Otoliths are ear bones in fish. Bony fish (not sharks or rays) have three pairs of otoliths:
- Sagitta: detects sound and converts sound waves into electrical signals (i.e., hearing);
- Asteriscus: detects sound and is involved in hearing; and,
- Lapillus: detects gravitational force and sound.
Sagittal otoliths are sometimes used for aging fish because they add “growth rings,” similar to tree rings for periods of faster and slower growth. Before using these growth rings, or annuli, as an age estimate, fisheries scientists must validate that each annulus is equivalent to an annual ring. There are a number of ways to do this, including raising a fish in an experimental setting, “tagging” an otolith with a fluorescent dye at a known age, and marginal increment analysis (measuring the distance from the last annulus to the edge of the otolith at different months during the year; if the distance peaks only once a year, the annulus is a yearly measure).