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Rockweed gunnel (Apodichthys fucorum)

Above: Fastosarion brazieri, commonly known as the chameleon semi-slug, and an unidentified species of semi-slug from the genus Sheldonia

The term "semi-slug" is used to describe an intermediate stage of evolution that occurs as snails evolve into slugs. Nearly 1,000 different species of semi-slug are currently known to exist, and these bizarre little creatures can be found on at least four continents.

Above: Fastosarion brazieri and Varadia amboliensis

Each species of semi-slug is technically still classified as a snail, but its shell is noticeably reduced, becoming smaller and more internalized as the species evolves. A semi-slug officially becomes a regular-slug once its shell is no longer visible at all.

As this article explains:

If life were simple, there would be snails and slugs. Snails carry their homes on their backs; slugs are naked and embarrassed. But life isn’t simple, so of course there’s secret option #3 – the semi-slug, a bizarre creature that sits exactly between the snail and the slug.

Above: genus Satiella and genus Euaustenia

This article describes the distinction between a snail, a slug, and a semi-slug:

In contrast to snails, which have an external shell large enough to accommodate the body, or slugs, in which the shell is completely internal or absent, semi-slugs have an external shell, but the shell is too small to accommodate the animal’s entire body.

Above: Megaustenia siamensis

The evolutionary process that causes the shell to recede is known as limacization, and it's especially common in moist, low-calcium environments where a snail's shell may be more of a burden than a benefit:

Terrestrial slugs are not a monophyletic group, but a case of convergent evolution in which the slug form evolved from different lineages of land snails that gradually lost their shell through a process called limacization. Limacization resulted in adaptive radiation in land snail lineages, as slugs became adapted to diverse moist and protected spaces, such as crevices in rocks and wood debris. The loss of the shell also allowed for more movement and less calcium dependence, making slugs more successful as pests.

Above: Gaeotis nigrolineata, also known as the Puerto Rican semi-slug, has a neon green shell that is almost completely internalized, but the shell is clearly visible through the semi-slug's translucent body

Some semi-slugs have shells that are still opaque and largely visible, with a patch of flesh covering only the outer edges of the shell, while others have shells that are more significantly reduced, transparent, and/or concealed.

Above: Ibycus rachelae, commonly known as the green-shelled semi-slug, and a species of semi-slug from the genus Durgella

This topic was mentioned in my previous post about Ibycus rachelae, but I wanted to write a more detailed post about semi-slugs, because they're just so fascinating and weird.

Above: a black-and-white semi-slug from subfamily Sheldoniinae

Above: Fastosarion brazieri again, but this one is especially spiky for some reason

Sources & More Info:

Australian Geographic: Meet the Semi-Slug, a Snail without a Home

Carnegie Museum of Natural History: What's So Good about Being a Slug?

Frontiers: Terrestrial Slugs in Neotropical Agroecosystems (PDF)

iNaturalist: Photos 1, 2, 3, 4, 5, 6, 7, 8, 9, 11, & 12

The Shell-Makers: Introducing Molluscs: On Becoming Sluggish

Land Snails and Slugs of Sabah and Labuan, Malaysia: Semi-Slugs