A handful of tiny, tooth-like scales recovered from ancient reef sediments is offering scientists an unprecedented way to estimate how many sharks once lived above coral reefs—and the results suggest that some conservation targets may be dramatically wrong. In a study published in Science, researchers from the Smithsonian Tropical Research Institute (STRI) used fossilized shark scales, known as dermal denticles, to reconstruct shark abundance on both sides of Panama’s Isthmus. Their findings reveal that prehistoric reefs in the Gulf of Panama supported approximately 20 times more sharks than reefs on Panama’s Caribbean coast, despite the two regions being separated by only about 100 kilometers and sharing many of the same shark species. The discovery also exposes how strongly ocean productivity can shape shark populations and how misleading modern comparisons may be when managers try to determine what a “healthy” reef should look like.
Sharks are continuously shedding dermal denticles throughout their lives. These microscopic structures cover the animals’ bodies and perform several functions, including reducing drag as sharks swim, protecting the skin, and influencing how water moves across the body. Unlike ordinary fish scales, dermal denticles are made from a hard, tooth-like material and often possess distinctive shapes that vary among species and ecological types. Once shed, they sink into reef sediments, where their mineralized structure can persist for millions of years. By counting denticles in sediment layers whose ages can be determined, researchers can estimate how frequently sharks occupied the surrounding reef environment at different points in the past. The method functions as a biological archive, preserving evidence of shark communities from periods long before systematic fishing records existed.
The research team applied this approach to sediments deposited between approximately 7,000 and 3,000 years ago, a period that predates intensive industrial fishing, and compared those ancient layers with sediments deposited during the past century. Across 157 samples, the scientists recovered 3,497 denticles. The number of scales in each sample was adjusted using models that accounted for factors such as sediment age, sampling effort, and variation in deposition. This allowed the researchers to compare denticle accumulation rates rather than simply counting raw fragments. A higher accumulation rate indicates that more sharks were likely swimming over or near a reef, shedding denticles that eventually became incorporated into the seafloor. Because the approach captures the combined signal of many animals over time, it can reveal broad population baselines that are impossible to reconstruct from scattered historical observations.
The first striking contrast appeared during fieldwork. In Bocas del Toro on Panama’s Caribbean coast, the team found roughly 50 shark denticles in 10 kilograms of reef sediment. A much smaller sample from a Pacific reef contained about 200 denticles in only one kilogram of sediment. As more material was processed and the data were analyzed, the initial impression became statistically robust. Before humans began fishing at large scales, reefs in the Gulf of Panama supported roughly 20 times more sharks than comparable Caribbean reefs. Over the following centuries, the two coastlines diverged even more. Denticle accumulation on the Caribbean side declined by about 75 percent, while accumulation on the Pacific side remained statistically similar to its prehistoric baseline when averaged across the last hundred years. Today, Pacific reef sediments yield approximately 100 times more denticles than Caribbean sediments.
That result is especially surprising because Panama’s Pacific coast has experienced much heavier fishing pressure. More than 98 percent of the country’s fishing activity occurs there, meaning the Pacific reefs have faced substantially greater direct human exploitation than their Caribbean counterparts. The explanation, according to the researchers, lies in the physical and chemical differences between the two marine environments. The Gulf of Panama is strongly influenced by seasonal upwelling, a process in which winds and ocean circulation bring cold, nutrient-rich water from depth toward the surface. These nutrients stimulate plankton growth and support a productive food web capable of sustaining large predator populations, including sharks. The Caribbean coast of Panama is comparatively nutrient-poor. Its clear tropical waters may resemble the classic image of a coral reef, but lower external productivity means that more of the ecosystem’s energy is stored within reef organisms themselves, leaving shark populations smaller and potentially more vulnerable to fishing.
