On 15th November 2006 I visited the South Forty Foot Drain at Swineshead Bridge, Lincolnshire (TF218429) to collect an invertebrate sample for the Environment Agency’s biological monitoring programme. At this site the river was approximately 12 metres wide and over one metre deep. The substrate was predominantly silt with some boulders. Some sections of the bank were artificially reinforced and there was a substantial amount of detritus along the margins. The marginal vegetation was relatively sparse and consisted primarily of Glyceria maxima and Carex sp. I took a sweep sample consisting of several netsweeps through the margins with some disturbance of the substrate.
While sorting through the sample in the laboratory I found a number of small mussels with a brown periostracum and lacking the distinctive dark bands of the Zebra mussel, Dreissena polymorpha. On internal examination a triangular tooth was observed on the septum, a feature indicative of the Dark false mussel Mytilopsis leucophaeta (Killeen et al. 2004). Externally, the periostracum was not as coarse as in the photographs of M. leucophaeta in Killeen et al. 2004. In total, I found seven specimens of M. leucophaeta in the sample. Subsequent to this, on 30th November 2006, one of my colleagues, Chris Extence, found one specimen of M. leucophaeta in the South Holland Main Drain at Clifton’s Bridge, Lincolnshire (TF380189).
Prior to these discoveries M. leucophaeta was known from only two sites in Britain, Roath Dock in Cardiff and Cliffe Fort Lagoon in Kent. These new records from Lincolnshire are the first from river habitats in Britain.
At both sites the rivers showed evidence of saline ingress. Although the sites are not tidal there are sluices downstream of the sites which leak. The South Forty Foot Drain at Swineshead Bridge was monitored quite regularly during the second half of 2006. Between 14th July and 27th October, 11 readings were taken. At the top of the water column salinity varied from a low of 0.58 ppt (0.058%) to a high of 7.8 ppt (0.78%). At the bottom of the water column salinity varied from a low of 0.60 ppt to a high of 18.6 ppt. Both sites are also sampled monthly as part of a regular monitoring programme. Approximate salinity levels for each site for 2006 have been calculated: for the South Forty Foot site the minimum and maximum salinity levels were 0.20 ppt and 7.01 ppt. The average over 12 months was 1.96 ppt. For the South Holland Main Drain site the minimum and maximum salinity levels were 0.89 ppt and 5.62 ppt. The average over 12 months was 2.44 ppt.
Other molluscs present in the sample were Bithynia tentaculata, Radix balthica, Lymnaea stagnalis, Physa sp. and Potamopyrgus antipodarum in the South Forty Foot Drain, and Radix balthica and Pisidium casertanum in the South Holland Main Drain. The samples also contained crustaceans commonly found in the saline reaches of rivers, Gammarus tigrinus and Corophium multisetosum in the South Forty Foot Drain and G. tigrinus in the South Holland Main Drain.
It appears that Mytilopsis is a relatively recent arrival to the Wash drainage system. Shipping from Holland into the ports of Sutton Bridge on the River Nene, and Boston on the River Witham are a possible source of the introduction.
Reference
Killeen, I., Aldridge, D. & Oliver, G. 2004. Freshwater Bivalves of Britain and Ireland. AIDGAP Series, Field Studies Council.