The first time I held a shark vertebra in my hand, I was surprised by how small and delicate it looked. People tend to imagine sharks as all teeth and muscle, but hidden inside their bodies are fragile structures that literally hold them together. For decades, scientists have tried aging sharks the same way dendrochronologists read tree rings: by counting bands in calcified tissue. One pair of bands, one year of life. Simple enough, right?

A new study on the critically understudied speartooth shark, Glyphis glyphis , suggests that scientists may need to rethink how we estimate age in at least some sharks and rays. And considering that more than one-third of all chondrichthyans, the group that includes sharks, rays and chimaeras, are threatened with extinction , getting age right is kind of a big deal! Age estimates influence conservation status assessments, estimates of population recovery and fisheries management decisions. If you do not know how quickly an animal grows or how long it lives, you cannot accurately predict how vulnerable it is to overfishing.

Traditionally, as was said above, shark age estimation relies on counting visible bands in vertebrae. These bands are usually viewed under transmitted light optical microscopy, or TLOM. Basically, researchers are looking for alternating opaque and translucent zones that are thought to represent annual growth cycles. The problem is that these bands are not always easy to interpret; in tropical species, where environmental seasonality is less dramatic than in temperate waters, the bands can be faint, compressed or inconsistent. Different scientists can even come to different age estimates when looking at the same vertebra! “Given the urgent need to protect them, age estimation is a pivotal piece of information for reconstructing shark habitats and life stages, and for developing appropriate strategies and policies for their protection,” the researchers said in a University of Melbourne article .

The researchers behind this new study, led by Hilary M. K. Lewis of the Australian National University and James Cook University, decided to approach the question differently. Instead of focusing only on visible growth bands, they looked at the chemistry inside the vertebrae of a shark living in a river. “ In our study , we applied geochemical techniques to analyse the geochemistry of the vertebrae of the Speartooth Shark ( Glyphis glyphis), a vulnerable river shark species . It is estimated that fewer than 2,500 mature individuals remain in the wild,” the team explained. This species is particularly interesting because it is euryhaline, meaning it can tolerate a wide range of salinities. Juveniles spend much of their lives in riverine and estuarine habitats influenced by dramatic wet and dry seasons. Those seasonal changes turn out to leave behind chemical fingerprints.

“For the very first time, we combined two complementary analytical geochemistry techniques. One uses X-rays (micro-XRF) , and the other uses laser beams (LA-MC-ICP-MS) to characterise the micro-scale chemistry of shark vertebrae,” the researchers said. “A focused laser beam removes material from the shark vertebrae sample, with the resulting aerosol being transported into the Mass Spectrometer (MC-ICP-MS) for ionization (gaining a positive or negative charge) and then detection of elements.” In simpler terms, these methods allow scientists to map the distribution of elements and isotopes within shark vertebrae at very fine scales. And one element stood out: strontium. Strontium naturally occurs in water, and its isotopic composition changes depending on environmental conditions. In the Adelaide River, seasonal rainfall dramatically alters the chemistry of the water; during the wet season, heavy rainfall washes material from ancient rocks into the river, changing the ratio of strontium isotopes. During the dry season, marine influence increases and the isotopic signature shifts again. The team found repeating patterns of strontium concentration and isotope ratios along the vertebral growth axis, lining up closely with annual wet and dry seasons, effectively creating a timestamp linked to environmental cycles. And here is the kicker: those chemical bands did not consistently match the traditional visible bands counted under a microscope.

This suggests that at least in speartooth sharks, visible vertebral bands may not reliably represent annual growth. Some bands may form because of structural or physiological processes unrelated to time itself. The chemical signatures, however, were anchored to predictable seasonal rainfall patterns, making them far more reliable indicators of age between birth and around 11 years old.

So, what does that mean and why does that matter? Well, if scientists have underestimated the age of certain sharks, then they may also have underestimated how slowly those sharks grow or how long they take to reproduce; for example, a species assumed to mature at eight years old might actually mature much later. That changes population models, recovery timelines and conservation priorities. It also raises broader questions about how we study long-lived marine animals. We often use techniques that have been used for decades in science because they’ve worked on animal x, y, and z. But how many assumptions in ecology are built on methods that work well for some species but poorly for others? The answer is probably a few. Maybe even a lot. Studies like this force us to confront the fact that the convenience of using a “tried and true” method may be leading us to false data for certain animals who cannot handle mistakes when it comes to their conservation.

Speartooth sharks are incredibly difficult to study in the wild. They live in murky rivers, are naturally rare and spend much of their lives hidden in turbid water. Thus, every new piece of information helps fill in major gaps in our understanding of the species. The study also revealed fascinating details about speartooth shark life history — the chemical signatures suggest that pregnant females likely remain in lower estuarine environments with stronger marine influence, while pups are born around the onset of the wet season. In fact, researchers could even identify the birthmark region in the vertebrae more clearly using elemental analysis than with traditional microscopy.

While this technique is not a universal solution, the approach works particularly well in environments where seasonal chemical changes are strong and predictable. Marine species living in relatively stable oceanic conditions may not show the same dramatic isotopic fluctuations but the broader concept remains important: environmental chemistry can serve as a biological clock.