The muddy bottom of Lake Pannon – a challenge for dreissenid settlement (Late Miocene; Bivalvia)

The Late Miocene dreissenids of Lake Pannon – a long existing eastern-central European lake – bear witness to two very different modes of life, which allowed these bivalves to successfully settle in what appears to be an inhospitable environment. One guild is restricted to the genera Mytilopsis Conrad, 1858 and Sinucongeria Lõrenthey, 1894. These representatives are extreme r-strategists which tend to produce temporal boom-and-bust populations during ‘time windows’ of improved ecological conditions. These opportunities are interpreted here to correlate with cyclic shifts in the epilimnion/hypolimnion relation and the corresponding oxygenation of bottom waters, events which are probably triggered by low-frequency orbital forcing. The second guild is composed solely of Congeria Partsch, 1836. The paradox of the proliferation of a single species, characterised by extreme size and weight, in a muddy, oxygen-deficient lake environment avoided by nearly all other molluscs points to a ‘hidden’ mechanism not accessible to other species. We tentatively link this mechanism with chemosymbiosis. Functional morphology in interplay with a highly sulphidic, over-saturated interstitial water support an interpretation of the large Congeria subglobosa as a successful ‘H2S pump’ model.