Cepaea nemoralis (L.)|
Cepaea hortensis (O.F. Müll.)
Class: Gastropoda – slugs & snails
Subclass or superorder: Pulmonata
British distribution: Both species widespread and generally common, C. nemoralis rare or absent in northern Scotland, C. hortensis extending north to Shetland.
World distribution: Both species widespread in N.W. Europe, C. hortensis extending into the Arctic, occurrence of both species elsewhere to be checked.
These two species are taken together here in view of their close similarity and possession of the same (or very similar) genetic polymorphisms. C. nemoralis is slightly the larger of the two when mature, and it characteristically has a dark lip to the shell. C. hortensis typically has a white lip to the shell and shows a preference for slightly damper places.
These differences are not, however, reliable, and C. nemoralis has an uncommon white-lipped morph. Their identities can be confirmed only by dissection. They commonly occur together and, consequently, these photographs represent individuals named on a basis of probabilty.
This morphological similarity between the species has implications for the many genetic studies of wild populations - identifications are unreliable when based on shells alone.
|Presumed Cepaea nemoralis (unbanded yellow morph), Wicken Fen, Cambridgeshire, June 2002.|
|Presumed Cepaea nemoralis (1-banded yellow or tawny morph), Shewalton Pits, Ayrshire, May 2009.|
|Presumed Cepaea nemoralis (unbanded brown morph), Shewalton Pits, Ayrshire, May 2009.|
|Presumed Cepaea hortensis (5-banded yellow morph), Wicken Fen, Cambridgeshire, June 2002.|
|Presumed Cepaea hortensis (banded yellow morph, bands non-pigmented),|
Freshwater Bay, Isle of Wight, May 2004.
Both species occur in very varied habitats, including woodland, hedges, fens, open grassland and dunes. C. nemoralis is evidently the commoner of the two in warmer, dryer habitats in the south.
They are highly polymorphic in shell colour and banding, and light and dark body colour morphs also occur. C. nemoralis has been (supposedly) much the more studied and it occurs in three main colours: brown, pink and yellow, with brown dominant over the other colours, and pink dominant to yellow. More than one allele is known for each of these colours and there is also a 'tawny' morph that occurs in sand dune populations. Bands may be present or absent (unbanded dominant to banded) and there is further variation in number and pigmentation of bands. The polymorphisms continue to be much studied and refinements of knowledge regularly appear in such journals as Heredity and the Biological Journal of the Linnean Society. A useful recent summary, with abundant references to original literature, has been given by Backeljau et al. (2001).
|Cepaea shells (C. nemoralis or mostly so, bottom right is C. hortensis), various sources.|
Banded morphs (especially banded yellow) are known to predominate in grassy habitats, where the bands are presumed to provide camouflage amongst the linear, often vertical shadows, whereas unbanded morphs (especially unbanded pink and unbanded brown) show preference for wooded sites. However, polymorphism is retained in these populations since they often occupy marginal or boundary habitats (notably wood and scrub margins, and roadsides with long grass backed by hedgerows). Heterozygote advantage is also involved.
|• ||Backeljau, T., Baur, A., & Baur, B. (2001). Population and conservation genetics, in Barker, G.M. (ed.) The biology of terrestrial molluscs, CAB International, Egham.|