mucins. Mucins can have different compositions, but they’re all glycoproteins, which are protein chains with sugars sticking out.

Other mucus can involve water-loving sugar polymers called glycosaminoglycans, or GAGs. And slime is so slimy because of how these polymers are arranged. In a solid, molecules are more organized and tightly packed, while in a liquid, molecules are arranged more randomly and move around more.

Slimes are somewhere in between. The polymers are lined up, but they can glide past each other. They’re usually non-newtonian fluids, so stretching, pushing, or squishing changes how viscous they are, or how easily they flow. [MIK-seen glue-tin-O-sa] Now, if you were to crown the slimiest creature on the planet, it would probably be the hagfish.

Its scientific name, Myxine glutinosa, actually comes from Greek and Latin words for slime and glue. Hagfish are jawless sea creatures with long bodies, flat tails, and somewhat terrifying mouths with rows and rows of pseudo-teeth. They’re made from keratin, like our hair, instead of materials like dentin and enamel like many true teeth.

They mostly roam around the seafloor, scavenging for worms or eating dead fish from the inside out. And if a predator tries to slurp one up like a tasty sea-noodle, the hagfish fights back. It spews a bunch of gill-clogging slime, by excreting a concentrated mix from around 100 pores along the length of its body.

This pre-slime mix contains mucin vesicles that burst in the surrounding seawater, because of a process called osmosis. The concentration of salt in the vesicles is low relative to the seawater, so water rushes through the membrane. Plus, the mix also has fine, twisted, protein fibers that are unique to hagfish slime called skeins.

They give the slime strength, help it stick together, and trap a lot of seawater. In fact, by one group’s measurements, about 99.996% of hagfish slime is seawater. So, a little protein can go a long way.

The more a predator struggles, the more viscous the hagfish slime becomes, making it gag and back away. Or, if the predator keeps attacking, it might even suffocate and die. And to escape its own mucus-y mess, a hagfish will tie its body into a knot and physically scrape the slime off.

On land, leopard slugs don’t use slime for defense… they use a string of it to get busy, wrapping around one another while hanging from a tree branch. First off, slugs attract their mates by secreting chemicals called pheromones into their slime trail. Once they’ve found a partner, they climb a nearby tree by changing the viscosity of their slime using muscle contractions.

At rest, mucin molecules in the slime are lined up like in a solid, so the slime basically glues the slug to a surface. But when the slug’s muscles contract, they push on the slime. That rearranges the mucin molecules enough so that the slime flows more like a liquid, and the slug can crawl along.

Once slugs reach their branch of choice, they squeeze out a long string of slime. The molecules realign to form a strong, semi-solid rope, and then they can do their Cirque-du-Soleil mating feats. It’s a lot of effort, but at least one researcher thinks that leopard slugs need the help of gravity to extend their penises which inflate to become almost bigger than their bodies.

And they have matching genitals! Because most slugs, including these ones, have both sperm and eggs inside them and can fertilize themselves. So this ooze-filled ritual helps ensure that, when two leopards slugs mate with each other, they make a lot of contact and successfully trade sperm so both sets of eggs get fertilized.

Now, one of their cousins, the violet sea snail, also secretes slime from its foot. But instead of acrobatic mating, this sea snail uses slime to float on the ocean. Violet sea snails are pelagic, meaning they hang out on the ocean’s surface.

And I mean that literally — they’ll hang upside down from a little raft of bubbles. The bubbles are made by splashing its foot and trapping air in a tough mucus, which one researcher described as similar to bubble wrap. Scientists think that a violet sea snail’s bubble boat could’ve evolved from egg sacs that were held together by a string of goo.

Empty ones may have helped with buoyancy, so adding more mucus-covered air made this floating ability a more permanent thing. With its sturdy raft, the sea snail can set out in search of food, which is mainly soft-bodied, stinging hydrozoans like the Portuguese man o' war. Somehow it eats them without getting killed by toxins, though.

Maybe the snail sidles up next to its food while avoiding stinging parts, or somehow neutralizes the stinging cells. Either way, the mucus probably plays a part. So even though glue and boric acid slime tutorials are all the rage on YouTube, nature’s been doing some pretty cool slimy stuff all along.

Thanks for watching this episode of SciShow! If you want to learn more weird and wonderful facts about living things, you can go check out our sister channel Animal Wonders at youtube.com/animalwondersmontana.