The Florida Museum of Natural History reports on a method that could close a major gap in shark research: a new study in Molecular Ecology uses chemical marks on DNA to estimate an animal’s age. In the zebra shark (Stegostoma tigrinum), the resulting epigenetic clock predicts the age of individuals with known birth dates to within roughly two years.
Age is more than a biological detail. The age structure of a population indicates whether enough young animals are entering it, how many breeding adults remain and whether fishing pressure is sustainable. For threatened sharks, however, reliable and non-lethal ageing methods are often missing.
Why shark age is difficult to determine
Body size provides only a rough guide because growth slows as sharks mature. Counting growth bands in vertebrae can be more informative, but the bands are not equally reliable in every species and obtaining the vertebrae is lethal. Researchers are therefore often limited to bycatch, stranded animals or specimens that have already died.
A blood-based test can be used on living sharks and may allow the same individuals to be sampled again. That makes it especially useful for threatened species and long-term monitoring programmes.
Chemical traces of time in the genome
An epigenetic clock does not read changes in the DNA sequence itself. It measures chemical modifications, especially methyl groups attached to certain cytosine sites. These marks can influence whether genes are active without altering the underlying genetic code.
Methylation changes in recurring patterns over an animal’s lifetime. Large parts of the genome lose methyl groups with age, while selected regions gain them. A statistical model can combine the most informative sites into an age estimate. Such clocks exist for many mammals and several other vertebrates, but sharks and rays have remained largely absent.
Fifty-one sharks calibrate the clock
The team analysed blood from 51 aquarium-born zebra sharks whose birth dates were documented. The animals ranged from juveniles under one year old to sharks older than 28 years, covering almost the species’ known lifespan.
The zebra shark genome contains about 3.7 billion base pairs. Instead of testing all of them in future samples, the researchers identified fewer than 100 cytosine sites with the strongest age-related patterns. Even a sparse model using only ten sites sacrificed relatively little accuracy.
Depending on the model, the median absolute error in known-age animals ranged from 1.03 to 1.99 years. Predictions were best for younger sharks. In older animals, methylation changes tended to level off and the uncertainty increased.
A test with sharks born in the wild
Conditions in aquariums can affect biological ageing through diet, stress, disease and the environment. A clock trained on aquarium animals therefore cannot automatically be applied to free-living populations.
The researchers also examined 19 zebra sharks that were born in the wild and later moved to aquariums. Their exact birth dates were unknown, so body size and time in human care were used to calculate minimum ages. Against these estimates, the clock usually came within three to four years.
The result is promising, but not a final validation for wild populations. Some of the discrepancy may come from the model, while some may reflect the uncertain true age or a difference between chronological and biological age. More samples from free-ranging sharks and repeated measurements are now needed.
New information for fisheries and conservation
A practical age test could reveal whether a population is dominated by juveniles, breeding adults or older sharks. If fishing shifts that distribution, managers may detect the warning before simple sighting counts show a clear decline. This would be particularly valuable for the endangered zebra shark, whose populations face fishing pressure and the loss of reef habitat.
The method is not yet a rapid field test: blood still has to be collected, processed and analysed at selected DNA sites. Restricting the clock to a small set of reliable markers should nevertheless make larger population samples more affordable.
Researchers now want to test whether the same ageing signals occur in other shark species. Because sharks split from other vertebrate lineages around 400 million years ago, shared epigenetic patterns could also reveal how deeply the biological mechanisms of ageing are rooted in vertebrate evolution.
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