By Sam Lucas
Have you ever wondered if fish have ears or if they can hear? Fish do not have ears that we can see, but they do have “earstones” known as otoliths, located behind their brain. These internal structures help fish interpret sound vibrations, maintain balance, and orientate themselves within their environment – similar to the function of a human’s middle ear.
There are three types of otoliths each with a different shape: the sagitta, the asteriscus, and the lapillus. The shape and size of these three otoliths differ between fish species. However, these structures are only present in bony fish – cartilaginous (sharks, skates, rays) and jawless fish (lampreys, hagfish) do not have them.
Otoliths are composed of calcium carbonate and protein. As fish age, these materials are deposited at different rates, causing distinct bands to form on the otoliths, similar to the growth rings of a tree. The bands differ in color depending on the type of growth – a dark translucent band indicates a period of fast growth while a white opaque band indicates a period of slow growth. With the help of a microscope, scientists are able to estimate the age of fish by counting the opaque bands on the otoliths. Otoliths can either be studied as a whole or sectioned for a clearer view of the banding pattern. Information obtained from the otoliths combined with the length and weight of the fish allow scientists to better understand growth rate and trends of future generations.
The chemical composition of otoliths is also studied, allowing scientists to delve deeper into the life of fish. It can reveal information on the fish’s migration patterns, what it was eating, where it lived, the temperature of the water, the presence of pollutants in the waterbody and much more!