Don’t be fooled by this photograph looking across Lago di Maggiore from the village of Angera, where I was staying during a visit to the European Commission’s Joint Research Centre at Ispra, a few kilometres away. There had been two days of almost constant rain and temperatures little different to those in the UK and the sun came out just before we were due to head to the airport for the homeward journey. Before the taxi came, however, I found time for a short walk along the lake front, during which time a silty foreshore littered with empty shells caught my eye. The shells looked like cockle shells, common around our own coast yet this was a freshwater lake. I scrambled down for a closer look.
The shells belonged to a bivalve mollusc and were an ochre colour, with darker markings and a series of concentric ridges. I’m not an expert on molluscs but, after a little searching on the web, I think that they probably belong to the Asian clam (Corbicula fluminea), one of a number of alien mollusc species that has become established in Lago di Maggiore in recent years. The shape of the cell, and the markings rule out the zebra and quagga mussels (Dreissena polymorpha, D. bugensis) whilst another recent invader, the Chinese pond mussel (Sinanodonta woodiana) is much larger (up to 30 cm).
The foreshore at Angera littered with shells and (right) a close-up of the two halves of an Asian Clam shell. Each is about 2 centimetres long. February 2016.
There is an irony here, as I have written several blogs about an endangered freshwater bivalve (the pearl mussel – Margaritifera margaritifera, see “’Signal’ or ‘noise’?”) yet this group also contains some of the most notorious invasive species in the world. All originate from Asia but are now widespread in Europe and North and South America, where they cause a large amount of economic damage.
Like all bivalves, Asian clams are filter-feeders. They open their shells slightly and extend two tubes (“siphons”) from their bodies. Water is drawn in through one of these, across the gills, where particulate material is trapped, and out through the other. Fine cilia waft the particulates towards the clam’s mouth. In a lake such as Maggiore, much of the particulate matter will be suspended algae (“phytoplankton”) and prolific growths of these invasive bivalves can actually reduce the quantities of phytoplankton to such an extent that they have been proposed as solutions to eutrophication problems under some circumstances. Because the phytoplankton, in turn, contains a lot of the phosphorus in the water, this has been referred to as “biological oligotrophication”. The benefits are partly a matter of perception: the bivalves have not removed the nutrients and algae, they have merely relocated them to another part of the lake. That might bring short-term benefits, but it is not a solution per se.
Unlike zebra mussels, however, Asian clams have an alternate mode of feeding: they extent a muscly foot to pull themselves into the sediments which, in the process, throws up fine particles that can be sucked through the siphons. This “pedal feeding” could play an important role in the functioning of lakes due to the disturbance it causes, allowing oxygen to penetrate the surface layers of the sediment. As phosphorus compounds are generally less soluble in the presence of oxygen than they are in anaerobic conditions, this can further reduce the effects of eutrophication by “locking” the phosphorus into the sediments. The oxygen also fuels microbial processes, including the breakdown of nitrogen compounds limiting the production of nitrous oxide, a potent greenhouse gas in the process. Yet, ironically, the distribution of Corbicula is strongly influenced by temperature and global warming is likely to increase its range.
Local scientists have concluded that eradication of Corbicula and other alien molluscs from Lago di Maggiore is probably impossible as any attempt to remove them will probably disrupt other littoral organisms too. Yet, at the same time, the role that Corbicula plays in the food chain is not clear and it is possible that the spread of this and other bivalves will have knock-on effects for local fisheries. The first record of C. fluminea in Lago di Maggiore was made in 2010. That means that the lake I looked across this week might look like the lake I looked across on my first visit about 10 years ago, but it is nonetheless a different lake under the surface. It’s not the same lake but, as Heroclitus also reminds us (see “What Constable never saw …”), I’m not the same man either.
Kamburska, L., Lauceri, R. & Riccardi, N. (2013a). Establishment of a new alien species in Lake Maggiore (Northern Italy): Anodonta (Sinanodonta) woodiana (Lea, 1834) (Bivalvia: Unionidae). Aquatic Invasions 8: 111–116.
Kamburska, L., Lauceri, R., Beltrami, M., Boggero, A., Cardeccia, A., Guarneri, I.Manca, M. & Riccardi, N. (2013b). Establishment of Corbicula fluminea (O.F. Müller, 1774) in Lake Maggiore: a spatial approach to trace the invasion dynamics. BioInvasions Records 2: 105–117.
McDowell, W.G., Benson, A.J. & Byers, J.E. (2014). Climate controls the distribution of a widespread invasive species: implications for future range expansion. Freshwater Biology 59: 847-857.
Vaughn, C.C. & Hakenkamp, C.C. (2001). The functional role of burrowing bivalves in freshwater ecosystems. Freshwater Biology 46: 1431-1446.