~~~ Snail Slime 101 ~~~
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© 2007 Manxminx
Sometimes we hate it, but we can’t 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. It’s used in food manufacturing, moisturising creams and medicines. It’s a type of sugar, but it isn’t sweet. It helps slow down the aging process. Some birds use it to built their nests.
I’m talking about saliva, catarrh, and other such bodily fluids. They all contain this life sustaining fluid – mucus. Snail and slug slime? Yes, it’s 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 what’s 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 don’t 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
‘Mucopolysaccharide’. 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 ‘pedal mucus’ because it’s released from their foot) when mixed with water forms the thick jelly like material we call slime, a ‘
biopolymer gel’ with some amazing properties. It’s both a glue AND a lubricant! Also, because it absorbs and holds water, it’s very good at stopping things from drying out – hence it’s 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 ‘Adhesive Locomotion’, 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 wouldn’t – it would stick to the surface instead. Adhesive Locomotion can be described like this – 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 doesn’t move – it’s 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 it’s moving you’ll see a ‘wave’ motion to it’s skin. What’s actually happening is that it’s sticking and sliding at the same time with different parts of it’s underside.
Muscular contractions lead to waves that (i) compress the foot, forcing that part to break free from the slime and move forward – 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 – a snail is standing still on a flat surface, it’s head starts moving forward, but it’s shell and rear end stays still. Suddenly, the shell shoots forward and the whole snail is now moving. It’s 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 ‘sticky’. Biologists have managed to identify specific ‘glue proteins’ 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
scienceblogs.com/zooillogix/2007/07/interview_with_dr_corfield_muc.php
Gooey solution to a sticky problem
www.thefreelibrary.com/Gooey+solution+to+a+sticky+problem.-a0166751417
Rheological fingerprinting of gastropod pedal mucus and synthetic complex fluids for biomimicking adhesive locomotion
www.rsc.org/Publishing/Journals/SM/article.asp?doi=b615546dTuning
Gastropod Locomotion: Modeling The Influence Of Mucus Rheology on the Cost of Crawling (lauga, 2006)
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]
I never knew a substance could be both a glue and a lubricant at the same time. ;D