D. Medakovic á S. Popovic á B. GrzÏeta M. Plazonic á M. Hrs-Brenko X-ray diffraction study of calci®cation processes in embryos and larvae of the brooding oyster Ostrea edulis Received: 28 May 1997 / Accepted: 1 July 1997 Abstract X-ray powder diraction was used to study shell calci®cations of the oyster Ostrea edulis, sampled in the Limski Kanal, Istria (Adriatic Sea), in May 1992. All the developmental stages were followed, from the em- bryonic stage through the transition between the tro- chophore and veliger larva (prodissoconch I and II) and later, after swarming, the pelagic free-swimming larval stages, up to their settlement and attachment (from the D-shaped to the fully formed pediveliger larva), and ®- nally during metamorphosis and juvenile stages (disso- conch). In the ®rst gastrula stage, only an amorphous tissue is present (a periostracum and organic matrix). The beginning of shell formation (at the end of gastrulation) in early trochophores is manifested by the appearance of calcite (up to 1±7% of total volume) and then aragonite (about 1%). In the later stage of the veliger larva the fraction of calcite decreases as well as the amorphous fraction, while the fraction of aragonite rapidly increases. In the prodissoconch II stage and during the whole pe- lagic period aragonite is dominant, accompanied by a very small amorphous fraction and traces of calcite. The shell mineral composition does not change until meta- morphosis, whereupon the fraction of calcite rapidly in- creases and the fraction of aragonite decreases. The postmetamorphic valves of the juvenile and adult oyster consist mainly of calcite, except the resilium and my- ostracum which remain aragonitic, possibly as a contin- uation of the inner layer of the larval shell. Introduction Biomineralization is a complex combination of bio- chemical and physiological processes, depending on the endogenous activity of an organism and environmental in¯uences. In bivalve molluscs, biomineralization starts at early stages of development and continues during the entire life cycle. Biomineralization processes depend on various conditions. Those in¯uencing the type and structure of the biominerals are the following: super- saturation of medium with calcium ions, nature of the critical nucleus, the organic matrix, inhibitors of phase transitions, environmental temperature, salinity, pH, etc. (Wilbur 1964; Taylor et al. 1969; Lowenstam 1981; Watabe 1983; Wilbur and Saleuddin 1983; McConn- aughey 1989; Addadi and Weiner 1992). These processes take place in tissues and shells and manifest themselves in calci®cation, decalci®cation and in polymorphic transitions of calcium carbonate in particular shell layers. The shells of marine bivalve molluscs are built of calcium carbonate in the form of calcite, aragonite or vaterite, distributed in two, three or more separated shell layers that dier in size, orientation and way of packing of crystals. The fractions of mineral components in carbonate shells depend on many factors, being a char- acteristic for family, genus and bivalve species. Results of recent research show that crystal components in dif- ferent organisms are under biological control, and that crystal forms in speci®ed tissues are adapted to their function (Addadi and Weiner 1992; Albeck et al. 1993; Berman et al. 1993; Davidson et al. 1995). In many studies related to morphological develop- ment of the bivalve after fertilization, during embryonic and larval stages to the ®nal shell of the adult organism (Timmermans 1969; La Barbera 1974; Walne 1974; Carriker and Palmer 1979; Waller 1981; Kasyanov et al. 1983; Kasyanov 1984; Strathmann 1987; Widdows 1991), mineral changes have not been described com- pletely or have been studied for some parts of the larval Marine Biology (1997) 129: 615±623 Ó Springer-Verlag 1997 Communicated by O. Kinne, Oldendorf/Luhe D. Medakovic (&) á M. Hrs-Brenko Rud  er BosÏkovic Institute, Center for Marine Research Rovinj, 52210 Rovinj, Croatia S. Popovic á B. GrzÏeta á M. Plazonic Rud  er BosÏkovic Institute, Materials Science and Electronics Department, P.O. Box 1016, 10001 Zagreb, Croatia S. Popovic University of Zagreb, Faculty of Science, Department of Physics, P.O. Box 162, 10001 Zagreb, Croatia