For many years, my snailing activities were almost completely confined to terrestrial species, with only occasional observations of a few common freshwater species. By the ’nineties, however, I realised that this was just not good enough, and decided it was time to start recording freshwater species properly. An added impetus came when the non-marine Atlas was published (Kerney, 1999), which showed a lot of empty circles for freshwater species in south-east Scotland, where I live. Had there been mass extinctions? Only, apparently, of conchologists. Michael Kerney wrote these words in the introduction to the Atlas (page 23): “In south-east Scotland . . . most of the detailed freshwater recording was done by D.K. Kevan of Edinburgh in the years 1930–60; the apparent decline of many common freshwater species in this area . . . is therefore illusory.” I had to get out there and do something about it.
Where to start? Many ponds have disappeared over the years, and others are too small to be indicated on the standard 1:50,000 Ordnance Survey maps. There are, however, a number of larger lakes and reservoirs, not all of them easy to reach. Most promising of all, though, was the Union Canal, running from near the centre of Edinburgh to Falkirk. A visit established that it was a fairly simple matter to get snails by searching the marginal vegetation along the canal, which of course was easily accessible from the towpath. Armed with my net, I set off along the canal.
My activities along the canal, and in freshwater generally, might well have remained spasmodic but for a surprising finding in late summer 2002. Somewhere east of Linlithgow in West Lothian I found some snails that seemed to me a bit different from what I had seen before (remember that I had to learn freshwater species as I went!), and fortunately kept a few. Examining them again later, they appeared to be specimens of Bithynia leachii, a rare snail known only from two sites in Scotland (Kerney, 1999), neither of which was near the new site; this identification was confirmed by Geraldine Holyoak, then Non-marine Recorder. Was Bithynia leachii more widespread in Scotland, where it is evidently at the edge of its range, than had been supposed?
At this point it is probably helpful to say something about the Scottish canals. Scotland is not good country for canal building on the whole, being mainly mountain, boggy and thinly populated. The Central Belt, between Edinburgh and Glasgow, is however more favourable terrain and was heavily industrialised at an early date, so there was good economic justification for building a canal between the Clyde and the Forth, which allowed not only better transport within the country, but also improved communications between Glasgow and Europe, as the canal was big enough to take the small sea-going ships of the era. The Forth & Clyde Canal was opened in stages between 1777 and 1790, and ran from Bowling on the Clyde estuary, several miles west of Glasgow, via Falkirk, to Grangemouth on the Firth of Forth, a total length of 35 miles. It included over 30 locks to enable boats to climb from sea level at each end across the higher ground between. The canal skirted the north side of Glasgow, and the Glasgow Branch was constructed to bring the benefits of the canal nearer to the centre of the city. Not only were facilities for trade greatly improved, but shipping between the east and west coasts of Scotland no longer had to risk the dangerous seas round the north of Scotland (Hutton, 2002; www.scottishcanals.co.uk).
The other main canal that survives in the Lowlands is the Union Canal, which connected Edinburgh with the Forth & Clyde Canal at Falkirk. This was not opened until 1822, too late to realise its full potential as the railways came not long after, offering much quicker services. The Union Canal was a “contour canal”, all on the same level without any locks until it reached Falkirk, where it was connected to the Forth and Clyde canal by a flight of 11 locks (Hutton, 2002). There are, however, some fine aqueducts over several deep river valleys.
As with many canals throughout Britain, as other means of transport developed, use of the canals diminished. Various stretches became choked, others were closed or culverted, and the will to maintain them failed. However, in the late ’nineties the decision was made to clean out the canals, reconstruct missing sections, and restore navigation throughout under the title “Millennium Link”. In many places this merely involved dredging, but there were also locks to be repaired, as well as general maintenance. The biggest tasks, however, were the reconstruction of closed and culverted sections, and particularly construction of completely new sections of canal and building the Falkirk Wheel (figure 1), a unique device to connect the two canals instead of the long-defunct (and very time-consuming) flight of locks. The Falkirk Wheel is now a big tourist attraction in its own right.
