Salt Shakes Out the Differences Between Garden Slugs & Sea Slugs

So you’re out in your garden at night sprinkling salt on slugs and snails to kill them, and then the next day you come into The Maritime Aquarium (or go to the beach) and see all the snails that are living in … salt water.

And naturally this makes you wonder: why is salt lethal to garden slugs and snails but a normal part of the habitat of sea slugs and snails?

The answer has to do with differences in their body chemistry. And the process of osmosis.

Nature likes to even things out. To settle to the average. One way it does this is through osmosis: moving a solute (like salt) from one solvent (say, water or body fluids) into an adjacent solvent that has a lower salt concentration … to equalize the concentrations in both.

Hunh?

Here’s an example: in the Norwalk harbor, where fresh water from the Norwalk River enters Long Island Sound, the lighter-in-weight fresh water initially rides up over top of the heavier saltier Sound waters. Seriously. The salinity near the water surface is less than the salinity at the bottom.

But after a while, helped along by the stirring of waves, some of the salt moves into the fresh water and the salinity becomes similar (if not exactly alike) from top to bottom.

Here’s another example: you have salt in your body. (Ever tasted one of your tears?) When you take a long bath or swim in a freshwater pond, your fingers get wrinkled because some of the salt solution in your body moves out through your skin cells into the water, as nature tries to average things out through osmosis. You get prune-y. This doesn’t happen when you swim in the ocean, because the salinity of the ocean and the salinity in our bodies is pretty similar.

Now back to the slugs.  When you pour salt on the moist exterior of a garden slug or snail, it becomes WAY saltier outside than inside. Nature tries to fix this by pulling liquids out of the slug’s body to make the salt levels the same, inside and out. Imagine having half of the water inside your body violently leave through your skin cells. That’s what kills the slug.

On the other hand, sea slugs and snails – we’re going to generalize here – are born having body fluids with a salinity that’s similar to the ocean water they live in. Most would die by being plopped into fresh water.

(Obviously, though, many marine gastropods – like the mud snails and periwinkles that are common in Long Island Sound – do just fine where the water salinity may vary noticeably in the course of a single day, like in the Norwalk harbor. This may be because their outer “skin” is tougher and less permeable, so osmosis won’t occur as suddenly as with a terrestrial snail or slug. And/or they may have a physical adaptation that allows them to control their bodies’ salt content – a process called osmoregulation.)