We owe a lot of our modern technology to biological concepts adapted from the animal kingdom, but sometimes we miss useful solutions that are right in front of our noses – or rather, our ears. Otoliths are calcified structures that build up in the inner ear of vertebrates and are very sensitive to horizontal and vertical movement. Because of this, otoliths play an important role in the interpretation of speed, balance and direction, but much like the seasonal growth rings of a tree, otoliths also preserve information about the growth rate of fish throughout their life. However, recent work published by an international team of researchers from Norway, Denmark and the UK reveals that these otoliths can also record long-term metabolic activity in the manner of a personal fitness tracker, allowing scientists to monitor and predict how environmental change may affect the metabolism of wild fish populations.
The cornerstone of this impressive new technique is in measuring the relative concentration of two distinct variations of carbon with different atomic masses (isotopes) that comprise the calcium carbonate in the otoliths. Each isotope is acquired from a different source, with the slightly heavier isotope diffusing into the fish directly from the surrounding water and the slightly lighter isotope being released from the respiration of food for energy. As rates of respiration increase due to physical activity or to the presence of environmental stressors, the concentration of the lighter isotope in the blood increases relative to the concentration of the heavier isotope, which then becomes deposited in the otoliths as carbonate and provides a permanent record of the change in metabolism. While this carbon isotope–metabolism relationship seemed to be a theoretically valid proxy for measuring metabolic rate, it had not been experimentally tested until now.
Through a series of experiments, the team recovered and analysed the carbon isotopes from the otoliths of Atlantic cod (Gadus morhua) reared in captivity at temperatures ranging from 4 to 14°C and then investigated how the ratio of the heavy and light carbon isotopes in the otoliths varied compared with their metabolic rates. They found that the relative amount of light carbon isotopes produced by respiration and deposited in the otoliths increased as the temperatures rose from 4 to 14°C in line with the increase in the fish's metabolic rates. So, the proportion of light carbon isotopes in the otoliths is a good indicator of the fish's metabolic rate at the time that the calcium carbonate was laid down in the otolith. The team then collected and analysed the calcium carbonate content in the otoliths of wild Atlantic cod and four deep-water fish species (Baird's slickhead, blue antimora, roundnose grenadier and orange roughy). The ratios of the carbon isotopes in the wild fish otoliths agreed well with measurements made by other scientists of these fishes’ metabolic rates, suggesting that this new technique could provide a reliable estimate of the metabolic rates of wild fish – at least for the range of fish species tested so far.
Measuring metabolic rate in free-roaming wild animals is an important yet challenging task; as such, this new technique marks a significant step forward for the field, especially for those invested in monitoring today's rapidly changing aquatic ecosystems. Perhaps most interestingly, this technique allows for the collection of metabolic data not only from recently deceased fish, but also from the remains of much older fish. Researchers as far back as the 1890s have been collecting fish otoliths as a means of measuring their growth rates without knowing the new significance that their specimens would hold today. To this end, the team have started to analyse the otoliths of Greenlandic cod from time periods dating back to the 1920s. They hope that by looking to the past, they might be able to predict the effects of future environmental change on fish populations thanks to these historic metabolic records that have been, quite literally, etched in stone.
