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Earstones help the fish know up from down, determine their speed through the water and aid hearing. The researchers from UK, Norway and Denmark have now shown that it also acts as a fitness watch, revealing whether the fish had an active life or was completely relaxed.
The carbon in the earstone originates from two sources: the water the fish lives in and the food it eats to meet its energy demands. The carbon from both sources mix in the blood before some of it ends up in the earstone as calcium carbonate. By carefully analyzing the calcium carbonate, the researchers can determine the how much of the carbon originates from the water or the diet as the two sources have different proportions of the light and the heavy carbon isotopes. When the fish's metabolism increases, for example when it encounters warmer water or is more active, it burns off more food and the proportion of carbon from the food in its earstone increases.
Earstones - Layers - Rings - Trunk - Increment
Earstones grow new layers similar to the rings in a tree trunk. Every little increment in the otolith growth reflects the growth of the fish and can be used to document its age. This data can be recovered even after the fish has died. Now, by measuring carbon isotopes in the individual layers, the researchers can tell how the metabolism of a fish has changed on a monthly basis during its life, including how seasonal variations in temperature, food and the fish's behaviour have affected its metabolism.
"Our new results is the key to a treasure chest of new discoveries," says Dr Clive Trueman, from the University of Southampton, who is one of the co-authors behind the paper that is being published in the journal Communications Biology today.
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