When salt is applied to slugs, they appear to shrivel and dissolve. This “melting” effect results from fundamental biological and chemical principles. It’s a direct consequence of their unique physiology interacting with salt.
Why Slugs Are Vulnerable
Slugs are highly susceptible to salt due to their biology. Their bodies are primarily water, and unlike snails, they lack a protective external shell. This leaves their delicate internal tissues exposed.
Slugs breathe through their moist, permeable skin. This allows for gas exchange but also makes them vulnerable to water loss. They secrete mucus to maintain moisture and prevent desiccation. Despite this, their high water content and permeable surface make them vulnerable to dehydration.
The Science of Water Movement
The reaction of a slug to salt is rooted in a process called osmosis. Osmosis is the movement of water across a selectively permeable membrane, like a cell membrane, from an area of higher water concentration to an area of lower water concentration. This movement occurs to balance the concentration of solutes on both sides of the membrane. The membrane allows water molecules to pass through but restricts the movement of larger solute molecules, such as salt.
When salt is sprinkled on a slug, it dissolves in its moisture and mucus, creating a highly concentrated salt solution on the exterior. Inside the slug’s cells, water concentration is significantly higher than in this external solution. This difference creates a steep concentration gradient. Water rapidly moves out of the slug’s cells into the more concentrated salt solution outside, attempting to equalize concentrations. This passive process does not require the slug to expend energy.
From Salt to ‘Melt’: The Dehydration Process
The rapid exodus of water from the slug’s body due to osmosis leads to severe dehydration. As water leaves the slug’s cells, they shrivel and collapse, compromising essential bodily functions. This water loss causes the slug to visibly shrink and appear to “melt.” Bubbling is often observed as air is expelled from the shriveling body.
The “melting” appearance is not chemical dissolution, but a physical transformation from rapid water loss. The slug’s soft body, maintained by water content, loses integrity as water is drawn out. This disrupts internal functions, from muscle to nerve impulses, ultimately leading to its demise.