Non-marine Molluscs in
Shetland
Roger Tallack
The
non-marine molluscs (Slugs and Snails) of Shetland are not a subject to
immediately inspire interest, but this group of invertebrates are one of
the most interesting and most often overlooked. To date 62 species have
been positively identified in Shetland with a further 5 unsubstantiated
records. This represents approximately 31% of the UK fauna and includes
both the smallest - Punctum pygmaeum, a tiny disc 1.5 mm across -
and the largest - Limax cinereoniger, up to 30 cm of ash-black
slug. The latter is considered to be an indicator species of ancient
woodland, but in Shetland it is likely to graze on lichens on exposed
rocks. Molluscs are present in small numbers in most habitats in Shetland,
although the diversity is influenced by underlying geology and it is only
among dunes and on limestone that a wide range of species will be found.
The
earliest known record is for the large freshwater bivalve Margaritifera
margaritifera, one of the rarest species, apparently confined to a
single site near walls. There are specimens in the Royal Museum of
Scotland from 1849. Victorian conchologists were far more interested in
making unusual discoveries than in plotting distribution so many old
records are tantalisingly vague. An example is Discus rotundatus,
one of the commonest species in the UK and first recorded from 'Shetland'
around 1860 and again at the turn of the century. However it was not until
1993 that it was re-located and there is no way of knowing if this is the
same colony or a very recent introduction (the colony is actually located
on a concrete pillar in Weisdale!). The Victorians also had a tendency to
be rather imaginative with their identifications and records such as that
for Limax tenellus (a rare woodland slug) - 'Northmavine, Shetland,
on stones in a watercourse of a mountain-mill'- are best disregarded.
There
are many parts of Shetland where little or no recording has been done
including some parts of North Mainland, although the paucity of records
here may be due to the acid soil. Many of the outer islands have not been
surveyed at all. Surprisingly, Foula is quite well represented, simply
because the eminent conchologist Charles Oldham spent a week there in
1929, recording 16 species.
A Guide to
the Common Slugs and Snails of Shetland
The 12 species described below are those most likely to be found in
gardens, on walks or by the casual observer turning over stones and wood.
Arion
ater
A
rather variable slug, up to 15 cm in length. Usually black with striped
foot-fringe, but may be grey, brown or orange although I have not seen the
last form yet in Shetland. Common on moors but will also invade gardens
where its voracious appetite can decimate precious plants.
Arion
circumscriptus
A
small (5 cm), monochrome slug with a white sole. In fact it is a complex
of three similar species. Common and found almost anywhere.
Arion
hortensis
Another species complex. Recognised by its almost black colour and orange
sole. Usually less than 4cm long.
Limax
maximus
The
largest slug likely to be found in gardens, up to 20 cm when extended.
Pale grey with darker bands or spots. Two identifying features - the
tentacles are reddish-brown and the bands never extend over the mantle
(upper back).
Deroceras reticulatum
A
buff coloured slug frequently flecked with dark marks, up to 5cm in
length. Almost ubiquitous and the main culprit for damage in gardens.
Milax
budapestensis
A
medium-sized slug (up to 6 cm), grey with pale irregular spots. It has a
yellow or orange keel stretching from the edge of the mantle to the tail.
The sole has a dark central band which distinguishes this species from its
congener Milax sowerbyi. Both species are under-recorded in
Shetland because they tend to frequent gardens.
Cepaea
hortensis
The
5-banded snail. A common garden pest in most of the UK but very local in
Shetland, confined to a handful of sites on limestone and dunes.
Variations are small and there appear to be no unbanded specimens in
Shetland.
Arianta
arbustorum
Slightly larger than the previous species (can be over 2 cm) but usually
speckled with pale bands, which may be absent. Inhabits walls, rocks,
meadows and sometimes gardens. This and the preceding species are the
largest snails to occur in Shetland.
Candidula intersecta
Another variable snail about 10mm in width, sometimes white with brown
bands, although it is more frequently lightly banded and speckled. This is
very much a maritime species found on dunes, although several specimens
were located on an embankment at Channerwick (South Mainland), presumably
imported with sand used in cement.
Balea
perversa
This
species is unusual (perverse!) because the whorls on its shell are coiled
anti-clockwise (sinistral) with the aperture on the left. It has a
distinctive turret shape up to 1cm tall and is found on old walls and
rocks on limestone and serpentine.
Oxychilus alliarius
This
small snail (7mm) has a glossy brown (sometimes greeny-white) shell and a
very dark body. Its main distinguishing feature is the strong smell of
garlic emitted when handled. It is probably the most widespread snail in
Shetland.
Lauria
cylindracea
A
very small snail (up to 4mm), shaped like a squat turret. Very common on
walls and rocks, e.g. in Gilbertson Park, Lerwick.
Nature in Shetland
