Acicula fusca (Montagu, 1803) (Mollusca: Gastropoda, Architaenioglossa, Aciculidae) extant at Cleeve Hill, Gloucestershire VC33 SO92, with observations on its ecology and palaeogeography
Although the distribution map of Acicula fusca (Montagu, 1803) shown by Kerney (1999) now requires some updating, there remains a clear picture of a declining species over large parts of central and eastern England. In the oceanic west and south-west the populations are rather more stable and examples may be found in relatively open sites such as sea-cliffs. In Worcestershire, recent discoveries or confirmations of A. fusca come from Death's Dingle (SO66, 21st June 2003) and Seckley Ravine (SO77, 27th June 2004) (the records were published anonymously in Worcestershire Record 15 and 17 respectively) and to the south in Watsonian Worcestershire on the wooded north Cotswold escarpment near Snowshill (Knight, 2007).
Scrutiny of the distribution of A. fusca in Gloucestershire (http://data.nbn.org.uk; Kerney, 1999) provides a catalogue of ancient species-rich Cotswold woodlands such as Buckholt, Cranham, Siccaridge, Frith and Workman's to which the Worcestershire woodland sites are complementary in quality. A combination of scree, subangular rock clasts, artesian drainage and sometimes tufa is one which A. fusca is known to favour.
Acicula fusca on Cleeve Hill
Cleeve Hill is delineated on its westernmost edge by limestone crags and thermoclastic scree and is noteworthy for its biota including invertebrates. The only previous record of A. fusca on Cleeve Hill that I have traced was of one found by Peter Tattersfield on 26th April 1986 (SO92) and away from woodland the pulmonate A. fusca becomes much scarcer in the midlands where it is sensitive to anthropogenic and climatic impacts and to desiccation.
The live specimen of A. fusca cited here was observed on 12th April 2012 at a depth of 20 cms in thermoclastic scree at 290 m O.D. This is a habitat that requires to be investigated with care and I regarded further searches as counterproductive on the basis that the hold of A. fusca there would be tenuous at best. The nearest woodland to the site is a mature dominantly high-canopy beech wood 100 m to the south; this woodland is also recognised for its invertebrate interest but up to now has apparently not yielded A. fusca.
The habitat of Acicula fusca
Acicula fusca is regarded as an oceanic climate woodland snail (Kerney, 1977a; 1977b; Limondin-Louzouet & Preece, 2004) sometimes of wet spots (Janus, 1965). However, A. fusca has several Palaearctic congeners which characterise scree in the Alps and other mountain ranges (Kerney & Cameron, 1979). It may well be, at least in the midlands, that woodland preference of A. fusca is determined partly by the rarity of crags with actively developing scree such as exist on Cleeve Hill. Further north along the north Cotswold escarpment where scree is well-developed, it is more rarely active and is quite often fixed by vegetation and soils which render it unsuitable for the pulmonate A. fusca. This process of scree-fixing renders it suitable for a superficially similar but unrelated subterranean mollusc Cecilioides acicula (Muller, 1774) which occurs in such habitat on Cleeve Hill within 20 m of A. fusca and down slope of it. Cecilioides acicula is not anthropophobic and is widespread in Worcestershire even occurring in stable undisturbed loamy sediments in old urban gardens. With its requirement for airspaces, the open mantle cavity lung of A. fusca is able to function optimally in interstices which may also partly explain its occurrence on the lightly vegetated rock floors of Cotswold beechwoods.
The range of Acicula fusca
Fig.1. shows the west European range of Acicula fusca (after Boeters, Gittenberger & Subai 1989). Note that this is an indication of distribution not of frequency and that whilst appearing widespread in Britain, some of the map dots may represent one or a few specimens.
The range of A. fusca is determined by climate. It avoids climatic continentality (Pfleger & Chatfield, 1983) and like its Cleeve Hill associates Pomatias elegans (Müller, 1774) and Abida secale (Draparnaud, 1801) has a Palaearctic range (Fig. 1) largely governed by winter temperature (Boeters, Gittenberger & Subai, 1989; Limondin-Louzouet & Preece, 2004). Evans (1975) points out that A. secale is essentially west European but is able to withstand temperatures as low as -10oC in the Alps. It should be recalled however that A. secale, which is also unknown in Scandinavia, is able to utilise scree interstices to avoid marked fluctuations of climate in the same way as A. fusca. Further confirmation of the relict status of A. fusca comes from its fossil record which extends back into the Miocene of central Europe.
The modern geographical range of A. fusca (Fig. 1) is worthy of scrutiny from another standpoint. The populations focus in Britain and northern Spain and one is entitled to ask, if they are disparate, whether each is indicative of a past refugium. Were that to be so, where in north-west Europe did A. fusca survive the last ice age? South of the Euro-Britannic land bridge, in south west Ireland, in both or in neither?
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Fig_1 The west European range of Acicula fusca