Why Slugs Hate Salt
It’s a well known fact; slugs and salt simply don’t mix, and when the two come into contact the slug’s life expectancy is rapidly diminished to a few seconds. But why is that, I wonder...
The short answer
The slug contains a lot of water, which is essential for its survival. Sprinkling with salt draws that water out, and the poor creature dies of dehydration.
And the long answer...
It’s called osmosis
Osmosis is a natural biological phenomenon whereby water passes from one region of high water concentration, through a semi permeable membrane to a region of lower water concentration.
Salt is a desiccant, meaning that it absorbs water from things. That’s why consuming an excess of it makes you feel thirsty. So when applied to the moist surface of a slug, it immediately sucks up water from its skin, which is far more permeable than the skin of most other animals.
The process of osmosis is triggered and large amounts of fluid from inside the slug rush to the surface to restore equilibrium and dilute the concentration of salt. This results in the thick goo you see.
The unfortunate slug rapidly loses more fluid than it can tolerate, and quickly dies from dehydration. Personally I think this sounds a particularly tortuous way to die, even for a slug.
So why not humans
Why don’t you fizzle into a gooey puddle if you spill salt on your skin? Well, one reason is that in proportion, a slug contains far more water than you do. The other reason is that you have a much more impermeable layer of skin. You don’t see people oozing body fluids like a slug does!
But what happens when that skin is broken; when you have an open wound? What happens then if you sprinkle salt on it...
Plants and salt
Salt in the soil or on plant leaves triggers osmosis too, this time within the plant cells. Water is drawn out of them to restore the equilibrium, and that’s why leaves burn and the plant eventually dies.
If you must use salt on slugs, be sure to keep it well away from your plants. A concentration of salt in the soil can take years to disperse.
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When picked up or touched, the Black Slug will contract into a hemispherical shape and begin to rock from side to side. This behaviour confuses predators
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