Ever since I moved to Sheffield, about 12 years ago, I have accumulated casual records of snails in the city, and in the area around it. In addition, as recorder for the Sorby Natural History Society, I have led parties looking at many of the ancient woodlands that can be found within the city limits. Recently, I took a group of learners, sponsored by Yorkshire Museums, Libraries and Archives, into the ancient Ecclesall Woods, within the city boundaries. This confirmed the existence of a puzzle that has worried me for several years.
Some of these woods have very rich mollusc faunas, with more than 30 species found in quite small areas. The best so far is Little Matlock Wood, with 35, including Limax cinereoniger and Arion owenii. In none of these woods, nor in the city generally, are there any records of clausiliids, and there seem to be no older records either.
Apart from Balea perversa/heydeni, which are hard to find anyway, the only clausiliids that might be expected are Clausilia bidentata and Cochlodina laminata. Consulting Mike Kerney’s (1999) Atlas of the land and freshwater molluscs of Britain and Ireland, one can see that there are areas, especially in the midlands, and up to Sheffield, in which these species are missing, or have not been recorded recently. Kerney follows David Holyoak in attributing this rather strange pattern to the effects of industrial pollution, especially for the Balea species and C. bidentata. Acid rain in particular might have an effect. In Britain, C. laminata seems to have a distinct preference for baserich soils, and since Sheffield sits on the Carboniferous Coal Measures, mostly sandstones, which give rise to generally rather acid soils, its absence might have other causes. Nevertheless, most of the Sheffield woods have rich, damp spots within them, with plants like dog’s mercury and wild garlic, and (today at least) a good fauna of large helicoid snails.
However, over the last 30 years or so our air has become much cleaner, and it is clear that many species that were badly affected have made remarkable recoveries. Thus, early records indicate that Cornu aspersum (ex Helix aspersa), and the two Cepaea species were very uncommon in the city. Now, they are present in very large numbers, to the extent that I get continual requests to explain why they are so common. Similarly, Lauria cylindracea can be found on many walls, and in woodlands within the city. Recent records of invaders, such as Hygromia cinctella reinforce the impression that the city is now quite a congenial place for snails. I hear similar stories from other industrial cities in the northern half of England.
So, why have the clausiliids not returned? Will they do so eventually? The only clue I have comes from Anston Stones Wood, a magnificent ancient wood in a gorge cut through Magnesian Limestone about 20 miles east of Sheffield. This is one of the richest spots in the country, and has been looked at by many conchologists over the years, including Barry Colville and Adrian Norris. It holds 47 species. The only new record I was able to obtain recently was of Cochlodina laminata, just two half-grown but living specimens (Clausilia bidentata is abundant there). It looks as though it has just managed to recolonise the wood. As with other sites in the Eastern part of England, it used to have live Helicigona lapicida, which may have been similarly affected by acid rain, but only long-dead shells have been found in recent years. It may take a long time for recolonisation to occur.
Clausiliids are not generally thought of as garden species. I suspect that there are quite a lot of places within the city where C. bidentata in particular could live quite happily, if only it could get there; it is perhaps much less likely to do so than species that are often found in very “human” habitats. It is abundant and widespread in the limestone White Peak, just west of Sheffield, but these populations are separated from the city by high moorland in which it is hard to find anything but a few slugs. Natural, active dispersal is hard when suitable habitats are fragmented, and there are “deserts” in between.
I shall keep looking! One of the problems with much of our recording is that when we find a species new to our area, we cannot tell if it has really just arrived, or whether it was there all along, perhaps scarce, and nobody found it. It is famously impossible to prove a negative, but at least in Sheffield I can say that there are lots of negative records, so that arrival and spread can be studied with some confidence. It would be good to know what urban records we have nationally for clausiliids, and whether there is any evidence, anywhere, of a recovery.