One of the ways to study the reactions of marine invertebrates to the external effects of changes in temperature and salinity is the biogeochemical analysis of skeletal parts, which are consistently built up during ontogenesis and record a variety of information about these changes. The most studied shells of mollusks, sea urchin shells and skeletal parts of corals. Information about the chemical composition of modern and fossil mollusk shells is widely used in solving geological and biological problems, including determining the temperature and salinity of ancient marine basins, studying the diagenesis of carbonate sediments, and the biochemical evolution of invertebrates. X-ray diffraction analysis of the shell matter of didacnae belonging to the Cardiidae showed an aragonite composition. The quantitative determination of elements in mollusk shells by microprobe analysis of spot scanning and spectrometric method is carried out. Samples were taken in successive layers of shell growth within the annual ring, and the seasonal dynamics of strontium changes were detected. For Didacna, strontium is the main element-indicator of seasonal and ontogenetic growth, is included in the crystal lattice of aragonite and forms strong compounds in the process of shell formation during the life of these bivalves. The variability of seasonal, ontogenetic, and taxonomic differences in a number of indicator elements in living and Pleistocene bivalves of the genus Didacna was studied.