Molecular structure and composition of fish otoliths

Abstract

Recent and fossil otoliths from 25 different fishes have been studied for their amino acid content and for their C¹³/C¹² and O¹⁸/O¹⁶ distribution in the carbonate fraction. The selection includes specimens from a wide phylogenetic range as well as from various freshwater and marine habitats. All otoliths are composed of aragonite, and their total organic matter ranges from 0.2 to 10%. The organic matter is a protein (MW>150,000), which is characterized by a high abundance of acidic amino acids. In comparison to molluscs that exhibit a wide variety of different mineralized tissues which are species specific, the proteinaceous matter of all otoliths is chemically rather uniform. The high abundance of oxygen-rich amino acids accounts for the ease of mineralization of the organic template. Namely, oxygen supplied by carboxyl grops is used for the coordination of Ca⁺⁺ ions, resulting in the formation of metal ion coordination polyhedra. Carbonate groups linked via hydrogen bridges to the template will exchange their oxygen with that of the metal polyhedra to stabilize the structure; Ca⁺⁺O₉ polyhedra are the consequence. Subsequent nucleation and crystal growth will lead to aragonite. Oxygen and carbon isotope data indicate that the aragonite is formed close to isotopic equilibrium with the sea. This is surprising, because seawater has no direct access to the inner ear where the otolith orginates. Isotope data may serve a threefold purpose: (1) to determine the mean water temperature where the fish lived, (2) to distinguish between fresh water and marine fish in ancient deposits, and (3) to reveal information on migrtory tendencies of fish.