On a humid morning along the northeastern coast of Brazil — the kind where the air feels heavy with salt and mangrove tannins — fishers haul in their nets and longlines as they have for generations. The catch hits the bottom of their boat with a resounding smack, followed by the inconsistent beating of numerous animals thrashing due to the lack of water. Among the catch are stingrays, their bodies folded over, pushed inwards (or outwards, for some) from the burgeoning force of other fish in the net. For decades, scientists have studied animals like these, driven by persistent questions about the major gaps that still remain in our understanding of their basic ecology and biology. Thanks to advances in technology we are starting to get answers to some of these questions, like what are they eating and where are they going to find it? But the answers (in science, it seems) are never as simple as the question.

Stable isotope ecology has long relied on carbon and nitrogen to reconstruct the lives of marine animals. Carbon isotopes help trace where primary production begins, whether in mangroves, seagrass beds or offshore waters, whereas Nitrogen isotopes offer a window into trophic position, increasing as energy moves up food webs. Together, they have transformed how scientists understand diet, movement and habitat use in animals like sharks and rays. But ecosystems are not built on two elements alone… and animals are not passive recorders of what they eat. Their bodies transform, regulate and filter the elements they take in. This is where newer tools are beginning to reshape the field, including an emerging tracer that until recently sat on the margins of ecological research: zinc. See, zinc isotopes (expressed as δ66Zn) behave differently from carbon and nitrogen; instead of simply reflecting diet or baseline food web structure, they appear to carry a blended signal influenced by trophic interactions, environmental chemistry and physiological processing within the animal itself. So, they do not replace traditional isotopes but add another tool in this field.

In a recent study of three sympatric stingray species along a heterogeneous coastal system in northeastern Brazil, researchers examined zinc isotopes in muscle tissue alongside carbon and nitrogen isotopes. The species, Hypanus guttatus, Hypanus marianae and Hypanus berthalutzae , share overlapping geographic ranges (because of this, they are called “sympatric”) but differ in habitat use and feeding ecology, as they move through estuaries, shallow reefs and deeper coastal waters, sometimes side by side, sometimes apart, sometimes interacting in ways that are still not fully understood by science. When scientists measured δ13C and δ15N of these animals, the expected patterns emerged. Meaning there were marked differences in diet and habitat use, with some overlap that reflected shared environments and prey resources. Hypanus guttatus stood out as the most variable, consistent with its reputation as a generalist that moves across habitats. The other two species showed tighter clustering in isotopic space (i.e., where an organism sits when you plot its stable isotope values on a graph), suggesting more constrained ecological roles.

The δ66Zn values showed clear species-specific patterns, but not always in predictable ways! Hypanus guttatus again displayed broader variability, while the other species were more constrained, yet zinc did not neatly track trophic position or habitat use in the same linear way as nitrogen and carbon. Instead, it highlighted differences that likely arise from a combination of diet, physiology and environmental exposure. But why would zinc behave this way? One possibility is that it integrates more than just what an animal eats, meaning it may reflect how an animal processes nutrients, how it regulates metals in its tissues and how it moves through environments where baseline chemistry shifts from mangrove-lined estuaries to open coastal waters. In systems like this, where salinity, sediment input and productivity change over short distances, the chemical landscape itself is patchy. Basically, the fact that zinc did not always agree with carbon and nitrogen suggests that δ66Zn may be sensitive to ecological processes that are invisible to traditional tracers (offering a complementary lens rather than a replacement). This raises a broader question: if different isotopes capture different layers of ecological reality, what does a “dietary niche” really represent? Is it purely about what is consumed, or is it also about how organisms physiologically interact with their environment as they feed, grow and move? And if so, how much of what we interpret as ecological separation is actually biology filtered through chemistry?

Elasmobranchs (the group that includes sharks, rays and chimaeras) are among the most threatened vertebrates in the ocean. Fishing pressure, habitat loss and climate change continue to reshape their populations worldwide. Thus, understanding how species share or partition resources is central to identifying critical habitats and designing effective protection strategies. Adding new tracers like zinc isotopes can help sharpen existing methodology, with a more detailed isotopic picture hopefully helping in distinguishing species that appear similar at first glance, identifying shifts in habitat use across life stages and revealing hidden complexity in food webs that are under pressure from multiple stressors. But it’s currently unknown how consistent zinc isotope patterns are across regions or how they change with age, growth or reproductive state. It’s also unknown how much of the signal is ecological versus physiological noise, both issues that need to be addressed if this methodology is going to be used confidently in future studies.

This new work from Brazil will undoubtedly open the floor for more questions than answers at this time, similar to a Pandora’s box. But perhaps the most interesting find of this research is the fact that a single stingray carries within its muscle tissue a chemical archive shaped by diet, environment and physiology. Which begs the question: how many other hidden archives are we overlooking in the ocean?