The whale shark ( Rhincodon typus ) is the largest living fish in the world, reaching up to twelve meters in length. As if it were a fingerprint, these animals present a pattern of spots that allows each individual to be identified. In addition, they have a succession of bands on their vertebrae that, like tree rings, increase over time. In order to know the age of each shark, therefore, it would be enough to know how often their rings grow. There is no scientific consensus on this point: some studies suggest that a new ring forms every year, while others estimate that this occurs every six months.
Knowing the age of these animals, classified as “vulnerable” by the International Union for Conservation of Nature (IUCN) list that assesses the danger of extinction, is important because, the longer a species lives, the more vulnerable it is to threats posed by factors such as illegal hunting or warming oceans.
Now, work published in the journal Frontiers in Marine Science suggests that ring augmentation could occur every year. To reach this conclusion, scientists have taken advantage of the radioactive legacy left by the arms race that occurred during the Cold War in the last century.
The carbon footprint-14
After the Second World War, the great powers of the world carried out various nuclear tests, and one of the effects was that, temporarily, in the atmosphere the concentrations of the isotope carbon 14, a natural radioactive element that archaeologists and historians use as common way to date. The advantage of carbon-14 is that it decomposes at a constant rate and is also easy to measure, making it perfect for making age estimates.
This footprint formed after the nuclear explosions of the Cold War created a kind of label in living organisms, as the isotope moved through food webs and its signature still persists. As we have already discussed, carbon-14 decays at a constant rate, which means that the amount contained in a bone formed at one point in time will be slightly greater than that contained in the identical bone formed later.
Using this technique, the team of researchers measured the levels of the isotope in the growth rings of two long-deceased whale shark specimens that were preserved in Pakistan and Taiwan, and in this way were able to more accurately determine the frequency of creation of the same and, therefore, the age of the analyzed animal.
“We discovered that, indeed, each year a new growth ring is deposited, ” explains one of the researchers participating in the study. “This is very important, because if you overestimate or underestimate growth rates, you will develop a conservation strategy that will surely not work.”
In one of the individuals analyzed, the scientists were able to determine exactly that he had died at 50 years of age. In previous modeling studies it had been suggested that whale sharks can live for up to a hundred years, and thanks to this new technique it will be possible to more accurately date the age of other specimens to estimate the average longevity of the species. “Our study shows that adult sharks can reach a great age and that the long lifespan is probably a characteristic of the species. Now we have another piece of the puzzle,” concludes the expert.
