Mineral skeletogenesis in sponges

Sponges secrete a variety of mineral skeletons consisting of calcite, aragonite, and (or) amorphous silica that confer strength and protect them from physical perturbations. Calcification takes place in a solution of bicarbonate and calcium ions, which is Supersaturated with respect to both calcite and aragonite. In contrast, siliceous spicules are formed from an environment that is undersaturated with respect to silicon. Silification is the predominant process of biomineralization in extant sponges (92% of the species). The number of axes of symmetry in the large skeletal elements (megasclere spicules) is the main skeletal difference between the classes Hexactinellida (monaxons and triaxons) and Demospongiae (monaxons and tetraxons). Hypersilification occurs in both lithistid demosponges and hexactinellids, which are mostly confined to silicon-rich environments. Both siliceous and calcareous sponge skeletons are deposited within a well-defined restricted space by the so-called matrix-mediated mineralization. Both processes require organic molecules, which are secreted by a particular cell type (sclerocytes) and guide spicule formation. In Most Siliceous sponges, these molecules form a discrete filament, which is mainly triangular or quadrangular in cross section in demosponges and hexactinellids, respectively. No discrete axial filament has been reported for calcareous sponges. Silica polycondensation produces nanospheres to microspheres, which are arranged in concentric layers to form the spicules. The potential number of siliceous spicule types in a sponge species appears to be fixed genetically, but the environmental conditions (specifically the availability of silicon) may determine whether a genetically determined spicule type is finally expressed. In this study I review the Current knowledge on sponge skeletogenesis, from molecular, cellular, and structural points of view. The contribution of environment variables, as well as the proliferation and decay of the main skeleton types in the past, are also considered.