The Manchester Museum houses two noteworthy collections of mollusc shells made in the 20th century. One was assembled by A.J. Cain, at one time a Professor in the University. The other is from the personal collection of A.W. Stelfox, an Irish naturalist and Curator at the National Museum of Ireland. Each reflects important developments in the history of our knowledge of the animals concerned, especially the two species Cepaea nemoralis and C. hortensis.
These very similar species of helicid land snails are notable for being polymorphic for both banding and ground colour of the shell (Figure 1). They come from western Europe and are found, sometimes together, in a wide range of habitats, from deciduous woodland to sand dunes, low-lying sunny southern valleys to high altitudes on limestone mountains. Almost every colony recorded has individuals differing from each other in some combination of yellow, pink or brown ground colour to the shells, which are either bandless or have some combination of up to five bands or stripes running round them (Figure 2.).
The development of interest in Cepaea species is recorded in papers noted in the Zoological Record, a journal which provides lists of scientific papers published annually, with their titles and categories of subject matter. It began in 1864 (Gűnther, 1865), when there were no citations for the species in this genus, but by 1990 (Vol. 127) there were over 750 references (Figure3).
One of the early objectives was to establish that the two taxa were in fact distinct. They are, although tests published in 1908 showed that hybrids could be made with difficulty. The diversity of possible combinations of colour and banding in natural populations was examined, and the extent of geographical variation. The inheritance of the variant forms was studied (Figure 4).
These observations led to the question why such a high degree of inherited visual diversity should exist. It was assumed to be accidental, resulting from mutations. However, that was not likely to explain the range of frequencies observed, and attention turned to the ways in which selection could operate to account for patterns. The great increase in publications after 1950 resulted from this new direction of interest, and was largely generated by the work of A.J. Cain.
Arthur J. Cain (1921-1999; Clarke, 2008) was in the Zoology Department in Oxford shortly after World War II. Later, he was Professor of Zoology in Manchester (1964-68) and Liverpool (1968-89). He became convinced that the design and evolution of organisms was driven by selection and that randomly generated features were very rare. With geneticist P.M. Sheppard he set out to show that the shell colour and pattern of C. nemoralis provided camouflage against predators. Their first work (Cain & Sheppard, 1954), which took place in areas near Oxford, showed that the snails tended to match their backgrounds. They were attacked by birds, which left piles of broken shells belonging to conspicuous forms. Later, wider surveys produced a more complex picture. This led to studies in ecological genetics by different workers, a number of whom were Cain’s students. The Museum collection consists of samples collected by several of them in the three decades following the initial research. A 2008 citizens’ science project covering much of the European range (Silvertown et al., 2011), and study of the molecular control and gene expression (Davison et al., 2017) have maintained interest in the eco-genetics of these two species of Cepaea.
Arthur Wilson Stelfox, (1883-1972; McMillan, 1972) was a naturalist whose career was at the National Museum of Ireland in Dublin where he worked on a variety of topics, particularly on Hymenoptera but also on the bones of mammals found in early cave deposits. At home he was one of the pioneers in establishing the genetics of the shell characters in snails by breeding, beginning in 1909. The collection has material for C. nemoralis, including demonstration of linkage between the colour and banding loci. He also investigated the shell pattern of Cornu aspersum (then Helix aspersa) and carried out a long-term study which showed that the shape of the shells could be modified by selective breeding. The striking result was corkscrew shaped shells produced in a few generations (Figure 4; Cameron, 2016 p.111).
The Cain and Stelfox collections
The Cain material consists of shells collected by Cain and Sheppard and by a number of other collaborators and independent workers. It is kept in individual boxes in 207 numbered drawers. Each drawer gives details of collector, location and date. There are also several larger boxes of mollusc shells from archaeological sites. The main contributors were: R.W. Arnold, D.P.T. Burke, A.J. Cain, M.A. Carter, R.A.D. Cameron, B.C. Clarke, J.J.D. Greenwood, J.J. Murray, D.T. Parkin and P.M. Sheppard.
