Otolith

Otoliths are biomineralized structures composed primarily of calcium carbonate in the form of aragonite, though some species have vaterite or calcite. They are embedded in the gelatinous matrix of the macula, a sensory epithelium containing hair cells. In mammals, the otoliths are called otoconia and are microscopic (<30 µm), whereas in fish they are larger and are known as otoliths proper. The three pairs in fish are the sagitta, lapillus, and asteriscus, each located in different parts of the inner ear.¹

Otoliths provide inertial mass that displaces hair cell stereocilia during linear acceleration or changes in head position, thereby transducing mechanical stimuli into neural signals. In fish, the otoliths also vibrate in response to sound pressure, enabling hearing. The otolith organs—utricle and saccule—are particularly sensitive to low-frequency vibrations and are essential for postural control and orientation.²

Fish otoliths grow throughout the animal's life, depositing alternating layers of calcium carbonate and protein that form daily rings, similar to tree rings. These rings are used by fisheries scientists to determine age, growth rate, and population dynamics. Otolith microchemistry, particularly the ratio of strontium to calcium, can reveal migration patterns and environmental history.³

Otoliths are not only used for aging fish; they also serve as a record of environmental change. For example, otoliths from archaeological sites have been used to reconstruct past ocean temperatures. In some species, otoliths exhibit asymmetry, which can affect swimming performance. Additionally, otoliths are used in forensic science to identify fish species from stomach contents or archaeological remains. A notable edge case is the otolith of the deep-sea fish Macrourus, which can be used to estimate depth of capture due to its density.

In humans, otoconia are involved in benign paroxysmal positional vertigo (BPPV), a common disorder where displaced otoconia enter the semicircular canals, causing vertigo. Treatments such as the Epley maneuver reposition these crystals. Research on otoconia formation has implications for understanding osteoporosis, as both involve calcium metabolism.⁴

Aragonite

A crystalline form of calcium carbonate, the primary mineral in otoliths.

Vaterite

A less stable polymorph of calcium carbonate found in some otoliths.

Macula

A sensory patch of hair cells in the inner ear that contains otoliths.

Sagitta

The largest otolith in fish, located in the saccule.

Lapillus

An otolith located in the utricle of fish.

Asteriscus

An otolith located in the lagena of fish.

BPPV

Benign paroxysmal positional vertigo, a disorder caused by displaced otoconia.