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Recent and fossil otoliths from 25 differ f nt fishes have been studied for their amino acid content and for their C13/C12 and 018/016 distribution in the carbonate fraction. The selection includes specimens from a wide phylogenetic rcqgenas well as from various fresh water and marine habitats.A l l otoliths are composed of aragonite, and their total organic matter ranges from 0.2 to 10 percent. The organic matter i s a protein (MW > 150,000), which i s characterized by a high abundance of acidic amino acids.that exhibit a wide variety of different mineralized tissues which are species specific, the proteinaceous matter of a I I otol iths i s chemica I l y rather uniform. In comparison to molluscsThe high abundance of oxygen-rich amino acids accounts for the ease of mineralization of the organic template. Namely, oxygen supplied by carboxyl groups i s used for the coordination of Ca* ions, resulting in the formation of metal ion coordination polyhedra. Carbonate groups I inked via hydrogen bridges to the template w i l l exchange their oxygen with that of the metal polyhedra to stabilize the structure; Ca*Og polyhedra are the consequence. Subsequent nucleation and crysta I growth w i l l lead to aragonite.Oxygen and carbon isotope data indicate that the aragonite is formed close to isotopic equilibrium with the sea. This i s surprising because sea water has no direct access to the inner ear where the otolith originates.(1) to determine the mean water temperature where the fish lived, (2) to distinguish between fresh water and marine fish i n ancient deposits, and (3) t o reveal information on migratory tendencies of fish.Isotope data may serve a threefold purpose:

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