Activity of recent endolithic bivalves (Pholas) in carbonated boulders from the Biscay abrasion platform: morphology and microstructures

Along the Biscay (Armintza-Sopelana) abrasion carbonated platform, at least four different marine bioeroders have been identified (sponges, polychaete annelids, endolithic bivalves and sipunculids). The most abundant recent structures (Gastrochaenolites), correspond to endolithic bivalves as Pholas genera, which in many occasions have preserved their valves in a hard carbonated substrate. There exists a lithologic control (carbonated vs siliciclastic rocks) in the hard substrate. Below the 60% CaCO3 it is not possible bioeroder colonization, which can reach high values (70 units/100cm2) in appropriate conditions. The aragonite composition and the complex cross foliated and regular simple prismatic microstructures of the Pholas valves are characteristic. The dominant drilling mechanism is chemical from dissolution of carbonated hard substrat