The ecological distinction changes how scientists should interpret reef recovery. Conservation programs often use the healthiest nearby reef as a reference point, assuming that neighboring ecosystems should support similar numbers of predators or that fishing has affected them in broadly comparable ways. In Panama, that assumption could produce a severe error. If Pacific shark abundance were used as the recovery target for Caribbean reefs, managers might establish goals that are unrealistic for a naturally less productive ecosystem. Conversely, if the lower Caribbean baseline were applied to the Pacific, managers could underestimate the number of sharks that a productive upwelling system can support. The study indicates that ocean productivity must be considered alongside fishing history, habitat quality, climate, and species composition when setting population targets. A reef cannot be evaluated accurately without accounting for the energy available to its food web.
The fossil record does not mean that Pacific sharks are safe. The comparison with ancient sediments may conceal more recent changes because the modern reference samples span roughly a century. Targeted shark fishing in the Gulf of Panama accelerated during the 1980s, so a steep decline in recent decades could be diluted when combined with sediments deposited earlier in the twentieth century. Other research has already shown that shark populations in Pacific Panama are under pressure from intensified fishing. The region may also be entering a period of ecological change: the seasonal upwelling that supplies the Gulf with cold, nutrient-rich water failed for the first time on record last year. If overfishing reduces shark populations while climate-driven changes weaken the productivity that supports them, the Pacific coast could be pushed toward a condition not experienced for thousands of years. Its apparent resilience may therefore represent a capacity for recovery rather than proof that recovery is unnecessary.
Developing the denticle method required more than a decade of work. The project began with the unconventional idea that reef sediments might contain enough shark scales to reconstruct ancient predator communities. Researchers first had to determine which denticles could be reliably identified, how to extract them from complex carbonate sediments, and how differences in denticle shape reflected shark species or ecological lifestyles. False-color scanning electron microscopy helped reveal the fine architecture of the scales, including ridges, crowns, and other microscopic features that can vary among sharks. These characteristics allow researchers to examine not only how many sharks were present but also whether the composition of the shark community changed through time. The work began when lead author Erin Dillon was an intern at STRI in 2014, continued through her doctoral research at the University of California, Santa Barbara, and later developed during her postdoctoral fellowship in Panama.
The study’s most powerful implication is that the ocean floor may preserve a record of ecological loss that conventional monitoring cannot see. Sharks are among the most difficult marine predators to survey because they move across large areas, occur at low densities, and can disappear from heavily fished ecosystems before scientists establish reliable population baselines. Dermal denticles provide a new source of evidence, linking the physical history of sediments with the biological history of predator communities. In Panama, they show that geography alone does not determine shark abundance: the productivity of the surrounding ocean can create radically different natural baselines on nearby reefs. As scientists extend the technique to other regions, microscopic “shark dandruff” could help reveal how many predators ecosystems once supported, distinguish natural differences from human-driven declines, and identify recovery targets grounded in the ecology of each reef rather than in assumptions based on the nearest surviving population.
Subject of Research: Shark abundance baselines, fossil dermal denticles, coral reef ecology, ocean productivity, and conservation targets.
Article Title: Fossil denticles reveal how ocean productivity shapes shark baselines and recovery potential.
Web References: https://doi.org/10.1126/science.aec2144
References: Dillon, E.M., McCauley, D.J., Gonzalez, M., Connolly, S.R., de Gracia, B., Cybulski, J.D., Norris, R.D., Gómez, M.M., Garcia-Pérez, I., Leonard, N.D., Zhao, J., García-Méndez, K., O’Dea, A. 2026. “Fossil denticles reveal how ocean productivity shapes shark baselines and recovery potential.” Science.
Image Credits: Erin Dillon, Jorge Ceballos, Aaron O’Dea, and Ashley Diedenhofen.
Keywords: Sharks, shark denticles, shark dandruff, coral reefs, fossil sediments, dermal denticles, Panama, Gulf of Panama, Caribbean Sea, ocean productivity, seasonal upwelling, overfishing, marine conservation, reef ecosystems, Science.
