Is a Slug Slimier than a Snail? (asked by Anonymous)
Land snails and slugs like the ones you get in your garden are members of the class Gastropoda, along with sea snails, sea slugs and limpets. However, they do not represent distinct branches of the evolutionary tree – slugs are just species of snail that have lost their shell. There are even some snails that have a reduced shell – halfway between slug and snail. So this makes it difficult to answer the question.
However, there are some good reasons why slugs (shell-less snails) might seem to be slimier than their shelled counterparts.
Firstly, as a simple surface area relationship, slugs are likely to seem slimier because they have a larger area of sliminess to come into contact with. Gastropods produce mucus across their entire bodies. And because slugs have a higher surface area of exposed flesh, they tend to produce more mucus across their body. Finally, the composition of mucus varies across the body – it is stickier around the ‘foot’ and thinner across the body. This may mean that the slug slime you come into contact with tends to be thinner and slimier than the slime on the foot of a snail.
So are slugs slimier than snails? Well, sort of. Producing a thinner mucus across their bodies, slugs will tend to feel slimier to the touch if you pick one up, but on average, the slime on the foot of a slug (ie. if it crawls across you) would be no different than that of a snail, and is more likely to reflect the weather, time of year and the creature’s last meal than anything else.
What is Slug Slime?
Snail and slug slime is a type of mucus – an external bodily secretion, composed largely of water and protein. It’s pretty remarkable stuff – gastropod mucus possess two seemingly incompatible traits – it is extremely sticky but can also act as a lubricant. It’s unique properties are achieved by a network of heavy glycoproteins known as mucopolysaccharides within the water. Gastropod mucus varies between 91 and 98% water, combined with proteoglycans, glycosaminoglycans, glycoprotein enzymes, hyaluronic acid, copper peptides, antimicrobial peptides, and metal ions, to form a jelly-like substance. It is a hygroscopic substance, meaning it is physically altered to be more lubricating as it absorbs water. And it can absorb a lot of water – expanding from tiny grains of glycoprotein to over 100 times its initial volume in slime.The mucus acts to lubricate movement, adhere to surfaces, deter predators, deflect pathogens and attract mates. However, both within and between species, even within an individual’s lifetime and across its body, the exact composition of the mucus is highly variable.
As I mentioned above, gastropods produce a different type of mucus on their ‘foot’ than the rest of their body. Known as pedal mucus, the mucus for the foot is used as a lubricant to allow the animal to move over a variety of different terrains. However, it also has glue-like properties, enabling snails and slugs to defy gravity and climb walls or crawl across ceilings. In some gastropods, such as barnacles, the foot mucus is also used to form a temporary seal called an epiphragm onto the surface of a rock – anyone who has ever been rock-pooling will appreciate just how strong a barnacle’s grip is! The mucus produced elsewhere on the body is designed to be protective, and in land-dwelling gastropods such as land snails and slugs, it helps to prevent water loss from exposed areas.
Some species have been shown to vary the composition of their mucus depending on where they are, what they’re up to and even what they’ve been eating and what time of year it is. The common garden snail, Helix aspersa can produce two different types of mucus – one is adhesive while the other lubricates – depending on whether it wants to move around or stick to a rock. The two types of mucus vary in their protein content and this influences their elasticity and ultimately how sticky they are.
Snail Slime is Surprisingly Useful
We might not like snail and slug slime very much; it creeps up our windows and is horrible to touch, but we use it far more than you might imagine. Gastropod mucus is used in cosmestics, and medicinal properties were first reported by the Ancient Greeks, who used it to treat ulcers and to sooth coughs. It contains antioxidants and has been suggested to stimulate collagen formation and skin repair mechanisms, making it a potentially powerful treatment for skin ageing and cancer. It also has antibacterial properties, and can act as a local anaesthetic. Together, the anaesthetic and antimicrobial properties of slug slime combined with its hydroscopic nature may make it an ideal treatment for superficial burns and rashes, and other minor but very painful wounds. In trials, slug slime has proved an effective protective layer for damaged skin, reducing pain and providing a water-resistant, antibacterial covering for the wound. One study found that slime from the common garden snail stimulates wound repair, further adding to it’s repertoire of wound-healing properties. Finally, scientists from Massachusetts Institute of Technology have recently succeeded in engineering a strain of bacteria that produce glue based on snail slime. I say based on, because the researchers decided to do one better than nature, modifying the slime with genes from a marine bacterium, so that it no longer dissolves underwater. The result is a super-strong underwater glue that could be used to repair ships and oceanic machinery and even heal wounds.
Want to Know More?
- Pawlicki et al (2004) The effect of molluscan glue proteins on gel mechanics Journal of Experimental Biology
- Davies and Hawkins (1998) Mucus from Marine Molluscs Advances in Marine Biology
- Cottrell et al (1993) Evidence for glycosaminoglycans as a major component of trail mucus from the terrestrial slug, Arion ater L. Comparative Biochemistry and Physiology Part B: Comparative Biochemistry
- Bretz and Dimock (1983) Behaviorally important characteristics of the mucous trail of the marine gastropod Ilyanassa Obsoleta (Say) Journal of Experimental Marine Biology and Ecology
Featured image used under a creative commons license from Wikimedia commons. Original image by Xauxa