MINERAL-COMPOSITION OF NUDIBRANCH SPICULES

Doridacean opisthobranchs have calcium carbonate spicules in their mantle, foot, gills and rhinophores, which are generally fusiform or spheroid in shape. There is surprising little knowledge of their structure, and little information is available on their mineral composition, generally considered to be calcite. In order to study their compositional variability and the possible role of environmental temperature in their calcification, the spicules of several doridaceans, collected at different latitudes, were analysed using a diffractometer (XRD) and a SEM microscope equipped with an EDS spectrometer. The study yielded the following results, concerning the minero-chemical composition of the spicules: 1) the fusiform spicules are mainly composed of calcite (CaCO3) and brucite (Mg(OH)(2)); with a small percentage of fluorite (CaF2). The smaller spherules are almost pure calcite; 2) the mineral composition of fusiform spicules of species collected at different latitudes showed inter- and intra-individual differences, but the Ca/Mg ratio does not seem to vary according to the environmental temperature. In fact, this ratio reflects the volume percent of calcite and brucite in a crystalline aggregate, and consequently, temperature has a poor influence on spicule composition. On the other hand, differences between foot and mantle, not only within the same species but also in the same specimen, suggest that the animal itself can influence the ratio of calcite and brucite nucleation. In this case, the different pH in mantle and foot tissue could induce a different up-take and use of calcium and magnesium; 3) the presence of pure calcite spherules suggests that they could be related to variations of the ion concentration in the skin caused by pH variations due to the dermal gland activity or, more simply, could be considered as a calcium reservoir and source for the production of the slender spicules.