a full moon duplicates
over abstract mountain peaks
years of growth revealed

In fishes, nestled along each side of the brain are otoliths or “ear bones”. Though technically calcium carbonate rocks, otoliths in fishes are similar to ear bones in humans – they help a fish hear and balance. Fishes have 3 pairs of otoliths: large sagittae which are relatively easy to locate and tiny lapilli and asteriscii which if located are followed by quiet celebrations under the warm glow and constant hum of fluorescent lights in a windowless lab whilst dissecting your nth fish.

Otoliths come in all shapes and sizes. They are even invisible in sharks…because sharks do not have calcified otoliths. You can however find otoliths that once belonged to a fish in the stomachs of sharks (and penguins, seals, dolphins and shrimp!) because otoliths do not decompose as quickly as true bones. Otolith shape and size can also help ID what species of fish a predator consumed.

Mostly commonly associated with otoliths is a fish’s age. As fish grow, new layers of calcium carbonate are added to the otolith. When an otolith is sliced a series of rings are revealed and each ring represents one year – analogous to rings of a tree trunk but not always as easy to see. 

You can also find out whether a fish has been swimming in lakes and rivers or the ocean based on an otolith’s chemical composition. Let’s consider the metal strontium – high levels are found in the marine environment and levels are lower in freshwater. As a fish gets older strontium is incorporated into new otolith layers. Using fancy tools with long acronyms the levels of strontium can be measured in otolith layers – higher levels mean a fish was growing in saltwater, lower levels mean a fish was growing in freshwater. 

These fishy “ear bones” can also adorn human ears.

[update: new study finds farmed fish have otolith deformities]