The Eastern Atlantic-Mediterranean boring bivalve Gastrochaena dubia (Mollusca: Gastrochaenidae) has a characteristically ‘8’ shaped calcareous chimney which encircles the siphonal aperture. The ‘8’ length (major axis) is strongly correlated with the valve dimensions (length and height) as well as the volume of the chamber bioeroded by the mollusc. Thanks to the relative robustness of this calcareous structure, it is possible to obtain, using plastiline material directly underwater, moulds of the siphonal aperture. These moulds allow to draw either population structure and bioeroding activity of G. dubia. This new method of measure results to be cheap, accurate and thanks to its non destructive properties, applicable also in Marine Protected Areas (MPAs).