Saturday, 14 December 2013

Freshwater Clams and Mussels

It may be 2013 but here in La France Profonde there are still shops that close for 2 hours or even more at lunch time. Then when they open you may well wonder why you bothered waiting because in many instances they don’t actually have what you want or perhaps they have it but it’s 5 times the price you can buy it for from the UK.

Anyway, before I start moaning the point is that I spent about 45 minutes poking around by the river Charente near to Civray waiting for a shop to open. This wasn’t likely to be exciting in mid December but you never know what may turn up and it’s a lovely river. The alternative for killing time would be to look at Estate Agents windows or wander round LIDL and even I’m not that sad.

It’s a spot near a sluice where there are always one or two Grey Wagtails,  Motacilla cinerea, (note to self to have a look for nesting activity in Spring-time), and sure enough there was one there on the bank-side – always a welcome sight. The river here can be quite varied in nature, extremely fast flowing following heavy or prolonged rainfall or be almost as still as a lake during dry spells. 

Above - looking upstream.

Location of the river site just below the "v" in Civraisien.

On these upper reaches of the River Charente there are numerous weirs / sluices that obstruct the free movement of both boats and fish so I was somewhat surprised to discover large collections of shells in the shallows and some on the land as well. Of course shells in themselves wouldn’t be a particular surprise but these were Corbicula fluminea, the Asian Clam. First detected in France around 1980 they feed primarily on phytoplankton (algae), which they filter from the sandy or muddy bottom of streams, lakes, or canals. They self fertilise and somewhat scarily can produce between 34,500 and 47,500 individuals in a season, hence within 20 years they had colonised most French rivers, some lakes and a large number of canals. Along with various human means of being spread it seems likely they are transported when very small by being stuck to birds’ feet.

They will be taken for food by Musk rats, (another introduced nuisance species), and possibly by some birds although this would be unlikely in most river situations. Musk rats also take native species of freshwater mussels / clams as can be seen by these other shells in the photo below of what I think possibly are Potomida littoralis, Mulette des rivières., one of the more common river mussel species in western France.

The freshwater mussel begins life as a tiny larva, measuring just 0.6 to 0.7 mm long, which is ejected into the water from an adult mussel in a mass along with one to four million others. This remarkable event takes place over just one to two days, sometime between July and September. The larvae, known as glochidia, resemble tiny mussels, but their minute shells are held open until they snap shut on a suitable host. The host of the mussel larvae are juvenile fish. The chances of a larva encountering a suitable fish are very low and most will die. Attached to the gills of a fish, the glochidia live and grow in this oxygen-rich environment until the following May or June, when they drop off. The juvenile must land on clean gravely or sandy substrates if it is to successfully grow. Attached to the substrate, juvenile freshwater mussels typically burrow themselves completely into the sand or gravel, while adults are generally found with a third of their shell exposed. Should they become dislodged, freshwater pearl mussels can rebury themselves, and are also capable of moving slowly across sandy sediments, using their large, muscular foot.

Life cycle freshwater mussels