Bivalves with 'concrete overcoats': Granicorium and Samarangia

Two veneroidean bivalves Granicorium indutum from Australia and Samarangia qundrangularis from the tropical Indo-Pacific region, cement a thick, hard layer of sand over most of their shells. In Granicorium this layer forms lo w commarginal ribs while in Samarangia it forms more prominent radial featu res. Sand grains are cemented to the shell and to each other with growths o f a crystalline aragonitic cement similar in morphology to inorganic marine cements. Both species secrete mucus layers at the growing shell margin whi ch initially hold the sediment grains together and form a substrate for the nucleation and growth of calcium carbonate crystals. The ribs of Samarangi a are formed by the accretion of successive sheets of spherulitic growths. In G. indutum, the middle and outermost of two inner mantle folds are large , glandular and capable of considerable extension beyond the shell margin. Mucus secreted by the folds contains abundant bacteria and small calcium ca rbonate crystals. It is proposed that initial nucleation of the calcium car bonate cement takes place within this biofilm possibly mediated by the bact eria. The function of the sand layers is unknown but predation resistance a nd protection of the shells from endobionts are the most likely possibiliti es.