Lake dwelling pearl mussels (Margaritifera margaritifera L.) in the southwest of Ireland - a unique Irish phenomenon?

Margaritifera margaritifera is a freshwater pearl mussel that has a wide distribution across the planet. As a result of its sensitivity to pollution, a failure to recruit juveniles and the widespread practise of pearl fishing in the last century, pearl mussel populations are in serious decline across most of its range. This study reports the discovery of a substantial population of freshwater pearl mussels in a lake in SW Ireland. The discovery of pearl mussel in a relatively deep and still lake habitat questions many of the widely held views which suggest that pearl mussels are a species only found in oligotrophic, fast flowing, shallow streams and rivers. While there have been several anecdotal reports of pearl mussels occurring in lakes, this is the first study to have quantitatively assessed pearl mussels in a lentic freshwater habitat. The lakebed of Lough Cloon was remotely surveyed using a quadrat apparatus with a video camera mounted above it. Quadrats were randomly placed across the lake and were assessed for mussel density, substrate composition, depth, macrophyte cover, distribution pattern and mussel orientation by reviewing the recorded video footage. The population within the lake was estimated using inverse distance weighting of the mussel densities within the quadrats across the lake. It is estimated that between 150,000 and 200,000 mussels exist in the lake which confirms it as an important population within Ireland. This study also opens the possibility for the discovery of more lake dwelling pearl mussel populations across its range. The recorded maximum depth of pearl mussels in this study of 9 m is also the deepest reported depth for this species worldwide. Mussels were discovered in two areas of the lake. It is apparent, that environmental conditions differ considerably between the two areas. The first area is the deep benthic habitat (3-6 m) of the north-west area of the lake. The substrate of this habitat was predominately sand. Mussels in this area were observed in densities up to 43 mussels m-2. Unlike the clustered patterns observed in rivers, mussels in this deep habitat, were randomly distributed across the lakebed and were also orientated in completely random directions. The presences of an aquatic charophyte, Nitella sp. was also observed in this habitat. This charophyte was most abundant where mussel density was highest. In the deepest areas observed mussels were spread out, often 1 mussel m-2. At these depths, the minimum dissolved O2 concentration was 84.5%. The second area was the shallow north-east littoral habitat. Mussels in this habitat were observed in water 1-2 m deep and in some areas of this habitat, sand was scarce. Sand was generally less available in this habitat but where it did occur, mussels were observed in densities up to 118 mussels m-2. Mussels were also observed to inhabit areas where the aquatic macrophyte Isoetes lacustris was present. High densities of Isoetes did not appear to have a significant negative affect on pearl mussels in this area. Where sand was almost absent, the substrate was composed mostly of pebble gravel and mussels were often observed lying completely un-stabilised on the surface of this substrate. The discovery of pearls mussels in a lake habitat is a significant find that has implications for our understanding of the biological limitations of freshwater pearl mussels from habitat requirements, reproduction, feeding behaviour, as well as for the conservation of pearl mussel now and into the future. While the widespread presence of Margaritifera margaritifera in lakes has not yet been confirmed, this study has highlighted that freshwater pearl mussels are capable of surviving in habitats that were previously overlooked or considered unsuitable. There is an imperative, for efforts to be made to survey other lakes in Ireland for the presence of pearl mussels