Otolith

An otolith is a calcified structure in the inner ear of most vertebrates, involved in balance and hearing. In teleost fishes there are three pairs of otoliths—the sagitta (sagittae), the lapillus (lapilli), and the asteriscus (asterisci)—situated in the vestibular apparatus with sensory hair cells that detect linear acceleration and head orientation.

Otoliths are composed of calcium carbonate, typically aragonite, embedded in an organic matrix. They grow throughout life by the incremental deposition of mineral layers, forming a record of growth and environmental history. Their mass and density cause them to lag slightly behind head movements, bending the overlying hair cells and producing neural signals that contribute to balance and spatial orientation. They also influence hearing by responding to particle motion and by providing directional sensitivity to sound.

Age determination is a major use of otoliths. Growth increments, visible as rings or zones under microscopy, can be counted to estimate age in fish and other teleosts. This information supports stock assessment, population dynamics studies, and ecological reconstructions. Chemical composition and microstructure of otoliths can reveal environmental conditions experienced by the fish.

Variability across species and environments means otolith interpretation requires care. In fossils, otoliths preserve well and help in species identification and paleoenvironmental inferences. Limitations include potential dissolution or diagenetic alteration, which can obscure growth rings and chemical signals.