Apart from shells illustrating the selective breeding experiment in C. aspersum, the Stelfox material includes results of crosses relating to inheritance of colour and banding in that species and in C. nemoralis and C. hortensis. These date from 1910 to 1930 and are some of the earliest investigations of Mendelian inheritance in snails. There are ten drawers and associated notes.
The use of collections.
In line with new perspectives as to their social value museums have reacted with initiatives designed to reflect contemporary issues. In the 19th and early 20th centuries, by contrast, very large assemblies of specimens were often accumulated in the biological field. In Manchester, for example, the Entomology department houses over 2.5 million specimens and there are about three-quarters of a million mollusc shells. Collections were initially made in the spirit of taxonomic discovery and by amateur enthusiasts. They were undoubtedly valuable in helping us to understand richness and diversity of the natural world. In a museum today they illustrate past activities, interests and enterprise.
The shells of the snail species described here carry information about the genetic constitution of the animals that bore them. When collected at random from wild populations, as with most of the Cain material, they provide data on the frequencies of the different morphs at particular times in the past. Comparisons have been made between samples from archaeological sites and modern records to assess the effects of long-term changes in habitat and climate. For shorter periods estimates of the amount of selection producing the change can be made. Sometimes shell samples can be put to novel uses which were not the concern of the first breeder or collector. If shells from parents and offspring are available the inheritance of shell dimensions may be estimated. Carefully controlled collecting from natural populations sometimes allows fitness characters associated with pattern to be measured. The Cain collection was part of a study of the way in which the living animal can form a cell layer over nematode parasites within the shell as a protection against infection. References to examples are given below. Changing interests and techniques of analysis continually suggest new avenues of research.
Cain, AJ. Sheppard, PM. 1954 Natural selection in Cepaea. Genetics 39, 89‑116.
Cameron R. 2016. Slugs and snails. Collins New Naturalist, London. (Scalariform aspersum on p. 111.)
Clarke, BC. 2008 Arthur James Cain. Biographical Memoirs of Fellows of the Royal Society 54, 47-57.
Davison, A, Jackson, HJ, Murphy, EW, Reader, T. 2019. Discrete or indiscrete? Redefining the colour polymorphism of the land snail Cepaea nemoralis. Heredity 123: 162-175.
GüntherAlbert CLG (ed.) 1865 The record of zoological literature. Van Voorst, London.
McMillan N.F. 1972. Arthur Wilson Stelfox, 1883-1972. Journal of Conchology 27, 520-522.
Silvertown J. et al. 2011. Citizen science reveals unexpected continental-scale evolutionary change in a model organism. PLoS One 6(4): e18927.doi:10.1371/journal.pone.0018927
Published studies using material in shell collections
Cameron RAD. 2001. Cepaea nemoralis in a hostile environment: continuity, colonizations and morph-frequencies over time. Biological Journal of the Linnean Society 74: 255–264.
Cook, LM. 1965 Inheritance of shell size in the snail Arianta arbustorum. Evolution, 19, 86-94.
Cook, LM. 2007. Heterosis in Cepaea. Biological Journal of the Linnean Society. 90, 49-55
Currey JD, Cain AJ. 1968 Studies on Cepaea IV. Climate and selection of banding morphs in Cepaea from the climatic optimum to the present day. Philosophical Transactions of the Royal Society London Series B, 253, 483-489.
Goodhart, CB. 1956. Genetic stability in populations of the polymorphic snail Cepaea nemoralis (L.). Proceedings of the Linnean Society of London 167, 50-67.
Ozgo, M, Liew, TS, Webster, NB, Schilthuizen, M. 2017. Inferring microevolution from museum collections and resampling: Lessons learned from Cepaea. PeerJ, 2017(10). https://doi.org/10.7717/peerj.3938.
Rae, R. 2017. The gastropod shell has been co-opted to kill parasitic nematodes. Scientific Reports. https://www.researchgate.net/publication/318226430
Williams, A, Rae, R. 2016. Cepaea nemoralis (Linnaeus, 1758) uses its shell as a defence mechanism to trap and kill parasitic nematodes. Journal of Molluscan Studies: (2016) 1–2. doi:10.1093/mollus/eyv064