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Nature in Wales


N.s. Vol. 4, no. 1/2 1985

Reduction in the distribution of the land snail Theba pisana in the Tenby area, Dyfed

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Reduction in the distribution of the lancT
snail Thebapisana in the Tenby area, Dyfed
Robert H. Cowie
The land snail Thebapisana (Miiller) is widespread
and abundant in coastal regions of the
Mediterranean. Its range extends up the Atlantic
coasts of Spain, Portugal, France and Belgium. In
Britain it is restricted to a small number of colonies
in the south-west (Kerney 1976), some of which
exhibit unique shell pattern characteristics (Cowie
1984). Cowie (1982) surveyed these British
localities, paying special attention to the detailed
recording of the distribution in and around Tenby,
South Wales. This is the largest British colony and
lies within the boundaries of the Pembrokeshire
Coast National Park.
The only detailed account of molluscan
distributions in the Tenby area is that of Stubbs
(1900). The present paper, presenting the results of
the survey of T. pisana, thus updates the record for
this species. Reasons for the local distribution of T.
pisana, and changes in it since the turn of the
century, are discussed. Photographs of all the
localities mentioned are available in Cowie (1982),
with extended discussion of the origin and
distribution of T. pisana in the British Isles.
The Survey
The whole coastline from Manorbier Bay (SN 055
977) to Wiseman's Bridge (SN 147 063) was
Fig. 1 Map showing the Dyfed localities mentioned in the text,
excluding Stackpole Warren. Urban areas are shaded
searched, with the exception of the section fenced
off by the Ministry of Defence at Old Castle Head
(SN 073 969 to SN 082 974) (Fig. 1). No live
specimens or dead shells were found at Manorbier
or Penally, and only one empty shell was found (at
Giltar Point) between Manorbier and the Tenby
Burrows (all localities at which Stubbs (1900)
found T. pisana). T. pisana was present on parts of
the Burrows and in Tenby itself; these colonies are
described in detail below. On Caldey Island it was
abundant (about 100 snails per sq m in July 1979) on
the dunes behind Priory Bay, but only three live
specimens were seen on the sloping sands behind
Sandtop Bay, and none on the south side of the
island (cf. Stubbs 1900; Wintle 1925).
T. pisana was found at two further localities in
Dyfed. North of Tenby, at Saundersfoot, a locality
quoted by previous authors including Stubbs
(1900), only two very small colonies remained, both
in patches of vegetation of only a few square metres
at the edge of the beach. One of these (SN 138 051)
was at the top of the stony upper beach, beside the
garden wall of a modern apartment block. The other
(SN 140 055) was between the beach and the coast
path, with a large unmetalled car park adjacent.
Only about 20 snails were ever seen at either site.
West of Tenby, a rather sparse but widespread
colony was observed on the dunes at Stackpole
Warren (SR 978 943) (Fig. 2). This colony was
recorded originally by Oldham (1928), by Kerney
(1976) as extant after 1950, but not by Deblock
(1962) who also surveyed this area.
At Tenby all localities mentioned by Stubbs (1900)
were investigated, as were other areas which seemed
suitable for T. pisana. Fig. 3 indicates where T.
pisana was found. Although absent from the north-
and east-facing sides of Castle Hill, large numbers
(up to about 180 adults per sq m Cowie 1982) were
present on the south-facing side in the areas of
rough vegetation beneath the mowed area which
covers the top of the Hill. This was similar to the
situation described by Stubbs. The snails were often
associated with the umbellifer alexanders
(Smyrnium olusatrum L.), as they are elsewhere at
Tenby (Fig. 4). Similarly, snails were only found on
the south-west side of St. Catherine's Rock, the
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