Remains of a third canal, the Monkland Canal, exist to the east of Glasgow. At its fullest extent, the Monkland Canal ran down from Calderbank to join the Glasgow Branch of the Forth & Clyde Canal (Hutton, 2002), but today only short segments remain, the rest being filled in or culverted. Over a mile of the topmost section remains, but it is heavily overgrown and not navigable (figure 2). At the lower end of this section, the water disappears into a culvert and re-emerges over 2 miles away at Coatbridge, where it continues west for a mile or more before disappearing for ever under a railway line. This section looks much more like a normal canal. Nearby, in the centre of Coatbridge, is a grassy depression that tends to flood in wet weather and this too is probably a remnant of the canal.
In what was left of 2002, and throughout 2003 I walked or cycled the whole length of the three Lowland canals recording the freshwater molluscs, and Bithynia leachii in particular, at frequent intervals. Much of this was done by sweeping a net through the marginal vegetation, which consists predominantly of lush growth of the Reed Sweet-grass, Glyceria maxima (figure 3). G. maxima lines the canals throughout, except where heavy shade from overhanging trees prevents its growth, and in newly reconstructed sections of canal where a few clumps have been planted but have not yet spread far (figure 4). In a few places, snails could be seen on vertical walls, such as near locks, or crawling on the shallow stony bottom, but such sites produced only a small number of records. The results were described in a paper published in the Journal of Conchology (Sumner, 2006). This showed that many species were more widely distributed than indicated in the Atlas, and that Bithynia leachii in particular was distributed throughout most of the length of the Forth & Clyde, and Union Canals (Table 1). Altogether 17 species were recorded, and of these seven (Valvata cristata, Bithynia leachii, Lymnaea stagnalis, Planorbis carinatus, Planorbarius corneus, Acroloxus lacustris, Oxyloma elegans) were very much more widespread than had previously been recorded. On the other hand, certain species shown in the Atlas (Radix auricularia, Dreissena polymorpha) did not turn up during this study.
Inadequate recording is probably in part responsible for the differences between my study in 2002–3 and the Atlas maps, but it also seems likely that loss of industry along the canals, resulting in better water quality, and the reconstruction works connected with the Millennium Link, could have been favourable to the spread of freshwater molluscs. In any case, I decided to see if any species were still spreading through the canals, first by sampling each year stretches of canal that had lacked Bithynia leachii when I first looked at them, and then after 5 years, in 2008, walking the whole length of the canals once again.
Canal towpaths are probably one of the most public areas in which one can look for snails, and I was often engaged in conversation by various passers-by. Apart from small boys, who wanted to know “What yer catching, mister?”, and who lost interest as soon as they discovered it wasn’t fish, many of these people were genuinely interested in what I was doing. My response was usually along the lines that species were spreading since the restoration of the canals, and the numbers of molluscs indicated that the water quality was good. On one occasion, at Clydebank, a couple who kept a pet shop told me that terrapins could be seen a bit further along the canal. Indeed there were, a couple of red-eared terrapins (Trachemys scripta elegans) sunning themselves by the canal. They are, apparently, often released into the wild by pet owners who have been unable to cope with them, but fortunately it is too cold in Britain for them to breed successfully. It was in the same place that I was approached by a man who wanted to know “What the **** are you doing?”, a friendly greeting in the West of Scotland. After some discussion he went away under the impression that I was a professor searching for substances of potential pharmaceutical value in water snails. Sometimes, in the more built-up areas near the canals, I would go into a corner shop to get a snack to keep me going during the long days walking along the canal. My net often excited comment, and the people looking after the shop (who usually seem to have come from overseas) would often ask what I was doing. Once the man asked if they were poisonous, and I realised he thought I was looking for snakes, not snails! Another question was whether I ate the snails. In a shop in Coatbridge I was told that in Morocco they boiled the snails before eating them, but I didn’t stay to get the recipe. During all the years I was doing this study I managed to avoid falling in the canals, but I was once suspected of wanting to do something similar. At its eastern end, the Forth & Clyde Canal used to reach the Forth through what is now Grangemouth Docks, but this route was cut off many years ago by the construction of a motorway. Instead, a new bit of canal was built to connect with the River Carron on the same side of the motorway. One day, having sampled the canal all the way from Falkirk, I went past the marina at the end of the canal and looked over the edge. The tide was out, and on either side of the Carron was an expanse of unpleasant-looking mud. It wasn’t long before the man in charge of the marina came out, fearing that I was about to throw myself into the mud, though it is difficult to imagine anything less inviting! Once he had established that I wasn’t a potential suicide, we had quite a chat. It seemed that algal growth was becoming a problem in the canals, because they were no longer allowed to use “the granules” (the nature of which I didn’t discover). One wonders what the effect of an algicide (many of which contain copper) on molluscs might be, and how molluscs might respond to excessive growth of algae.
