A predatory (shelled) slug (Testacella haliotidea)

Testacella haliotidea
Yellow-grey shell slug (Testacella haliotidea).
Picture: M. Gurney (Flickr.com). Picture 1! Picture 2!

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 are the Daudebardias (Daudebardiinae), a group in the Gastrodontoidea superfamily. Another is the Testacellidae. From the tiny little shell at their tail end, two characteristic furrows trail towards the head. Testacellas are, however different from Daudebardias in their shell being so far reduced that it resembles a small shell of a sea-living ormer or abalone (Haliotidae).

  Testacella scutulum
Under ground and when feeding, Testacellas
withdraw their tentacles. (here T. scutulum).
Picture: Piergiorgio Di Pompeo (Source).

Testacellids live mainly underground, on one hand, to hunt their favourite prey, earthworms, on the other to hide from evaporation. They find their prey through 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 cut pieces off: The prey is swallowed whole and digested alive. Testacellids are often found in gardens and agricultural areas with an abundance of earthworms.

There are three species of shell slugs in Europe, all of them distributed 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 distributed and may even be found in Western and Southwestern Germany.

Conchological Society of Great Britain & Ireland: Identification aid: Testacella species.
Anderson, R. (2008): "An annotated list of the non-marine Mollusca of Britain and Ireland" (PDF).


Class: Gastropoda
Subclass: Pulmonata
Superorder: Eupulmonata
Order: Stylommatophora
Suborder: Sigmurethra
Infraorder: Achatinoinei
Superfamily: Testacelloidea
Family: Testacellidae Gray 1840

Source: Mollbase on http://www.mollbase.de/list/.