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 substrate