Science Spotlight: Banana Slug Slime

Spiky proteins called mucins allow banana slugs to crawl over all kinds of ground cover without slipping or sticking.

by Sarah McQuate

When hiking in the cool damp forests surrounding Santa Cruz, it’s hard to miss the yellow banana slug. It looks like its namesake—and it feels as slimy as a slice of ripe mango.

These slimy slugs are covered in mucus, which is similar to our snot. The oozy substance is composed of proteins called mucins, which have spiky sugar molecules on their edges. “They look a bit like bottle brushes, where the sugar spikes are the bristles,” says biomolecular engineer Christopher Viney of UC Merced. When many mucins pack tightly together, their bristles can interact to create specific patterns, such as snowflake shapes.

Slugs pump out the tiny mucins within concentrated but dry packets. Once these packets mix with water in the environment or from the slug itself, they swell up to 1,000 times their original volume to become mucus. Slug slime flows easily over most surfaces, says Viney—yet clings stubbornly to human fingers.

Banana slugs use their slippery mucus to crawl around on almost anything. Mucins reorganize themselves into different patterns to complement the surface a slug needs to crawl on, giving it a smooth ride over most jagged nooks and crannies.

As banana slugs meander through their environment, they leave a slime trail with secret messages for other slugs that come across it. “Slugs know how to follow it,” says Viney. “They know which direction to go. There is an unambiguous left and right.” He suspects the secret has to do with how the mucins left in the trail are organized. For now, only the slugs know how to break the code.

Note from the editors: Science Spotlights are a joint project of the UCSC Science Communication Program and Hilltromper. This article was published in November 2016.