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The aliZone.com - Snail Slime 101
The aliZone.com
Snail Slime 101
Sometimes we hate it, but we canít live without it. It sticks our cells together and lubricates our joints. Jellyfish and plants are full of it. It enhances male virility and acts as a barrier, protecting us from disease. Itís used in food manufacturing, moisturising creams and medicines. Itís a type of sugar, but it isnít sweet. It helps slow down the aging process. Some birds use it to built their nests.
Iím talking about saliva, catarrh, and other such bodily fluids. They all contain this life sustaining fluid Ė mucus. Snail and slug slime? Yes, itís got mucus in it.

Human saliva is 99.5% water, the other half a percent being made up of mineral salts, amylase (an enzyme that breaks down starches), Lysozyme (an enzyme that kills bacteria), and mucus (known as mucin). As you can imagine though, when we have a cold, what comes out of our noses contains a lot more mucus than whatís in saliva!

Snail and slug slime is similar to human saliva. It contains around 90% water, some mineral salts, and as much as 10% mucus. But snail and slug mucus is a little different to human mucus. After all, we donít use saliva to climb walls and stick to ceilings!

Chemically speaking, human and other vertebrate mucus is a glycoprotein, consisting of a protein backbone onto which polysaccharides (complex carbohydrates) have covalently attached. It can be referred to as a ĎMucopolysaccharideí. Invertebrate mucus though is better described as a protein-polysaccharide complex which contains predominantly complex carbohydrates with a small amount of protein.

Basically, snail and slug mucus (known as Ďpedal mucusí because itís released from their foot) when mixed with water forms the thick jelly like material we call slime, a Ďbiopolymer gelí with some amazing properties. Itís both a glue AND a lubricant! Also, because it absorbs and holds water, itís very good at stopping things from drying out Ė hence itís moisturising effect.

But how can a glue also be a lubricant? After all, lubricants are supposed to stop things sticking together! Welcome to the wonderful world of ĎAdhesive Locomotioní, where snails and slugs crawl on a thin layer of pedal mucus using muscular contraction and expansion which makes the structure of the gel alternatively rupture and reform.

Imagine sliding the palm of your hand over a smooth surface. If the surface was oily then your hand would slide quite easily. If it was sticky, then it wouldnít Ė it would stick to the surface instead. Adhesive Locomotion can be described like this Ė Our imaginary surface is covered with a substance that our hand sticks to (toothpaste or wet clay works well for this experiment). We slowly try to slide our hand across the surface. It doesnít move Ė itís stuck. We gradually increase the force and suddenly our hand breaks free and slides easily.

Snails and slugs move and stick to surfaces at the same time. If you look at the underside of a snail on glass while itís moving youíll see a Ďwaveí motion to itís skin. Whatís actually happening is that itís sticking and sliding at the same time with different parts of itís underside.



Muscular contractions lead to waves that (i) compress the foot, forcing that part to break free from the slime and move forward Ė the slime acting as a lubricant. The inter-waves (ii) are areas of lower force, where the muscle has relaxed, allowing the slime to act as a glue, sticking that part of the snail to the substrate.

Sometimes you can see the breakaway effect quite easily Ė a snail is standing still on a flat surface, itís head starts moving forward, but itís shell and rear end stays still. Suddenly, the shell shoots forward and the whole snail is now moving. Itís at that point when the shell shoots forward that the snail has exerted sufficient pressure to unstick itself.

It would appear that some types of gastropods can change the composition of their slime to make it more or less Ďstickyí. Biologists have managed to identify specific Ďglue proteinsí in and note that at times, the concentration of glue proteins in slime can vary upto between 10 and 50%, especially in the likes of Limpets and Periwinkles.

Out of interest, mucus production accounts for about a third of the total energy output of snails and slugs, far more than the mechanical energy required for locomotion, making snail and slug crawling the most energetically expensive type of locomotion known among both vertebrates and invertebrates.

References & Further Reading

Interview with Dr. Corfield, Mucin Biologist
http://scienceblogs.com/zooillogix/2007 ... ld_muc.php

Gooey solution to a sticky problem
http://www.thefreelibrary.com/Gooey+sol ... 0166751417

Rheological fingerprinting of gastropod pedal mucus and synthetic complex fluids for biomimicking adhesive locomotion
http://www.rsc.org/Publishing/Journals/ ... 546dTuning

Gastropod Locomotion: Modeling The Influence Of Mucus Rheology on the Cost of Crawling (lauga, 2006)
http://dspace.mit.edu/handle/1721.1/37326

Ewoldt, 2006. Rheology of Complex Fluid Films for Biological and Mechanical Adhesive Locomotion [pdf - 3.35 MB]

Pawlicki et al, 2004. The effect of molluscan glue proteins on gel mechanics [pdf]

Smith, 2002. The Structure and Function of Adhesive Gels from Invertebrates [pdf]