Precipitation of aragonite by calcitic bivalves in Mg-enriched marine waters
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To understand the relative importance of biological versus physicochemical control over biomineralization, we have tested if the chemical composition of the medium (i.e., the Mg/Ca ratio) can change the mineralogy of mollusk shells. The shells of mollusks are made of calcite and/or aragonite, which are by far the most common CaCO3 polymorphs. Several species of bivalves with predominantly calcitic shells have been cultivated in artificial seawater with a Mg/Ca molar ratio within the range of 8.3–9.2, well above the present value for seawater (5.2). Four out of six species used (the scallop Chlamys varia, the oyster Ostrea edulis, the saddle oyster Anomia ephippium and the mussel Mytilus edulis) survived long enough to secrete significant amounts of calcium carbonate. The deposits (sometimes extensive) formed on the interior shell surfaces were predominantly aragonitic. Three individuals of C. varia also increased their length by adding new shell at the margin. Contrary to the internal shell deposits, these margins were high-Mg calcite. This implies that the marginal mantle is able to exert a more strict control on the secreted mineral phase than the mantle facing the internal shell surface. This is the first report on an in vivo experimentally forced switch in bivalve shell mineralogy, from calcite to aragonite due to a change in water chemistry.
KeywordsCalcite Bivalve Aragonite Calcium Carbonate Nacreous Layer
We thank Amelia Ocaña and Luis Sánchez-Tocino (Departamento de Biología Animal y Ecología, Universidad de Granada) for aquarium facilities and Juana Cano (Centro Oceanográfico de Fuengirola, Málaga) for providing specimens. Two anonymous reviewers helped to improve the paper through its critical reading. This work was supported by the Spanish–Portuguese Integrated Action HP02-85, the Research Projects BOS2001-3220, CGL2004-00802 and REN2003-07375 (DGI, MEC) and by the Research Group RNM0190 (CICE, JA). C.J.-L. and A.R.-N. also acknowledge financial support through the Programa Ramón y Cajal (MCyT, Spain).
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