What are Otolith rings?

About Otolith rings

• The otolith is a stony lump in the fish ear.
• These are much like tree rings which reveal fishs’s age.
• Different forms or isotopes of oxygen in the otolith indicate the temperature the fish experienced when it was alive.
• Carbon isotopes reveal how quickly food was converted into energy. Fish carry their fitness trackers in their ears.
• They are commonly known as "earstones," are hard, calcium carbonate structures located directly behind the brain of bony fishes. 
• There are three types of otoliths, all of which aid fish in balance and hearing:
  • Sagitta: The largest of the 3 pairs of otoliths, sagitta is involved in the detection of sound and the process of hearing, or converting sound waves into electrical signals
  • Asteriscus: This type of otolith is involved in the detection of sound and the process of hearing.
  • Lapillus: This type of otolith is involved in the detection of gravitational force and sound
• Different species have otoliths of different shapes and sizes; and cartilaginous fishes, such as sharks, skates, and rays, have none. 
• Significance
  • Features of otoliths can be used to identify the species, size, age, growth rate, and season of death of an individual fish.
  • Analysis of the oxygen isotope values of fish otoliths can provide information on the temperature of the water in which the fish lived.
  • While studying concentrations of trace elements such as barium can indicate the salinity levels of the water.

Q1) What are isotopes?

Atoms with the same number of protons but different numbers of neutrons are called isotopes. They share almost the same chemical properties, but differ in mass and therefore in physical properties. There are stable isotopes, which do not emit radiation, and there are unstable isotopes, which do emit radiation. The latter are called radioisotopes.

Source: What stones inside fish ears are telling us about climate change