Over the five years since I first sampled the canal, it became clear that species such as Bithynia leachii continued to expand their range, and fill in most of the gaps that were present in 2003. As the vegetation developed in the newly reconstructed parts of the canals, these too became colonised. As late as 2008, fresh species turned up that I hadn’t previously recorded: Hippeutis complanatus at two sites in the Union Canal; Valvata piscinalis in the Forth & Clyde Canal on the west side of Glasgow; and Bithynia leachii in the Monkland Canal where I hadn’t found it in 2003. How do these molluscs spread? Sticking to the feet of birds or even large insects is commonly stated to be a mechanism (Rees, 1965), but in an elongated body of water like a canal drifting vegetation and other material must provide a good means of transport. Clumps of vegetation are often seen drifting along the canals (Fig. 5), and I found no fewer than nine species attached to such clumps, of which Acroloxus lacustris seemed to be the commonest. Lymnaea stagnalis and Radix balthica turned up on pieces of floating wood, and a specimen of Planorbarius corneus was seen floating along by itself, not attached to anything. Passing boats must also be an effective means of transport, and recently a specimen of Dreissena polymorpha, not recorded in the canals for some time, was found on a boat in the Forth & Clyde Canal (Adrian Norris, personal communication). Not all molluscs in the canals seem to be spreading, however. Various common freshwater species have local distributions and occur only in small numbers in the canals. Radix balthica, probably our commonest freshwater snail, was only found rather rarely. Gyraulus albus, another common species, was only found in the Forth & Clyde Canal, mainly in the Falkirk area. And Potamopyrgus antipodarum, which has spread so widely throughout the British Isles, is scarce throughout the canals, apart from a good population that seems to have become established in the Union Canal at Falkirk. In general the canals provide a fairly uniform habitat, shown by the presence throughout most of their length of several molluscan species and the uniformity of the vegetation, so it is difficult to see why some species remain localised. Continued monitoring of this rich and fascinating system may help to provide some answers – perhaps another survey in the future would be appropriate.
Hutton, G. (2002) Scotland’s Millennium Canals. Stenlake Publishing, Catrine, Ayrshire.
Kerney, M. (1999) Atlas of the Land and freshwater Molluscs of Britain and Ireland. Harley Books, Colchester.
Rees, W.J. (1965) The aerial dispersal of Mollusca. Proc. Malac. Soc. Lond. 36, 269–282.
Sumner, A.T. (2006) Distribution of certain molluscs in the lowland canals of Scotland. J. Conchol. 39, 221–228.
figure 1: The Falkirk Wheel
(Photo: Adrian T. Sumner)
figure 2: The topmost section of the Monkland Canal, almost completely choked with vegetation.
(Photo: Adrian T. Sumner)
figure 3: The Glasgow branch of the Forth and Clyde Canal, lined with Reed Sweet-grass, Glyceria maxima
(Photo: Adrian T. Sumner)
figure 4: A reconstructed section of the Union Canal at Wester Hailes, Edinburgh, showing the sparse marginal vegetation typical of these new parts of the canals with concrete banks
(Photo: Adrian T. Sumner)
figure 5: A clump of floating vegetation drifting along the Union Canal. Such clumps can transport several species of freshwater molluscs.
(Photo: Adrian T. Sumner)