I really shouldn’t be so discriminatory here. All gastropods (the class name for snails and slugs) are brilliant engineers. First of all, they grow their own home; a pretty impressive feat. But that’s a topic for another day … this story is about the incredible properties of their slime.
Snail slime does two jobs: it allows the snail to crawl across a surface, but also helps the little creature stick to that surface. This adhesive role is especially important for marine gastropods, otherwise they’d be constantly shoved around by water currents and swirling vegetation. But, when you think about it, these two functions (movement and adhesion) seem completely incompatible. How does a snail make slime that keeps it stuck to a rock AND allows it to wiggle around to find food?
Back in the 1980s, some clever biologists conducted a series of experiments to find out. They designed and built ingenious devices to test what happens to snail slime when different amounts of push are applied across the surface of a very thin layer, “under conditions as closely as possible approximating those under a crawling slug”. What they discovered is that these creatures generate a truly unique substance: as you push harder on the slime, it becomes runnier. When you stop pushing, it ‘heals’ and returns to something thicker. With very small pushes, the slime acts like a stretchy solid that snaps back into place, like gelatin, keeping the snail attached to the surface. With more sustained, stronger pushes, the slime acts as a runny liquid, like warmed honey, making it easier for the snail to move across the surface.
Brilliant, right? So, the next question is: How? What happens to the slime to change its runniness this dramatically? Well, for that we skip forward almost 40 years to a study by a group of engineers in China, published in 2018. This team collected mucus from snails crawling across smooth glass plates at angles ranging from flat on the lab-bench to completely vertical. As the angle increased, the structure of the slime changed significantly. The slime from the flat plate was made up of tiny little nanoparticles distributed more or less uniformly. The slime from a plate at 45⁰ showed the nanoparticles starting to clump up into microspheres and the slime from the vertical plate was thick with fibres formed from globs of microspheres attaching together. Nanoparticles – runny slime. Microparticles – less runny slime. Fibres – almost gel-like slime. This team is now working on making fibres based on their results for biomedical applications.
Want to make some ‘anti-slime’ at home? We usually call this Goop or Oobleck – 2 parts corn starch to 1 part water (plus food colour if you’d like). When you push gently it behaves like a liquid. When you hit it hard, it feels more like a solid. Squeeze some into a ball in your hand and throw it to a friend – solid in your hand, drippy and runny in the air, solid again when your friend catches it! The exact opposite of what a snail needs to stick to the surface and move around, but a little bit of fun for an afternoon during physical distancing.
(Oobleck gif from Giphy: https://gph.is/1Qm2Kmc)
The physical properties of the pedal mucus of terrestrial slug, Ariolimax Columbianus; M. W. Denny & J. Gosline; Journal of Experimental Biology 1980 88 375-393 (https://jeb.biologists.org/content/88/1/375)
Controlled self-assembly of glycoprotein complex in snail mucus from lubricating liquid to elastic fiber; T. Zhong et al; Royal Society of Chemistry Advances 2018 8 13806-13812 (https://pubs.rsc.org/en/content/articlelanding/2018/ra/c8ra01439f#!divAbstract)