Testacella in the Garden

Testacella , or the Shelled Slug, has a distinctive way of life living buried in the soil where it feeds on earthworms. Unlike the internal shell plates in the mantle at the anterior end of some slugs such as limacids, the shell plate of Testacella is attached to the tail on the outside. Most Testacella that I have found have been in my Alton garden, but before Conch. Soc. members start placing orders, I have to point out that their occurrence is sporadic and unpredictable.  However they are more likely to be found on the surface after very wet weather or under objects such as flower pots on earth. It was whilst I was moving pots around looking for specimens of any snails to take into a school in autumn 2009 that I came across the two illustrated examples that were different colours, perhaps polymorphic forms (figure 1). From the apricot-coloured foot sole I identified them as T. scutulum (figure 2).  The species distinction between T. scutulum  and  T. haliotidea seems to hinge on the colour of the foot sole, the lateral grooves combined before the junction with the shell in T. scutulum  and close together in T. haliotidea as illustrated in Cameron, Jackson and Eversham 1983. The latest Non-marine Atlas (Kerney 1999) has explanatory notes and does suggest that the two species may not be distinct in view of their identical distribution and habitat. What effect do the so-called species differences have on the animals’ daily function underground? Another difference outlined in Ellis’s British Snails (1926) refers to the penis sheath having a flagellum in T. haliotidea but not in T. scutulum. What difference does that make to its functioning?  Some DNA work is clearly needed to sort this out.  Both of these species are clearly different from the grey-brown T. maugei  that occurs in Wales and the west country, where I have encountered it twice in Cardiff in the 1970s.

A particular feature of Testacella is its diet – earthworms – a departure from that of most other slugs. Slugs, however are not a true taxonomic group, being derived from several different snail families that have undergone shell reduction. On one occasion, when digging the vegetable garden in my Alton garden in May 2005 I brought up an earthworm that had a yellow end and I looked at it more closely,  thinking that I had chopped the worm in half and exposed the intestine. Not so, it was a Testacella with its mouth wide open in the process of trying to eat an earthworm several times its own size. I went in to find a camera to record this but the two had by then separated (figure 3). In the great age of Victorian microscopy the tiny pointed radula teeth of Testcella that grip the slippery worm were something unusual to study.

_{References
Cameron, R A D, Jackson, N and Eversham, B, (1983). A Field Guide to the Slugs of the British Isles. Field Studies 5: 807-824
Ellis, A E, (1926/1969) British Snails, Oxford
Kerney, M, (1999). Atlas to the Non-marine Molluscs of the British Isles. Harley Books.}

_{figure 1: T. scutulum from an Alton garden showing the normal and the pale form.}

_{figure 2: T. scutulum showing the apricot-coloured foot sole.}

_{figure 3: T. scutulum with earthworm to which it was attached.}

_{figure 4: T. scutulum  crawling with the body expanded.}