This study describes the morphological properties and discusses the colloid-chemical mechanisms of the formation of hierarchically structured aragonite fibers in the exoskeleton structure of the Mediterranean zooxanthellate scleractinian coral Cladocora caespitosa. The study is based on a detailed structural and morphological examination of the coral exoskeleton and on a preliminary biochemical and molecular identification of the isolated soluble proteinaceus organic matrix. The biomineral structure was examined by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and atomic-force microscopy (AFM), while the isolated protein organic constituents were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and mass spectrometry (MALDI-TOF-MS). The SDS-PAGE analysis of the soluble protein matrix showed three major protein bands at 15, 41, and 80 kDa. Based on the MALDI-TOF-MS analyses, the identified peptides tend to exhibit an acidic character. The results obtained confirm and complement the existing hypotheses relating to the significant role of the soluble acidic protein matrix and the biologically induced colloid-chemical processes in the phase formation and growth of scleractinian submicrometer fibrous aragonite units. It was also shown that the general strategy for the morphogenesis of fibrous structured aragonite lies in the nanoscale aggregation and subsequent coalescence processes that occur simultaneously. The subsequent morphological conversion of the initially formed submicrometer fibrous aragonite units into well-defined, micrometer-sized, prismatic facets in the skeletal structures of the corals is demonstrated. (C) 2010 Elsevier Inc. All rights reserved.