Semi-slug

Image: Bernard DUPONT

You got your slugs, you got your snails, but don't forget your "a little from column A, a little from column B" semi-slugs!

Semi-slugs are snails who have a shell that's too small for them to retract into. They're dotted around the gastropod family tree, so there are several branches where a bunch of slugs, snails and semi-slugs are more closely related to each other than to any other slug, snail or semi-slug.

Image: Wikipedia
Amphibulima patula dominicensis

It would appear that getting rid of your shell is quite the thing for young snails about town, but it takes time. Rome wasn't levelled in a day! So it starts with subtle shell shrinkage...

Image source

And then just keep going! It doesn't take much shrinkage before the shell starts to look a little embarrassing, like you walked out in the middle of a haircut. The next step, therefore, is to hide the thing.

Image: Thomas Brown

Slugs and snails have what's called a mantle. It contains many of the vital organs and it creates the snail's shell, which means a snail's mantle is almost completely covered by the shell. Semi-slugs turn the tide - their mantle is unleashed and starts to swallow the shell whole.

Image: Bernard DUPONT
Yellow-shelled Semi-slug (Parmarion martensi)
Barely any yellow-shell left to see!

The process continues over the generations. Some species look like they're a slug who ate a snail several sizes too big. It's as if their guts are splitting, exposing a tiny sliver of shell.

Image: Luis Daniel Carbia Cabeza

And on it goes! The mantle finally meets in the middle, the shell is completely covered and we have a Semi-slug with a bulging, internal shell.

But why stop there? Surely the shell should continue to shrink so it doesn't cause such an unsightly bulge?

Image: Cody Hough
Grey Field Slug (Deroceras reticulatum)

If you take a completely ordinary slug that's very obviously a slug and nothing but a slug, and look at it closely. Very closely, now. The kind of close look that requires a scalpel...

Image: Welter Schultes, Francisco
Grey Field Slug shells

You might find a shell! A tiny, thin shell embedded deep in the mantle and so flimsy as to be completely useless. It's a vestigial shell. Other slugs don't even have that, they've lost their shell and every vestige of its existence.

Video: Benjamin Blonder

You might ask why this would happen. Why would you get rid of a strong, thick shell that protects you from predators and stops you from drying out?

Image: H. Zell
This semi-slug shell is cheap and useless. You get what you pay for

The snail's shell is certainly useful, but it has to be paid for. Snail's require a lot of calcium from nearby rocks to produce a strong shell, so ridding themselves of this requirement allows slugs and semi-slugs to live in new areas and focus their energies on other things.

It also means they can squeeze into nooks and crannies or burrow underground more efficiently. Not only does this let them find food snails can't reach, it also means they can still protect themselves from predators and dry weather even without a shell.

Image: Parks Victoria

So while semi-slugs are quite clearly mad, be assured there's method in it!