The depth of water and the inclination angle of the coast and, in particular, the kind of substratum determine the distribution and population density of date mussels. Oolitic limestones are twice to three times as highly populated as dolomites.The growth takes place in stages, between periods of rest. 10 to 20% ofLithophaga lithophaga having a length of about ∼30mm grew by 0.4 to 3.1 mm in the period of 12 weeks in the substratum. 1 to 2% of mussels with a size of 50 to 60 mm grew by 0.1 to 1 mm outside the substratum. Small mussels grew more frequently and to relatively greater extent than big ones. The first 10 mm of growth in length occurs in the first three years, the next 10 mm after 5 years, etc. so that the biggest mussels (80 mm) will be about 80 years old.The boring activity depends on the condition of the animal and, largely, on the kind of substratum. The boring rate on prepared stones in the open was 0.09 to 0.25 ml in 12 weeks forLithophaga lithophaga with a size of 30mm, that is 0.5 to 1 ml per year. In the laboratory, this amounted to only a third.The depth of the etching effect depends on the granule size in the substratum. The crystals released from the substrate are removed as pseudofeces via the ciliary epithelium. The amount of the fine sediment may be calculated roughly from the volume of the hole.Finally, an attempt is made to apply the experimental findings to the conditions prevailing in the sampling area. It is true that the destruction of the coast by date mussels is restricted to relatively small areas, but it may amount to more than 40 cm/m² at a depth of 1 to 2 meters and continue for a period of 1000 years on the steep slope of oolitic limestones.