A predatory (shell) slug (Testacella haliotidea)

Shell slug (Testacella haliotidea, Testacellidae).
Source: KILLEEN (1992).

Snails, whose way of life includes burrowing underground, might begin to reduce their cumbersome shell in their evolution and become slugs. Between snails with a complete shell and slugs without one there are numerous different grades of shell reduction, one of which are the so-called shell slugs or half-slugs – slugs with a tiny little rudimentary shell at the end of their tails, into which they cannot withdraw.

One family of shell slugs is the Daudebardiidae, a group, which belongs to the Vitrinoidea superfamily of glass snails. Another is the Testacellidae. From their tiny little shell at the tail end two characteristic furrows trail towards the head.

Testacellids live mainly underground, on one hand to hunt their favourite prey, earthworms, on the other to hide from evaporation. They find their prey by their very well developed sense of smell. With dagger-like radula teeth pointing backwards, they manage to catch the earthworm and hold it, even if it tries to pull back into the surrounding earth. Testacellids do not have an upper jaw, like for example Helix pomatia, because they, usually eating their prey in whole, need not to cut pieces off: The prey is swallowed in whole and digested alive.

Dalmatian predator (Poiretia cornea, Oleacinidae).
Picture: Klaus Bogon.

Testacellids are often found in gardens and agricultural areas, where earthworms are abundant.

There are three species of shell slugs in Europe, all of them spread around Western Europe, with a centre of distribution in France and the British Isles:

Testacella haliotidea Draparnaud 1801, the shelled slug,

Testacella maugei Férussac 1819, Mauge's slug, and

Testacella scutulum Sowerby 1821, the shield slug.

The species Testacella haliotidea is most widely spread and may even be found in Western and South-western Germany.

In contrary to the Daudebardiids, to which they resemble externally, the Testacellid shell slugs are not related to the glass snails (Vitrinoidea), but they belong to the Oleacinoidea, a snail group otherwise occurring in the Tropic. Apart from the Testacellidae, the Oleacineoidea superfamily also includes other predatory groups like the Oleacinidae family, among them the wolf snail (Euglandina) and the Dalmatian predator (Poiretia).